Landing Gear Problems: Guidance for Flight Crews
Landing Gear Problems: Guidance for Flight Crews
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Flight crews encountering problems with the operation of landing gear must firstly, Fly the Airplane, then follow the appropriate EICAS or ECAM procedure, Emergency or Abnormal Checklist (EAC)/Quick Reference Handbook (QRH), Operations Manual and AFM direction and guidance for the resolution of problems and the continued safe conduct of the flight.
This article covers some of the extensive range of airmanship considerations associated with landing gear problems that will vary from type to type. Crews should study Landing Gear emergency procedures in depth and regularly. When a landing gear problem does occur, it may well be as part of a larger failure scenario that has led to a loss of hydraulics, electrics and/or engines. Time available may also be limited if the failure becomes apparent towards the very end of a flight.
Problem Identification
- Consider any unusual occurrences prior to manifestation of the problem. For example an unusual noise or slow movement of landing gear on a previous sector. Is there any previous history of gear malfunctions in the aircraft Technical Log? If so, establish whether rectification was carried out or "no fault found" was recorded - the latter may indicate an intermittent fault.
- When surface temperatures are low enough for frozen deposits to be present, consider whether the mechanisms might have become frozen during flight as a result of prior extended taxiing through slush or wet snow. Flight above the freezing level for prolonged periods may result in the landing gear becoming frozen in the retracted position. This is less of a problem on most modern commercial transport aircraft, where the gear bays are largely enclosed by doors unless retraction/extension is in progress, but is a relatively common occurrence on aircraft such as the LOCKHEED C-130. If it is suspected that taxiing through slush or wet snow may have led to deposits adhering to the landing gear assemblies, then it may be advisable to cycle the gear after initial post take off retraction to try and shed any deposits.
- In an environment characterised by high levels of dust or fine sand, especially if the aircraft is not regularly exposed to it, consider if the landing gear mechanism may be jammed as a result of insufficient lubrication and contamination by sand and dust.
Management of Aircraft Systems
Resolving landing gear problems can involve disabling features that control the configuration of other systems, for example a proximity switch on a landing gear leg that will signal whether a system should be in ground mode or air mode. It may also be necessary to inhibit part(s) of the Ground Proximity Warning System (GPWS). Such consequential matters need to be understood and properly briefed at appropriate stages of a flight where they are likely to have an effect, as will those aspects listed under Flight Planning Considerations (see below), which can apply whenever the landing gear has malfunctioned in flight.
Landing with Gear in an Abnormal Position
The crew should have studied the Operations Manual and QRH landing gear abnormalities in depth before ever meeting this problem. This is particularly important when changing aircraft types as procedures can vary considerably.
Consider jettisoning fuel (or burning the fuel) down to safe reserves, prior to commencing the approach in order to minimise fire risk and to reduce the approach speed. It is important to brief the cabin crew and passengers and to ensure the cabin and flight deck are fully secure. Securing flight deck articles may take longer than might be anticipated.
Following a landing gear problem that has not resulted in a leg collapse on landing, it may be appropriate to remain on the runway until getting the landing gear “pinned” before attempting to taxy clear. Cabin crew briefings must cover all the possible outcomes, which could include evacuation on the runway or an eventual taxi clear/to normal parking. It may not be necessary to evacuate the aircraft after landing, even if part of the structure is in contact with the ground; if there is no fire present, it may be better to disembark the passengers via steps.
Since there are usually three ways to get landing gear down, the normal system, an alternate system and a freefall option, it is essential to try all of these before preparing to land with abnormal gear status i.e. with one or more legs indicating not locked down.
If the problem affects one set of main landing gear only, for some types it may be recommended to land with all the other main gear retracted. However, the general consensus for larger aircraft is to land on all available landing gear, because even if the landing gear collapses, it will absorb some of the aircraft’s energy and momentum and it is far better the impact is absorbed by the landing gear, than by the fuselage. In all cases, crews must be familiar with and follow the instructions and guidance in the AFM or Operations Manual for their particular type.
If the problem relates to the nose landing gear only, it may be appropriate to hold the nose off the runway initially. If practicable, shifting the centre of gravity aft (within the allowable limits for landing) by repositioning passengers or freight will assist, as will not arming autobrakes and using wheel brakes in a manner compatible with elevator authority/runway length. However, it is important to lower the nose smoothly onto the runway well before elevator authority is lost and to follow guidance in the AFM or Operations Manual.
In all cases when landing with abnormal gear, use reverse thrust only in accordance with the manufacturer’s guidelines. Particular care is required if the drill advises shutdown of some engines prior to landing, or shutdown of all or some of the engines at touchdown or at some point during the landing roll out; the crew must understand fully the implications of losing engine-driven systems and take particular care not to be distracted from the main task, which is to bring the aircraft to a safe stop.
In the past, a low level ‘flypast’ has sometimes been flown to allow persons on the ground to inspect the landing gear. In certain circumstances, if the inspection is made by appropriately qualified technical personnel, this can be valuable. However, it is a pointless exercise if merely seeking to establish whether one or more landing gear legs are ‘down’ since the issue is not usually whether they are down but whether they are locked down, which cannot be reliably determined by persons on the ground whether qualified or not. The ‘flypast’ option is proscribed or severely qualified in many Operations Manuals because it is not covered in training and would constitute an unmitigated heightened risk in most Safety Management Systems.
Planned flight with Landing Gear in the Down Position
Such a flight must only be undertaken in accordance with the procedures and limitations in the AFM or Operations Manual. It will usually also require the express approval of the Flight Operations Department of the Aircraft Operator as well as appropriate entries in the Aircraft Technical Log which allow the aircraft to be released to service, usually in accordance with the Minimum Equipment List (MEL). Depending upon aircraft type, pre-flight preparation may include maintenance action to remove some gear doors or making specific switch selections in the avionics compartment. In some cases, there may be a requirement to fit the undercarriage ground lock pins or similar devices to ensure the undercarriage remains down & locked.
Limitations on speed (VLE)may affect other airspace users and result in exclusion from certain airspace and higher flight levels. Check with the air navigation service provider (ANSP) and ensure that a new or amended Flight Plan (FPL) is filed which clearly indicates any performance restrictions.
Flight Planning Considerations
Whether a flight with gear down is planned or occurs as results from a decision to continue a flight following a failure to lock landing gear up after take off, the following need to be considered:
- Structural Considerations: Flight with the gear down is likely to involve limitations on both indicated airspeed and cruising altitude.
- Crew Considerations. Extended flight with the gear locked down is very noisy, impacting on both the environment within the cabin and on pilot fatigue levels. Increased vibration is also a factor and will increase crew fatigue.
- ATC Issues: ATC will need to be advised of any reduced cruise speed and maximum altitude unless these if these will mean that the figures filed in the FPL are no longer applicable.
- Take –Off and Climb Performance: Planned gear down operation may cause the rate of climb on departure after passing screen height (after which the gear would normally be retracted) to be seriously degraded. The degree of performance loss could be such that meeting the remainder of the net take-off flight path terrain clearance requirements and/or vertical profile of any standard instrument departure could be impossible at take-off mass close to maximum. Planned gear down operation may require a significant reduction in the calculated performance limited take off mass (PLTOM) allowed.
- Landing Climb Performance: Prior to an approach with the gear remaining extended in the event of a go-around/missed approach, the effect on the landing climb performance needs to be considered. Usually the landing climb maximum mass will be much reduced to ensure that the minimum climb gradient requirements will to be achieved. On a long sector this will not usually cause much difficulty as the reduced PLTOM along with the fuel burn en-route would means that the landing climb performance will be adequate. On short sectors where the fuel burn is less the limit to the maximum landing climb mass may be more restricting that the limitation to PLTOM
- Flight Time: Consider the impact of reduced speed on flight times and expected arrival time at the potentially revised destination and alternates - will they be open? Is new en route wind velocity data needed? Consider whether previous performance calculations are still valid such as reduced cruising speed, engine out terrain clearance requirements (drift down), or ETOPS.
- Fuel Planning: Increased drag, and flight at lower than normal cruising levels, will result in a significantly higher fuel burn. Fuel consumption with the landing gear locked down is significantly higher than most pilots realise and potential range can be decreased by 2/3rds, due to both limitations on operating altitude and operating speed. It may be necessary to make a refuelling stop en-route if it is still intended to continue to the original destination. Note that most FMS will not give reliable fuel predictions in such a situation. Crews should compare actual fuel consumption and groundspeed achieved figures.
- Flight in Icing Conditions. Flight in icing conditions should usually be avoided because any build up of frozen deposits on the extended undercarriage will increase drag and aircraft weight and may also affect the centre of gravity. It should be noted that at a slower speed, ice accretion on the airframe may occur at an increased rate.
Accident and Incident Reports
On 29 September 2019, an Airbus A330-200 received simultaneous indications of low pressure in two hydraulic systems soon after takeoff. An emergency was declared, and a return to land was followed by a stop on the runway due to a burst main wheel tyre. A manual valve for one of the hydraulic systems located in the left main gear wheel well had completely detached and impact-damaged a pipe in a nearby but separate hydraulic system. Both systems lost their fluid with valve detachment attributed to fatigue failure of the attachment screws, a risk addressed by an un-adopted non-mandatory Service Bulletin.
On 18 June 2021, a Boeing 787-8 being operated by British Airways was being loaded for a cargo flight at Heathrow whilst line engineering carried out checks required to permit despatch with a deferred minor defect for later rectification. The check required cycling the landing gear with locking pins inserted so that only the bay doors cycled but when this was done, the nose gear retracted and the front of aircraft dropped to the ground causing significant damage to the airframe and minor injuries to two people. The nose gear downlock pin had inadvertently been inserted into the wrong hole.
On 3 December 2021, a Boeing 737MAX-8 released to service with antiskid and autobrake systems inoperative in accordance with Minimum Equipment List procedures then operated two sectors. On the return to Singapore, both left main landing gear tyres were sufficiently damaged during landing to cause the bursting of one and deflation of the other. The cause of this was failure to deploy the speedbrakes manually as required. A similar error on the previous sector did not have the same outcome because the relatively more positive touchdown enabled automatic speedbrake deployment and wheel spin was accompanied by simultaneous manual braking.
On 7 June 2021 an Embraer ERJ170 had just commenced its descent towards destination when both primary and secondary pitch trim systems failed resulting in excessive nose-down pitch control forces and an inoperative autopilot. The flight was completed without further event with the Pilot Flying using both hands on the control yoke to control pitch attitude manually. During the landing roll the nosewhweel steering system also failed. The pitch trim failure was attributed to probable jamming of the trim actuator due to water, possibly condensation, freezing within it. The steering system fault was attributed to a completely unrelated sensor failure.
On 6 August 2021, an Airbus A319 experienced uncommanded loss of both Flight Directors and the Flight Mode Annunciator and disconnection of both autopilot and autothrust in the climb. After levelling at FL350, significant inertial reference position inconsistencies were observed. A precautionary PAN was declared and the flight was completed. Investigation found that the cause was a momentary abnormal vertical shock load transferred to Inertial Reference System equipment through an overextended nose gear shock absorber by a sharp jolt during takeoff caused by a runway patch repair. Sensitivity of the particular inertial reference system installed on the aircraft was noted.
