Nose landing gear retraction

Citation

(2000), "Nose landing gear retraction", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 5. https://doi.org/10.1108/aeat.2000.12772eab.013

Nose landing gear retraction

Nose landing gear retraction

Keywords Safety, Flight safety, Landing gear, Accidents

A DC-8 routinely operated cargo flights into Manston. Because of space restrictions on the stand, the normal practice with this aircraft was for it to position on the taxiway after landing and for a tug to manoeuvre the aircraft thereafter, the aircraft being finally pushed back on to the parking area. On the day of the accident, the aircraft had landed normally and had positioned as usual on the taxiway where it was met by the tug. A DC-8 nose landing gear ground lock pin, carried for the purpose on the tug, was then installed and the aircraft was towed without incident before being pushed back on to the stand. The nose landing gear pin was then removed by the tug crew, the tow bar disconnected from the nose leg, and the torque link reconnected. The tug then departed and unloading of the cargo commenced.

The sequence of events was such that the inbound flight crew vacated the aircraft in due course, and the outbound crew boarded. The flight engineer immediately positioned himself at his station on the flight deck and therafter, throughout the relevant period, was occupied almost exclusively with the refuelling of the aircraft and balancing of the fuel load. The first officer (FO) was unable to occupy his seat upon boarding because it was occupied by the avionics engineer, who was busy changing the GPS unit which was situated on the right side of the pedestal, abutting the main instrument panel and immediately beneath the landing gear selector lever. On seeing this, the commander left the flight deck so as to keep out of the way of the avionics engineer, and the FO checked the Jeppeson library for the same reason. The problem with the GPS, as it had been diagnosed originally, required a replacement memory card but the card which had been supplied for the purpose was for a different model of GPS unit, and was not compatible. It had been decided, therefore, to change the whole GPS box for a type which was compatible with the replacement card, the two types of unit being comparable in all other respects. The avionics engineer reported that, having pulled the circuit breaker powering the GPS system, he prepared to remove the GPS unit but was prevented from doing so by the position of the landing gear selector lever. He had changed GPS units previously on DC-8-50 series aircraft without such a problem. However, the landing gear lever on this DC-8-60 aircraft is significantly longer than on the -50 series, and in the DOWN position the end of the lever physically prevented the GPS unit from being lifted clear of its housing.

The hydraulic systems were unpressurised at this time, and the avionics engineer decided that he would have to raise the landing gear lever out of the DOWN position sufficiently to allow the GPS unit to be lifted clear. Before doing so, however, he left the flightdeck and inserted ground lock pins into all three landing gears, using the aircraft set of pins. He then returned to the flight deck where he pulled the gear lever out of the DOWN detent and, lifting it just sufficiently to provide the necessary clearance, removed and replaced the GPS box. So far as he could recall, he then returned the landing gear lever to the DOWN position, in its detent, before resetting the associated circuit breaker and checking the operation of the GPS unit. He then left the flight deck, retrieving the pins from the landing gears, and returned them to their stowage on the aircraft before moving to the back of the flight deck, where he stood waiting. Meanwhile the commander and FO had re-entered the flight deck and resumed their seats. Neither pilot recalled the position of the landing gear lever at this time, although the FO did recall seeing that the GPS unit was lit; therafter, his attention was focused on the overhead radio panel, which was of a different configuration from other aircraft in the fleet. The flight engineer had remained at his station throughout, occupied with his re-fuelling tasks.

Injuries and damage

Shortly after the flight crew had re-entered the flight deck, the ground engineering supervisor boarded the aircraft, having been asked by the engineer servicing the hydraulics reservoir to restore hydraulic power, so that the associated main landing gear door could be closed. (The reservoir was located within the main wheel well and the gear door had to be lowered, using a service panel control valve, to gain access.) On entering the flight deck the supervisor announced his intention, asking "OK to put the hydraulics on?", or words to that effect. Although the pilots each recalled mention having been made of "hydraulics", neither gave explicit permission for hydraulic power to be brought online; however, the request was not refused. (The flight engineer was still occupied with re-fuelling at this time, and did not recall any mention of hydraulics.) The supervisor then selected the auxiliary (electrically powered) hydraulic pump to ON using the switch on the flight engineer's overhead panel. Some 20 to 30 seconds later, the nose landing gear retracted and the nose dropped violently to the ground. The commander immediately looked across and saw that the landing gear lever was in the UP position. He therefore moved it back to the DOWN position, and instructions were given to put the ground lock pins in the main landing gears.

The sudden dropping of the nose on to the ground resulted in those standing on the flight deck (i.e. the avionics engineer and the supervisor) being thrown violently off their feet. A scream was also heard from the main deck and the avionics engineer immediately moved back to investigate. However, having reached the vestibule area, he was suddenly overtaken with extreme back and neck pain, and found himself unable to continue. The scream had emanated from the loadmaster who had been involved in moving the palletised loads into the L1 doorway, and who had been standing between the pallet being moved and the forward cabin bulkhead when the nose gear had collapsed. When the nose had suddenly dropped, he found himself against the bulkhead with the pallet, which weighed approximately two tonnes, sliding towards him. With his back against the bulkhead, he had brought one knee up in front of him in an effort to brace himself against the approaching pallet; however, the pallet drove his knee back, dislocating his hip.

He was subsequently taken to hospital where he underwent an operation to re-locate his hip. He then spent four weeks in traction followed by six weeks' physiotherapy. The avionics engineer was also hospitalised for three days, having suffered bruising of his lower spine and "whiplash" injuries to his upper spine. The supervisor attended hospital as an out-patient, for assessment of whiplash-type injuries. In addition, two ground staff engaged in unloading operations also suffered minor injuries. One of them had been ahead of the pallet, and was pushed forward through the L1 door into the forward vestibule, suffering bruising to his knee. The other, who had been positioned aft of the pallet and was pushing it forward while holding on to its netting, suffered a fractured thumb as the pallet suddenly moved forward. The engineer working on the No. 3 engine had just stooped beneath the forward fuselage and was making his way back across the apron when the nose fell to the ground.

The aircraft suffered major structural damage in the vicinity of the nose wheel bay, and to the nose gear doors. In addition, a pivot housing which formed part of the "weight-on-wheel" sensing mechanism of the nose landing gear had fractured, and the fuselage skins suffered minor damage in several areas caused by contact with ground equipment as the aircraft shifted.

Technical investigation

The nose landing gear retracts forward and, consequently, in the event of a retraction on the ground there would be mininimal loading on the retraction actuator even with the weight of the aircraft resting on the gear, due to the ability of the nose wheels to roll freely forward. Retraction of the nose landing gear would therefore have been expected if, with no ground lock pin installed, the system was selected to UP with the hydraulic system pressurised. (The main landing gears retract sideways, and consequently they would not be likely to retract, due to the lateral resistance from tyre friction.) A typical delay period before an unpressurised system reaches a working pressure, after switching on the auxiliary pump, is of the order of 20 to 30 seconds. In this accident the landing gear ground lock pins had been removed and the nose landing gear retracted some 20 to 30 seconds after the auxiliary hydraulic pump had been energised. Immediately afterwards, the landing gear lever was observed in the UP position. The evidence therefore indicated that the nose landing gear had been retracted by the retraction system, as a result of the selector lever having been in the UP position.

The design of the landing gear and related systems on the DC-8-60 is such that the action of pulling the landing gear lever back to disengage it from the DOWN detent will change the state of a set of microswitches in the gear indicator light circuit, causing the three gear "down and locked" green lights to extinguish and the single "gear in transit" red light to illuminate; no warning horn should sound. Both the green and red landing gear lights are positioned close by the landing gear lever. If, therefore, the gear lever had been out of position, and this had not been noticed by any of those on board, it is conceivable that an abnormal gear indication would also have gone unnoticed.

Consideration and experimentation established that it appeared possible for the landing gear selector lever, having first been raised sufficiently to permit removal of the GPS box, to have moved without further intervention, into a position which would have resulted in a retraction of the nose landing gear when the hydraulic system became pressurised.

The investigation identified a lack of coordination between the various personnel working on the aircraft during the turn-around, and a commensurate failure to fully supervise or control the various activities taking place. These aspects were fully investigated and drawn to the attention of the operator at an early stage of the AAIB investigation, and the company also carried out its own independent internal investigation. The company subsequently reported that it had taken the following actions in the light of this accident:

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  Steps were taken to clarify and explain the terms of reference of the engineering supervisor, and the responsibilities of all individuals engaged in turn-around duties.

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  A quality notice was raised to address the problem caused by the need to move the landing gear selector lever when changing a GPS box on DC-8-60 series aircraft.

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  The implication of modifications will in future be subject to scrutiny by a Modifications Committee.

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  A quality notice was raised addressing the procedures to be adopted for checking the correct positions of aircraft controls and indications prior to the application of hydraulic power.

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  The procedures covering control of the GPS Data Card system were reviewed by the operations department and, with the involvement of the technical records supervisor, were revised. A notice to crew on this subject was raised.