Failure of nose landing gear downlock link

Citation

(2002), "Failure of nose landing gear downlock link", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 4. https://doi.org/10.1108/aeat.2002.12774dab.013

Failure of nose landing gear downlock link

After an uneventful positioning flight, a Piper PA-34 Seneca aircraft was landed at Stapleford on the tarmac portion of Runway 22; the landing was made with full flaps and full aft elevator was applied. The main wheels touched down softly and the nosewheel was then lowered to the ground. Gradual braking was applied and, at the same time, full back elevator was re-applied. However, the nose continued to lower until it came into contact with the runway. The pilot then, immediately, shut down both engines and the electrical system and closed the fuel cocks. The aircraft slid to a stop still on the tarmac section of the runway, and the pilot left the aircraft. The aircraft was recovered from the runway and the nose landing gear was pulled down. Initial examination by the engineers who recovered the aircraft revealed that the upper, adjustable eye-end of the lower downlock link assembly had broken in its threaded portion. The nose landing gear was made safe by bracing the drag link into the 'over-centre' position.

The nose landing gear of the Seneca is of the forward retracting type which, when extended, has the wheel axle forward of the oleo pivot. When retracted, the gear is held up by hydraulic pressure in the actuator and, when extended, it is held in the down position by a geometric mechanism. There are no locking hooks for either position. The primary brace against collapse, when the nose landing gear is extended and under load, is the drag link assembly. When the landing gear is fully extended, the drag link centre pivot should be offset below the line between its two end pivots and, in this position, the fixed stops of the drag link centre joint, which limit the over-centre travel of these links, should be in abutment.

The overall geometry of the landing gear is such that aircraft weight on the nose-wheel applies a compressive load to the drag link assembly which tends to drive it more firmly into the safe 'over-centre' condition when the gear is properly extended. Conversely, it will tend to cause the drag link to fold and the gear to retract, if the load is applied when the drag link assembly is in an 'under-centre' condition.

The downlock assembly, which forms the geometric lock to keep the drag links in the extended position, also acts as an integral part of the retraction/extension mechanism. The retraction actuator attaches to the centre pivot bolt of the two part, articulating, downlock linkage. During the retraction cycle, the first movement of the actuator causes the downlock linkage to pull the drag link out of the over-centre condition; during the extension cycle the final movement of the actuator causes the downlock assembly to push the drag link into the fully over-centre position. There is a downlock spring which pulls the downlock centre pivot aftwards, assisting the downlock assembly into the 'gear locked down' position, particularly during 'free-fall' extensions (Figure 3).

Figure 3 Cross section of lower downlock link (actual size)

This lower part of the downlock assembly is a spring strut (illustrated) which has spring force of about 2–3 1bs and is compressible by about 0.06 inch. The spring travel is limited by a cross-pin fitted through the shank of the lower eye fitting, running in a control aperture in the skirt of the lower downlock link body. This aperrture is described in the Service Manual as a 'slot'. The length of the lower downlock link is adjustable and is correct if, when the drag link assembly is driven to the fully over-centre position, the lower downlock link is almost fully compressed; the clearance of the cross-pin from the upper end of the slot, established by the rigging procedure, is a half turn of the adjustment thread, which is about 0.018 inch.

Examination and analysis

Further examination of the failed landing gear by the AAIB showed that the upper end of the sprung lower downlock link had failed in a single bending overload event, at a point where its adjustment threads entered the lock-nut at the top of the link body. The cross-pin and the slot in the skirt of the link body also exhibited evidence of high compressive axial loading of the link, which took the form of severe bearing crushing of the upper edge of the aperture and bending of the cross-pin. Although there were slight wear marks indicating that the link had been extended to its maximum length in service, this link had been fitted, new, only five flights before the accident flight, and thus evidence of the most compressed length regularly achieved during the landing gear extension cycle had not yet developed. There was evidence that there had been some distortion of the upper downlock link, consistent with severe upward loading of the downlock link centre pivot. The lateral stagger of the joint between the two parts of the downlock link results in twisting forces being applied to the upper link and the bulkhead support structure as a result of high compressive loads in the link assembly.

A review of the procedure for rigging the nose landing gear was made by the AAIB and several matters of concern noted. Also, from the number of nose landing gear collapses involving Senecas which had been investigated by the AAIB, it appeared that the type might be more prone to nose gear malfunctions than other similar types in the same weight category. A statistical comparison over the last 15 years, using data from the CAA occurrence reporting system, was made, relating the number of occurrences to the number of aircraft years on the UK register for each type. The survey was restricted to types of which there were, on average, 20 or more on the UK register; there were, on average, 118 Senecas on the register in each year representing just under one third of the significant light twin fleet.

The result of this survey indicated that nose landing gear collapses on the PA 34 Series fleet were more than twice as frequent as the average on the remainder of the light twin fleet and nearly three times that of the numerically similar PA 23 Aztec series (average of 125 on the register in each year). Over the 15 years of the survey there were 35 recorded collapses on Senecas against 13 for Aztecs. If the analysis is taken over the last ten years the results, although marginally better for the Seneca, are broadly similar, the Seneca representing just over one third of the fleet and accounting for half of the nose landing gear collapses.

Although, over the 15 year period, the majority of all collapses had occurred during the landing phase, of the collapses which occurred during take-off, one out of four involved Senecas, and of collapses during taxying, two out of three.

The PA 34 Seneca type has persistently suffered a noticeably higher rate of nose landing gear collapses than all but one of the other numerically important light twin types. The AAIB has investigated four of the most recent ones in detail and a common feature observed in all cases has been the failure, in bending, of the upper rod end of the lower download link. Crushing damage to the upper arc of the 'slot' has also been a regular finding.

It is unclear, because of the distortions which have occurred during the subsequent collapses, whether these failures are the result of actual misrigging or of maladjustment which has developed as a result of accrued crushing damage on the upper arc of the download link slot. The accumulated evidence from the previous AAIB field investigations into PA 34 nosewheel collapses, however, indicates that both the instructions for rigging the nose landing gear mechanism and the robustness of the downlock link require to be addressed. The susceptibility of the upper end of the 'slot' to crushing damage, which will lead to increasing effective misrigging of the downlock with the number of landings since the last time checked, is also an issue of robustness. The consistency of the rigging instructions, particularly those related to the over-centre travel of the drag link assembly, should be improved and a maximum limit for this over centre should be specified. The wear limits allowed in the various pivot points of the nose landing gear should also be reviewed, since this affects the degree of 'hammering' damage inflicted on the 'slot'.

Safety action

A recommendation was received from the FAA on 29 January 2002 indicating some proposed action on two previous AAIB recommendations, which are summarised below:

Safety Recommendation 2000-45It is therefore recommended that the New Piper Aircraft Company, should review and amplify the instructions for rigging the nose landing gear downlock mechanism contained in the Piper PA 34 Maintenance Manual.

Safety Recommendation 2000-46The FAA and the CAA, in conjunction with the New Piper Aircraft Company, should investigate the causes of reported cases of Piper Seneca nose landing gear collapse. Consideration should be given to design modification which should minimise movement of the drag brace resulting from loads applied to the nose landing gear, and to ensure sufficient force is applied to the drag brace to retain it in the locked condition.