SOW Attachment 1 - ICAO Aerodrome Design Manual Part 6.pdf

International Civil Aviation Organization

Approved by the Secretary General and published under his authority

Aerodrome Design Manual

First Edition — 2005

Doc 9157

AN/901

Part 6 Frangibility

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AMENDMENTS

Amendments are announced in the supplements to the Catalogue of ICAO Publications; the Catalogue and its supplements are available on the ICAO website at www.icao.int. The space below is provided to keep a record of such amendments.

RECORD OF AMENDMENTS AND CORRIGENDA

AMENDMENTS CORRIGENDA

No. Date Entered by No. Date Entered by

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FOREWORD

Proper design and installation of visual and non-visual aids (e.g. approach lighting towers, meteorological equipment, radio navigational aids) are prerequisites for the safety and regularity of civil aviation. At airports, various visual and non-visual aids are located near runways, taxiways and aprons where they may present a hazard to aircraft in the event of accidental impact during landing, take-off or ground manoeuvring. All such equipment and their supports shall be frangible and mounted as low as possible to assure that impact will not result in loss of control of the aircraft.

Much of the material included herein is closely associated with the specifications on frangibility of visual and non-visual aids contained in Annex 14 — Aerodromes, Volume I — Aerodrome Design and Operations and Volume II — Heliports. The purpose of this manual is to assist States in the implementation of these specifications and thereby help to ensure their uniform application.

Annex 14 — Aerodromes contains both Standards and Recommended Practices for objects to be made frangible. However, since the objective of this manual is to provide guidance on frangible design, distinction between the two types of specifications will not be made.

The manual incorporates guidance material on the design, testing and installation of frangible structures at airports and heliports and is based on the conclusions of the Fifth and Sixth Meetings of the ICAO Frangible Aids Study Group held in 1998 and 2003, respectively, as well as on current practices of several States.

It is intended that the manual be kept current. Future editions will be improved on the basis of the results of the work of regulators, industry and airport operators as well as on the basis of experience gained and comments and suggestions received from users of this manual. Readers are therefore invited to give their views, comments and suggestions on this edition. These should be directed to the Secretary General of

ICAO.

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TABLE OF CONTENTS

Page

Chapter 1. Introduction..................................................................................................................... 1-1

1.1 Definitions ............................................................................................................................. 1-1

1.2 What is frangibility................................................................................................................. 1-1

1.3 Obstacles to be made frangible ............................................................................................ 1-1

Chapter 2. Siting considerations .................................................................................................... 2-1

2.1 Siting of equipment ............................................................................................................... 2-1 Runway, stopway and taxiway edge lights ........................................................................... 2-1 Approach lighting system...................................................................................................... 2-1 Visual approach slope indicator systems.............................................................................. 2-1 Signs and markers ................................................................................................................ 2-2 Wind direction indicators (wind cones) ................................................................................. 2-2 ILS localizer .......................................................................................................................... 2-2 ILS glide path antenna system ............................................................................................. 2-2 MLS approach azimuth equipment ....................................................................................... 2-3 MLS approach elevation equipment ..................................................................................... 2-3 Anemometers........................................................................................................................ 2-4 Ceilometers........................................................................................................................... 2-4 Transmissiometers................................................................................................................ 2-4 Fencing ................................................................................................................................. 2-4

2.2 Preferred siting of equipment components ........................................................................... 2-5

Chapter 3. General design considerations.................................................................................... 3-1

3.1 Operational requirements ..................................................................................................... 3-1 Approach lighting systems.................................................................................................... 3-1 Wind direction indicators....................................................................................................... 3-1 ILS localizer .......................................................................................................................... 3-1 ILS glide path ........................................................................................................................ 3-1 MLS approach azimuth equipment ....................................................................................... 3-1 MLS approach elevation equipment ..................................................................................... 3-1 Anemometers........................................................................................................................ 3-2 Ceilometers........................................................................................................................... 3-2 Transmissiometers................................................................................................................ 3-2 Fencing ................................................................................................................................. 3-2

3.2 Environmental service conditions ......................................................................................... 3-2 Wind loading ......................................................................................................................... 3-2 Jet blast................................................................................................................................. 3-3 Vibration ................................................................................................................................ 3-3

3.3 Frangibility requirements....................................................................................................... 3-3

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Page

Chapter 4. Design for frangibility ................................................................................................... 4-1

4.1 Design philosophy................................................................................................................. 4-1

4.2 Failure mode ......................................................................................................................... 4-2

4.3 Impact mode ......................................................................................................................... 4-2

4.4 Energy transfer ..................................................................................................................... 4-2

4.5 Frangibility concepts ............................................................................................................. 4-3 General ................................................................................................................................. 4-3 Frangible connections........................................................................................................... 4-3 Frangible members ............................................................................................................... 4-4 Frangible mechanism............................................................................................................ 4-5

4.6 Break-away or failure mechanisms....................................................................................... 4-5

4.7 Material selection .................................................................................................................. 4-5

4.8 Electrical components........................................................................................................... 4-6

4.9 Design criteria for frangibility ................................................................................................ 4-6 Elevated runway and taxiway edge lights............................................................................. 4-6 Taxi guidance signs .............................................................................................................. 4-7 PAPI/APAPI and T-VASIS/AT-VASIS................................................................................... 4-8 Approach lighting systems.................................................................................................... 4-8 Supporting structures............................................................................................................ 4-8 ILS/MLS structures and other non-visual aids ...................................................................... 4-16

Chapter 5 . Testing for frangibility.................................................................................................... 5-1

5.1 General ................................................................................................................................. 5-1

5.2 Testing procedures ............................................................................................................... 5-1 Elevated runway and taxiway edge lights............................................................................. 5-1 Taxiing guidance signs ......................................................................................................... 5-2 PAPI/APAPI and T-VASIS/AT-VASIS................................................................................... 5-3 Approach lighting towers and similar structures ................................................................... 5-3 Wind direction indicators/transmissometers/forward-scatter meters.................................... 5-5 ILS/MLS structures ............................................................................................................... 5-6

5.3 Tests by manufacturers and testing organizations ............................................................... 5-6

Chapter 6. Numerical simulation methods for evaluating frangibility......................................... 6-1

6.1 General ................................................................................................................................. 6-1

6.2 Analyses................................................................................................................................ 6-1

6.3 Finite element analysis (FEA) approach............................................................................... 6-2

6.4 Hybrid approach.................................................................................................................... 6-2

6.5 Verification by computational analysis.................................................................................. 6-3

Chapter 7. Installation, inspection and maintenance .................................................................... 7-1

7.1 General ................................................................................................................................. 7-1

7.2 Installation ............................................................................................................................. 7-1

7.3 Inspection and maintenance................................................................................................. 7-1

References

1-1

Chapter 1

INTRODUCTION

1.1 DEFINITIONS

Frangible object. An object of low mass designed to break, distort or yield on impact so as to present the minimum hazard to aircraft.

Impact load. A sudden application of a load or force by an object moving with high velocity.

Break-away or failure mechanism. A device which has been designed, configured and fabricated in such a way that it is very sensitive to one type of loading, usually resulting from a time dependent dynamic impact, but immune to the normal environmental and operational loads imposed on the mechanism during the lifetime of the structure. The “break-away mechanism” can be designed in conjunction with the joints of the structure and/or designed independent of the joints of the structure.

Impact energy. The energy required for an object to break, distort or yield when subjected to an impact load.

1.2 WHAT IS FRANGIBILITY

At airports, various visual and non-visual aids (e.g. approach lighting towers, meteorological equipment, radio navigational aids) are located near runways, taxiways and aprons, where they may present a hazard to aircraft in the event of accidental impact during landing, take-off or ground manoeuvring. All such equipment and their supports should be frangible and mounted as low as possible to ensure that impact does not result in loss of control of the aircraft. This frangibility is achieved by use of lightweight materials and/or the introduction of break-away or failure mechanisms that enable the object to break, distort or yield under impact.

1.3 OBSTACLES TO BE MADE FRANGIBLE

1.3.1 Obstacles are defined as all fixed objects, or parts thereof, that are located on an area intended for the surface movement of aircraft or that extend above a surface intended to protect an aircraft in flight.

The first objective should be to site objects so that they are not obstacles. Nevertheless, certain airport equipment and installations, because of their function, must be located in an operational area. All such equipment and installations as well as their supports should be of minimum mass and frangible in order to ensure that impact does not result in loss of control of the aircraft.

1.3.2 Annex 14 — Aerodromes, Volume I — Aerodrome Design and Operations, Chapter 5, specifies that elevated approach lights and their supporting structures should be frangible except that, in that portion of the approach lighting system beyond 300 m from the threshold:

a) where the height of a supporting structure exceeds 12 m, the frangibility requirement should apply to the top 12 m only; and

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b) where a supporting structure is surrounded by non-frangible objects, only that part of the structure that extends above the surrounding objects should be frangible.

1.3.3 Annex 14, Volume I, Chapter 9, specifies that any equipment or installation required for air navigation purposes which must be located:

a) on a runway strip (instrument or non-instrument); or

b) on a runway end safety area; or

c) on a clearway and which would endanger an aircraft in the air; or

d) on a taxiway strip or within the distances specified in Annex 14, Volume I, Table 3-1, column 11);

should be frangible and mounted as low as possible.

1.3.4 Annex 14, Volume I, Chapter 9, also specifies that any equipment or installation required for air navigation purposes which must be located on or near a strip of a precision approach runway category I, II or III and which:

a) is situated within 240 m from the end of the strip and within:

1) 60 m of the extended centre line where the code number is 3 or 4;

2) 45 m of the extended centre line where the code number is 1 or 2; or

b) penetrates the inner approach surface, the inner transitional surface or the balked landing surface;

should be frangible and mounted as low as possible.

1.3.5 Furthermore, Annex 14, Volume I, Chapter 9, specifies that any equipment or installation required for air navigation purposes which is an obstacle of operational significance in accordance with 4.2.4, 4.2.11, 4.2.20 or 4.2.27 of the Annex should be frangible and mounted as low as possible.

1.3.6 Airport equipment and installations which, because of their particular air navigation function, have to be located in an operational area include:

— elevated runway, taxiway and stopway lights

— approach lighting systems

— visual approach slope indicator systems

— signs and markers

— wind direction indicators

— Instrument landing system (ILS) localizer equipment

— ILS glide path equipment

— ILS monitoring antenna

Part 6. Frangibility Chapter 1. Introduction 1-3

— Microwave landing system (MLS) approach azimuth equipment

— MLS approach elevation equipment

— MLS monitoring antenna

— radar reflectors

— anemometers

— ceilometers

— transmissometers

— forward-scatter meters

— fencing

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Chapter 2

SITING CONSIDERATIONS

2.1 SITING OF EQUIPMENT

2.1.1 Guidance or specifications on the siting of navigational aids are contained in Annex 10 — Aeronautical Communications, Volume I — Radio Navigation Aids, and Annex 14 — Aerodromes, Volume I

— Aerodrome Design and Operations, and Volume II — Heliports, and their related manuals. These should be taken into account when siting navigational aids. In general, equipment and security fencing should be sited as far away from the runway and taxiway centre lines as practicable.

Runway, stopway and taxiway edge lights

2.1.2 Runway, stopway and taxiway edge lights should be placed along the edges of the area declared for use as runway, stopway and taxiway, respectively, or outside the edges of these areas at a distance of not more than 3 m. Similarly, runway threshold and end lights should be placed in a row at right angles to the runway axis as near to the extremity of the runway as possible and, in any case, not more than 3 m outside the extremity. Elevated runway, stopway and taxiway edge lights are obstacles and therefore should be frangibly mounted.

Approach lighting system

2.1.3 An approach lighting system can only be located along the extended runway centre line.

Annex 14, Volume I, specifies three types of approach lighting systems, i.e. simple, precision approach category I and precision approach categories II and III. All approach lighting systems begin at a specified distance from the runway threshold and extend outwards in the direction of approach to the runway. Where the threshold is at the extremity of the runway, the entire lighting system is elevated and the lights may constitute an obstacle to air navigation. Where the threshold is displaced from the extremity, that part of the system between the displaced threshold and the runway extremity is usually inset and as a result, the lights do not constitute an obstacle.

Visual approach slope indicator systems

2.1.4 A visual approach slope indicator system is required to be installed at a specified location near the runway. Annex 14, Volume I, includes specifications for two types of visual approach slope indicator systems: T-VASIS and PAPI (T-visual approach slope indicator system and precision approach path indicator, respectively). These systems consist of elevated light units located either on one or both sides of the runway at specified distances beyond the threshold. The number of light units involved and their disposition depend on the type of system. In general, the light units are located between 15 m and 42 m from the edge of the runway.

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Signs and markers

2.1.5 Signs and markers need to be located as near to the edge of the pavement as their construction will permit for easier visibility by the pilot of an aircraft. Those located near a runway or taxiway need to be sufficiently low to preserve clearance for propellers and for the engine pods of jet aeroplanes. Those farther from the runway or taxiway need to be larger in order to provide for inscriptions large enough to be read by the pilot.

Note.— For further information on location of the visual aids discussed in 2.1.2 to 2.1.5, please refer to Annex 14, Volume I, Chapter 5.

Wind direction indicators (wind cones)

2.1.6 A wind direction indicator should be located such that the indicator is visible from aeroplanes in flight or on the movement area. This can be outside the areas mentioned in 1.3.3 and 1.3.4. A further consideration in selecting the site is that the indicator is free from the effects of air disturbances caused by nearby objects.

ILS localizer

2.1.7 The preferred location for the localizer antenna array is on the extended runway centre line beyond the far end of the runway. This location permits the radiated on-course signal to overlie the runway centre line. The following factors govern site selection:

a) coverage requirement;

b) type of localizer array;

c) obstacles or vertical reflecting surfaces within the desired localizer coverage volume;

d) obstacle clearance and missed approach criteria;

e) location of monitoring antenna; and

f) technical siting considerations.

ILS glide path antenna system

2.1.8 The lateral displacement of the ILS glide path antenna system should not be less than 120 m with respect to the runway centre line. The longitudinal location should be selected to place the ILS reference datum as close as possible to the recommended nominal value of 15 m above the threshold. In general, the following factors govern site selection:

a) desired operating limits with respect to approach speeds and rates of descent of aeroplanes;

b) the position of obstacles in the final approach area, the aerodrome sector and the missed approach areas, and the resulting obstacle clearance limits;

c) runway length available;

Chapter 2. Siting considerations 2-3

d) location of monitoring antenna; and

e) technical siting considerations.

Note.— For further guidance on siting discussed in 2.1.7 and 2.1.8, please refer to Annex 10, Volume I, Chapter 3 and Attachment C.

MLS approach azimuth equipment

2.1.9 The preferred location for the approach azimuth equipment antenna (analogous to the ILS localizer) is on the extended centre line beyond the stop end of the runway and on the extended centre line before the threshold for the back azimuth equipment antenna, if provided. The following factors govern site selection:

a) need for collocation with existing ILS localizer antenna array;

b) obstacle clearance and missed approach criteria;

c) multipath considerations;

d) possible interference problem when the MLS has to be located in the approach lighting area;

e) location of monitoring antenna; and

f) technical siting considerations.

MLS approach elevation equipment

2.1.10 The approach elevation equipment antenna (analogous to the ILS glide path) should be located offset of the runway. The location is selected so that the asymptote of the minimum glide path crosses the threshold at the MLS approach reference datum. The following factors govern site selection:

a) need for collocation with existing ILS glide path antenna;

b) obstacle clearance and missed approach criteria;

c) multipath considerations;

d) location of monitoring antenna; and

e) technical siting considerations.

2.1.11 In the case of ILS/MLS collocation, the MLS approach elevation equipment antenna should be positioned forward of the ILS glide path and outboard (further away from the runway centre line) or inboard (nearer to the runway centre line) of the ILS antenna.

Note.— For further guidance on the siting discussed in 2.1.10 and 2.1.11, please refer to Annex 10, Volume I, Chapter 3 and Attachment G.

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Anemometers

2.1.12 In view of the usually flat, open aspects of most aerodromes, generally speaking, the surface wind flow over the runway or runway complex may be considered homogeneous. Surface wind observations should be representative of conditions at a height of 6 m to 10 m above the runway, and this usually means that the height of the anemometer mast itself is between 6 m and 10 m. Under normal circumstances, therefore, anemometers can be located outside the runway strip and should not penetrate a transitional obstacle limitation surface nor infringe taxiway strips. Where it is necessary to locate anemometers inside the strip in order to provide representative observations for landing and take-off operations, it is most unlikely (although not totally impossible) that local conditions would require siting closer than 60 m from the runway centre line. Hence, on runway strips that include a precision approach runway and where local conditions require an anemometer to be located inside the strip, the anemometer need not penetrate the inner transitional surface and the obstacle free zone. Consideration should be given to using frangible approach lighting towers for the anemometer masts.

Ceilometers

2.1.13 Observations of height of cloud base required for landing operations should be representative of the approach area, but in the case of precision approach runways, it should be representative of the middle marker site of the instrument landing systems. Measurement of the height of cloud base for precision approach runways should be made automatically by a ceilometer located near the middle marker. Where for any reason this is not possible, the ceilometer should be located within the runway strip but, except in very exceptional local circumstances, it need not penetrate the obstacle free zone. Where a ceilometer is used on non-precision or non-instrument approach runways, representative observations of height of cloud base can normally be obtained by locating the instrument outside the strip. The ceilometer rarely exceeds 1.5 m in height and usually comprises transmitter and receiver units.

Transmissometers

2.1.14 Transmissometers normally comprise transmitter and receiver units mounted on pylons approximately 1.5 m to 4.5 m high and separated along a baseline of 10 m to 200 m. Up to three sets of units may be required per runway. The transmissometer units should be located no farther than 120 m from the runway centre line. This means that transmissometers must be located within the runway strip. However, it would only be in very exceptional local circumstances that they would need to be located less than 60 m from the runway centre line and thus penetrate the obstacle free zone.

Fencing

2.1.15 Fencing should be provided on an aerodrome to deter the inadvertent or premeditated access of an unauthorized person onto a non-public area of the aerodrome. Fencing should also be provided to prevent the entrance into the movement area by animals large enough to be a hazard to aircraft. In general, the fence should be sited as far away from the runway and taxiway centre lines as practicable.

2.1.16 The fence should be provided with gates to allow for vehicle access to the movement area and for convenient access to areas outside the airport boundary by rescue and fire fighting vehicles. Gates, in particular heavy, remote-controlled gates, should be positioned outside of operational areas and as far away as possible from the runway or its extended centre line to minimize the structural damage to an aeroplane in the event of it colliding with a fence or its gates. Furthermore, so-called “crash gates” should be used to provide rescue and fire fighting vehicles easy access to areas outside the airport boundary.

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2.2 PREFERRED SITING OF EQUIPMENT

COMPONENTS

2.2.1 In cases where frangible design of equipment would be impracticable or would jeopardize the operational performance to stipulated requirements, the object should be relocated or otherwise positioned so as to not present a hazard to aircraft.

2.2.2 In systems design, consideration should be given to the possibility of arranging components in such a way as to limit the number and/or mass of obstacles on those areas that must be maintained free of all objects, except for frangible equipment and installations required for air navigation purposes (1.3.3 and

1.3.4 refer).

2.2.3 A review of relevant accident data reveals that a majority of the accidents in the overrun area occur within a distance of 300 m from the runway end. All equipment located within this area should, therefore, be of low mass and frangible. Where practicable, all equipment located beyond a distance of 300 m from the runway end should be of low mass and frangible. The available accident data also indicate that a majority of accidents occur where the aeroplane comes to rest within the graded portion of the runway strip. All equipment located within this portion of the strip should, therefore, be of low mass and frangible.

Where practicable, all equipment located within the non-graded portion of the runway strip should be of low mass and frangible.

2.2.4 Where the function of equipment requires siting in an area that presents a hazard to aircraft, those components which can be moved outside the hazard area should be moved.

2.2.5 Where collocation is essential, an effort should be made to place components below the surface of the ground, where practicable.

2.2.6 Due to its heavy mass, the transmitter housing for ILS installations cannot be made frangible.

Therefore, when planning for the installation of an ILS, the location of the transmitter housing for the localizer as well as for the glide path should be carefully considered. In no instance should the transmitter housing for the ILS localizer be located within the runway end safety area (or the extension thereof within a distance of 300 m from the runway end). Where practicable, the transmitter housing for the ILS glide path should be located outside the runway strip. In any event, the lateral displacement of the transmitter housing for the ILS glide path should not be less than 120 m with respect to the runway centre line.

2.2.7 MLS installations, including both the currently designed azimuth antenna and the elevation antenna are heavy equipment and cannot be made frangible. Therefore, these installations should be positioned so as to present the least hazard to aircraft. The MLS azimuth antenna should be located as far away as practicable from the runway end and, in any event, not closer than 300 m. Where practicable, the MLS elevation antenna should be located outside the runway strip.

2.2.8 Existing structures located within a distance of 300 m from the runway end not meeting the frangibility requirement, such as an existing non-frangible ILS localizer antenna array, should be replaced by a frangible structure or relocated beyond a distance of 300 m from the runway end. Similarly, structures located within the graded portion of the runway strip not meeting the frangibility requirement, such as an existing non-frangible ILS glide path antenna, should be replaced by a frangible structure, if practicable, and relocated within the non-graded portion of the runway strip. In this context, it should be noted that, generally, the lateral displacement of the ILS glide path antenna system should not be less than 120 m with respect to the runway centre line (2.1.8 refers).

3-1

Chapter 3

GENERAL DESIGN CONSIDERATIONS

3.1 OPERATIONAL REQUIREMENTS

3.1.1 It is normal for a frangible structure to deflect when exposed to environmental loads. However, it is important that deflection of the structure remain within limits so as not to affect the signal quality of the aid which the structure supports. To this end, 3.1.2 to 3.1.10 contain guidance on permissible deflection limits, i.e. deflection tolerances in respect of aids that are installed on tall poles or structures.

Approach lighting systems

3.1.2 When subjected to the envisaged environmental loads, the deflection of the structure should be such that the deflection of the light beam does not exceed ±2 degrees in the vertical axis and ±5 degrees in the horizontal axis.

Wind direction indicators

3.1.3 It is unnecessary to establish deflection tolerances in respect of this aid.

ILS localizer

3.1.4 When establishing deflection tolerances for the structure, the applicable system monitoring limits for each installation and facility performance category of operations should be taken into account.

ILS glide path

3.1.5 When establishing deflection tolerances for the structure, the applicable system-monitoring limits for each installation and facility performance category of operations should be taken into account.

MLS approach azimuth equipment

3.1.6 When establishing deflection tolerances for the equipment, the recommended permissible operating tolerance for beam accuracy should be taken into account.

MLS approach elevation equipment

3.1.7 When establishing deflection tolerances for the equipment, the recommended permissible operating tolerance for beam accuracy should be taken into account.

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Note.— For further guidance on deflection tolerances discussed in 3.1.4 to 3.1.7, please refer to Annex 10, Volume I.

Anemometers

3.1.8 This equipment comprises a wind speed sensor and a direction sensor, usually collocated on the same mast. The mast should be subject to minimum vertical deflections so as to ensure that the sensors are always in equilibrium. This is necessary for the wind speed sensor (or propeller) to ensure that the response time is not adversely affected, and for the direction sensor (wind vane) to ensure that it does not have a preferred null position but has a single equilibrium position with respect to each wind direction.

Ceilometers

3.1.9 The structure has to be sufficiently stable in order to provide for measurement accuracy but less so than for transmissometers.

Transmissometers

3.1.10 Precise alignment of the transmitter and receiver are required in order not to compromise measurements. In view of this, the structure has to be sufficiently stable with minimal deflection in order to provide for measurement accuracy when subjected to envisaged environmental loads.

Fencing

3.1.11 The fence and its gates have to be sufficiently stable to serve the purpose and cannot be made frangible without detrimental effects to their intended function. However, the structure should be segmented and the design such that the failure mode of the structure would be “windowing” in the event of an aeroplane colliding with the fence.

3.1.12 Notwithstanding the above, a lightweight frangible fence should be installed when located between frangible approach lighting towers or when located to protect the ILS critical and sensitive areas against unlawful interference.

3.2 ENVIRONMENTAL SERVICE

CONDITIONS

3.2.1 Although required to be of a frangible design to minimize hazard to aircraft in the event of impact, the object must also be capable of withstanding the environmental conditions to which it may be exposed during normal service. The following identifies several conditions that should be addressed by the designer.

Specifics on these as well as other conditions can be found in the pertinent documents of the authority having jurisdiction.

Wind loading

3.2.2 The object should be sufficiently strong and rigid to meet the operational requirements of its normal service at the specified level of wind speed (e.g. 140 km/h (75 kt) with 12.5 mm of ice cover). In

Chapter 3. General design considerations 3-3 addition, the object should be capable of surviving a higher level of wind speed (e.g. 210 km/h (113 kt)). In designing, the wind loading should be based upon that historical projection (e.g. 50-year mean recurrence interval).

Jet blast

3.2.3 Loading generated by jet engine blast should not cause failure or permanent deformation. The exhaust contours of the design aircraft should be applied. The actual loading depends on the distance and orientation of the object from this aircraft.

Vibration

3.2.4 The components of the structure, which form the supporting means for the object, should be designed so that no member or combination of members vibrates at or close to the resonating frequencies induced by the aerodynamic response to wind forces, jet blast, earthquakes, etc.

3.3 FRANGIBILITY REQUIREMENTS

3.3.1 Equipment and its supports, located in areas described in section 1.3, should be frangible in order to ensure that they will break, distort or yield in the event that they are accidentally impacted by an aircraft. The design materials selected should preclude any tendency for the components, including the electrical conductors, etc., to “wrap around” the colliding aircraft or any part of it.

3.3.2 A frangible structure should be designed to withstand the static and operational wind or jet blast loads with a suitable factor of safety but should break, distort or yield readily when subjected to the sudden collision forces of a 3 000-kg aircraft airborne and travelling at 140 km/h (75 kt) or moving on the ground at 50 km/h (27 kt).

3.3.3 The frangibility of the design should be proven either by means of full-scale tests, computer evaluations or by calculations based on comparison with similar already approved structures possibly supported by additional component tests.

4-1

Chapter 4

DESIGN FOR FRANGIBILITY

4.1 DESIGN PHILOSOPHY

4.1.1 Equipment (and its supports) located near runways and taxiways should be designed to be frangible in order to limit the hazard to aircraft accidentally impacting them from any direction either in flight or during ground manoeuvring. Impact may affect flight safety in three ways:

a) the aircraft may lose momentum;

b) the aircraft may change direction; and

c) the aircraft may suffer structural damage.

4.1.2 The amount of momentum lost is mathematically governed by the integral of force over time.

This implies that both the magnitude of the impact load and its duration should be as minimal as possible.

4.1.3 The structural damage to the aircraft is related to the amount of energy it requires to move the obstacle, or part of it, out of the way and should therefore be limited. This energy can be broken down into the following components:

a) energy to activate break-away or failure mechanisms;

b) energy required for plastic and/or elastic deformation of the obstacle, or part of it; and

c) energy required to accelerate the obstacle, or part of it, up to at least the aircraft velocity.

4.1.4 The energy required to activate break-away or failure mechanisms depends on the efficiency of their design and on the number of mechanisms to be activated. The energy absorbed by plastic or elastic deformation of the structure is strongly dependent on the choice of material: the amount will be higher for ductile materials with high-yield strains. The (kinetic) energy required to accelerate the obstacle, or part of it, is dependent on the aircraft velocity, which is not a design variable, and on the mass to be accelerated.

Therefore, the mass should be limited, for example, by using low-mass materials and/or by limiting the amount of structure to be accelerated, which can be accomplished by incorporating suitably located break-away or failure mechanisms in the structure.

4.1.5 The structural damage to the aircraft is also related to the contact area between aircraft and obstacle through which the energy transfer takes place. A larger contact area is shown to prevent obstacles cutting deeply into the aircraft structure. This has an implication on the structural geometry of the obstacle.

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4.2 FAILURE MODE

4.2.1 In order to meet the frangibility requirements, different failure mechanisms can be applied. For example, structures can be of modular design, which on impact “open a window” for the aircraft to pass through, or of a one-piece design, which on impact do not disintegrate but are entirely deflected away by the aircraft.

4.2.2 In the case of a modular design, the structure should contain break-away or failure mechanisms which, apart and together, require only a minimum amount of energy for their activation. This concept permits moving the least amount of mass out of the way of a colliding aircraft. The sequence of events is easier to predict as the structure behaves in a brittle way, disintegrating preferably at small deflections. It also reduces to a minimum the possibility of a “wrap-around” effect. However, in this case, detached fragments may be impacted by other parts of the aircraft passing the location of impact slightly later.

4.2.3 In the case of a one-piece design, the frangibility must be guaranteed by a complete failure of the structure, which is achieved by the random failure of structure members, instead of by failure of predetermined break-away or failure mechanisms. This implies that eventually the entire structure will be involved in the impact, resulting in a relatively high value of the kinetic energy required to move the structure out of the way. Therefore, this type of failure mechanism seems to be suitable only for lightly loaded structures, i.e. those meant to carry low-mass equipment. Moreover, due to the continuous nature of the structure, the sequence of events is difficult to predict and the tendency to “wrap around” the aircraft should be considered an additional hazard.

4.3 IMPACT LOAD

The impact load is a rapidly changing dynamic load of short duration. Typical loading and response times are in milliseconds. The impact load influences the frangibility performance in two ways. First, the maximum impact load may adversely affect the structural integrity of the aircraft. Second, the integral of the impact load over the duration of the impact leads to a change of momentum (including direction) of the aircraft.

4.4 ENERGY TRANSFER

4.4.1 During an impact, energy will be transferred from the aircraft to the obstacle. As the damage to the aircraft is proportional to the energy transferred, it should be limited. The energy required is estimated as follows:

a) The energy required to cause a break-away mechanism to fracture is determined in a laboratory on a component scale; this amount of energy must be multiplied by the number of mechanisms to be broken;

b) The energy required for plastic and/or elastic deformation is calculated or determined by simple tests; this energy is often negligible when stiff and brittle materials are applied in a modular design;

and

c) The kinetic energy required for acceleration of the fragments, or the total structure in the case of a one-piece design, is calculated using the known mass and the representative aircraft velocity.

4.4.2 The estimation should be done for all different scenarios of an aircraft impacting the structure.

Chapter 4. Design for frangibility 4-3

4.5 FRANGIBILITY CONCEPTS

General

4.5.1 The frangible structure should include concepts such as low-mass members, brittle or low-toughness members and connections, and/or suitable break-away mechanisms. Various design concepts exist, each with its own advantages and disadvantages. Designs may incorporate one or more concepts in order to ensure frangibility.

Frangible connections

4.5.2 In a frangible connection design, frangibility is incorporated in the connection, which carries the design load but fractures at impact. The structural member is not designed to break but rather to transfer the impact force to the connection. A stiff, lightweight member provides efficient load transfer to the connection and minimizes the energy absorbed from bending and mass acceleration. The connection should break at low energy levels, as determined by impact tests. Types of frangible connections include neck-down or fuse bolts, special material or alloy bolts, countersunk rivets or tear-through fasteners, and gusset plates with tear-out sections. Some of these are described as follows:

a) Fuse bolts. Failure of this type of connection is induced by providing a “stress raiser”, due to removal of material from the bolt shank. One method used to achieve this is to machine a groove to reduce the bolt diameter or to machine flats in the sides of the bolt, making it weaker in a specific direction. Shear strength is maintained and tensile strength is reduced by machining a hole through the bolt diameter and locating it out of the shear plane. Fuse bolts must be carefully installed to ensure they are not damaged or overstressed when tightened. The problem with fuse bolts is that the stress raiser may shorten the fatigue life of the bolt or may propagate under service loads and fail prematurely. Fuse bolts with machine grooves are commercially available. See Figure 4-1 for an example of the application of such fuse bolts.

b) Special material bolts. Use of fasteners manufactured from special materials eliminates the need for extensive machining or fabricating and allows the basic design to consist of conventional cost-effective techniques. The fasteners are sized to carry the design loads but are made from material with low-impact resistance. Materials such as steel, aluminium and plastic should be selected based on strength and minimum elongation to failure. Aluminium bolts of ANSI alloy designation 2024-T4 are recommended because they are as strong as stainless steel bolts but have only 10 per cent ultimate elongation compared to 50 per cent for stainless steel. Plastic bolts may have low elongation values, but their strength would have to be established by testing. Since frangibility is based on material selection, it is extremely important to purchase hardware with guaranteed compliance of physical properties.

c) Tear-through fasteners. Fasteners such as countersunk rivets can be used to sustain shear loads but tear through the base material if the impact force creates a tension load. The hole in the base material is accurately machined to grip a minimum amount of the area under the head of the fastener. The taper of the countersunk head also helps initiate the pull-through. This technique relies heavily on the manufacturing process and requires extensive quality inspection.

d) Tear-out sections. Connecting gusset plates can be designed with notches that will tear out with the member. In this type of connection the fastener does not break but instead is used to pull out a section of the gusset plate. Fatigue life and manufacturing quality are the primary design considerations.

4-4 Aerodrome Design Manual

Figure 4-1. Fuse bolt

Frangible members

4.5.3 In this design, the structural member is required to fail and not the end connection. The member should achieve a segmented-type separation along its length, thereby minimizing the amount of mass acceleration and reducing the potential of a wrap-around effect. Brittle materials such as plastic, fibreglass or other non-metals are more likely to be used than metals. The main advantage with frangible members is that impact forces do not have to be carried back to the connection in order to fail the section. This means that energy is not absorbed by bending the member as in a frangible connection design. The disadvantage is that special, non-metallic materials require extensive testing to establish properties to be used for deflection analysis of the structure. The analysis should also be confirmed by doing full-scale load tests on the structure. Non-metals must also contain ultra-violet inhibitors for protection against the environment.

4.5.4 Plastic extrusions or fibreglass-moulded sections are available in angular or tubular shapes.

Members can also be specially fabricated to incorporate built-in break points. This is done by bonding one material to another at points along the length of the member. The bond line then becomes the fracture initiation point in the member.

Chapter 4. Design for frangibility 4-5

Frangible mechanism

4.5.5 Frangibility can be incorporated into the support structure by means of a mechanism that slips, breaks or folds away on impact and removes the structural integrity of the support. A frangible mechanism can be designed to withstand high wind loads but remain very sensitive to impact loads. Frangible mechanisms tend to be directional in strength, i.e. they carry high tension and bending but very low shear.

4.5.6 Friction joints used as frangible mechanisms can supply high strength normal to the sliding surface but slip when the force is applied parallel to the sliding surface. In a support structure, impact forces are predominantly horizontal. Friction joints should be designed so that the slip plane is horizontal and complete failure occurs if impacted in any direction in that plane. This is achieved by using flange-type couplings on the ends of tower legs or interconnected tubes that slide apart on impact.

4.5.7 “Swing-away” support members can also be used as frangible mechanisms. These are incorporated into the structure to provide stability but if broken away on impact, leave the structure unstable and allow it to fracture. This type of design, however, may require large amounts of mass to be moved out of the way before failure.

4.5.8 Any design using frangible mechanisms has to ensure that no slippage or change in shape occurs from cyclic loading. For example, in a design using interconnecting tubes, vortex shedding on a tube caused by jet blast or wind could loosen or separate it from its counterpart.

4.6 BREAK-AWAY OR FAILURE

MECHANISMS

The location of break-away or failure mechanisms should be such that disintegration results in components of predictable mass and size, which, in case of a secondary impact, do not present a greater hazard than they present as part of the undamaged structure.