West Atlantic AT72 enroute on Mar 7th 2023, rudder restriction

Last Update: November 28, 2024 / 12:10:48 GMT/Zulu time

Bookmark this article

Photo of West Atlantic G-NPTF, ATR ATR-72-200 — West Atlantic G-NPTF, ATR ATR-72-200 (Photo credit: Alec BHX/KKC / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Mar 7, 2023

Classification
Incident

Airline
West Atlantic

Flight number
NPT-07B

Departure
East Midlands, United Kingdom

Destination
Belfast International, United Kingdom

Aircraft Registration
G-NPTF

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A West Atlantic Avions de Transport Regional ATR-72-200, registration G-NPTF performing flight NPT-07B from East Midlands,EN to Belfast International,NI (UK), was enroute when the aircraft suffered restricted rudder travel. The aircraft continued to Aldergrove airport for a safe landing on runway 25 about one hour after departure.

The UK AAIB rated the occurrence a serious incident and opened an investigation, also stating that the rudder travel restriction continued after landing and during taxi.

The aircraft is still on the ground in Belfast 9 days after the landing.

On Jan 17th 2023 the aircraft had suffered electrical issues keeping the aircraft on the ground for about a week, see Incident: West Atlantic AT72 at East Midlands on Jan 17th 2023, electrical issues.

On Mar 7th 2024 the AAIB reported: "During the landing flare the flight crew encountered a rudder control restriction and used nosewheel steering for directional control on the runway. Examination of the aircraft the following day revealed that the rudder was almost immoveable, either using the rudder pedals in the cockpit or by physically pressing on the rudder itself. The investigation is nearing completion and has focused on determining the reason for the restriction in the rudder control system."

On Nov 28th 2024 the AAIB released their final bulletin concluding the probable causes of the serious incident were:

Following an extensive history of reports of stiffness within the rudder control system, the flight crew experienced rudder stiffness during the full and free control check prior to the flight. Aware of the recent maintenance interventions which were considered to have resolved the problem, the flight crew elected to continue with the flight. They subsequently encountered excessive rudder stiffness during the landing flare which rendered the rudder pedals almost immovable.

Two support bearings on the rudder rear quadrant shaft were found to be corroded. Trapped moisture in the aircraft’s rear bay probably contributed to the condition of the bearings. Unable to rotate freely, the bearings would have resisted the movement of the rudder rear quadrant shaft leading to the stiffness. Other anomalies observed in the rudder control system may have contributed to the stiffness, but to a lesser extent.

A Service Bulletin published in 1993 existed to replace the affected bearings with corrosionresistant equivalents, but had not been embodied on G-NPTF. In February 2024 the manufacturer updated the IPD to allow interchangeability for some flight control bearings (including those on the rudder rear quadrant) with corrosion-resistant bearings, as an alternative to the Service Bulletin.

The AAIB analysed:

Background to the occurrence

Over a period of approximately one month several of the operator’s pilots had intermittently reported stiffness within G-NPTF’s rudder system on three occasions. Each report appeared to indicate the degree of stiffness was increasing over time, despite prompt maintenance intervention on each occasion. The resulting maintenance ranged from functional and operational tests to replacement of the rudder damper and, following the most recent occurrence, replacement of the TLU, RCU and the rudder damper for a second time and an extensive period of troubleshooting which included guidance from ATR. The operator indicated that these actions had, at least to some extent, alleviated the perceived stiffness in the rudder system.

Flight crew’s acceptance of aircraft for flight

ATR indicated that the primary mitigations for any stiffness, resistance or jamming in the rudder control system are the full and free control check conducted before flight and the rudder jam procedure, if the condition is encountered in flight. The rudder jam procedure had limited relevance in this case, as the aircraft was already in the landing flare when the rudder stiffness was encountered.

During the full and free movement check of the flight controls after engine start, both flight crew commented that the rudder was very stiff to move. Despite this observation they continued with the flight. This meant that the last chance to prevent the aircraft flying with the stiff rudder was missed. Had the crew opted to return the aircraft to the stand, it is possible that further engineering investigations might have identified there was a significant issue with the rudder control system. However, the previous extensive engineering attention and the repeated clearance of the system as having no faults meant the flight crew were ready to accept the aircraft for the flight to Belfast despite feeling that the rudder was very stiff.

Condition of rudder control system following the occurrence

Examination of the aircraft the day after the occurrence confirmed the presence of significant resistance in the rudder system, with the rudder being extremely difficult to move both when using the rudder pedals and by hand. While there was no moisture or ice accumulation evident in the rear bay during the initial aircraft examination, subsequent examination after the aircraft had been parked outside for several days revealed an accumulation of water and condensation in the rear bay.

By isolating the command and actuation sides of the rudder system and disconnecting the rudder pedal and autopilot yaw cables from the rear quadrant, the predominant source of the stiffness/friction was determined to originate from the rudder rear quadrant shaft. Removal of the shaft revealed that both rear quadrant shaft support arm bearings were in a degraded condition.

Some residual stiffness remained in the rudder/actuation circuit and the investigation identified that the aft attachment bolt for the rudder damper had been over-torqued. Its installation had contributed to stiffness with the rudder circuit, albeit to a much lesser degree that the degraded bearings.

Additionally, examination of the rudder and vertical stabiliser identified the presence of moisture and degraded sealant, and the grease on the rudder pivot (hinge) points had a degraded appearance.

Rear quadrant support bearings

The predominant source of stiffness/friction in the rudder system was determined to be the degradation of the rear quadrant support bearings. Both bearings showed evidence of corrosive attack. The No 2 bearing was completely seized when examined in the laboratory and there was a complete absence of fresh grease, despite being a sealed bearing. The No 1 bearing was rough when rotated and had also suffered from corrosion, although to a lesser extent.

The degradation of the bearings would have substantially reduced or prevented their ability to rotate freely and thus resisted the movement of the rear quadrant shaft, which would have resulted in the difficulties reported in the rudder operation.

SB 72-27-1020 was issued by ATR in March 1993 recommending replacement of steel flight control bearings with corrosion-resistant stainless steel bearings but had not been embodied on G-NPTF.

Following this event, ATR took steps to ease the installation of some post-mod flight control bearings, including the rudder rear quadrant bearings, as an alternative to the SB. This change took effect in February 2024. This means that it will be possible for operators to replace the original steel bearings on the rear quadrant shaft and in other flight control Type F bearing locations, on an on-condition/opportunity basis, without the need to embody the entire SB.

Moisture ingress

While G-NPTF’s flight control bearings were not corrosion-resistant, many older aircraft in the ATR 42/72 fleet similarly equipped with the original steel bearings, continue to operate without reported problems. Regardless of whether original steel or the post-mod corrosionresistant bearings are installed, bearings perform better when operated in a mostly dry environment.

Ordinarily, internal bearing components should not be exposed to moisture since the bearings are sealed and covered with grease; in this case, the presence of excessive moisture in the rear bay undoubtedly contributed to the corrosion on the bearings. The rear bay is not intended to be a fully sealed area and it is not unusual to encounter moisture here, but not to the extent observed on G-NPTF. Degraded and missing sealant on the vertical stabiliser provided a path for moisture ingress.

The horizontal and vertical stabilisers and the rudder are areas of the aircraft subject to external de-icing, and pressurised jets are sometimes used to ensure de-icing fluid reaches the upper part of the rudder. ATR is aware of reports de-icing fluid residue being found in the rear bay in the past. Some of the moisture accumulations in G-NPTF’s tailcone area had a gel-like consistency, visually consistent with a mixture of water and glycol-based deicing fluid. It is therefore probable that de-icing fluid entered this area as well as rain and could also have contributed to the corrosion.

Once in the rear bay, accumulated water/de-icing fluid was unable to effectively drain away due to an absent drain hole in the door frame, which obscured the corresponding drain hole in the door. An additional fastener hole had been added instead of the drain hole, at some point in the aircraft’s history, but the investigation did not determine when. The resulting trapped moisture would have created an environment conducive to corrosion.

The manufacturer considered that any obstruction of the rear bay drain holes should have been detectable by two routine inspections of fuselage drain ports, which required ensuring that drain holes were clear of debris and unobstructed. While these inspections had been performed on G-NPTF, the inspection tasks assume that the drain holes are present and correctly located. On G-NPTF, it is feasible that the absence of the drain hole in the door frame would not have been detected, particularly if the inspections were performed with the rear bay door in the open position.

Following the occurrence, the operator resealed the vertical stabiliser on G-NPTF and restored the drain hole in the rear bay door frame. ATR has undertaken to remind operators of existing maintenance requirements and best practice regarding rear bay sealing to minimise moisture ingress and glycol contamination in a customer communication.

Why was the cause of the rudder stiffness not identified sooner?

The first crew report of rudder stiffness was made on 4 February 2023. Given the extent of the corrosion exhibited on the rear quadrant shaft support bearings, it is likely that this degradation would have developed over an extended period of time. Despite this, it seems that it was only in the month leading up to the incident flight that the condition of the bearings became such that the friction in the rudder control system was detected by flight crew. The most recent routine visual inspection (of the rudder cable circuit) was performed approximately one year earlier, under the previous operator’s tenure with no issues identified.

Neither the rudder rear quadrant shaft nor its support bearings were specifically examined or considered during the maintenance interventions which took place in response to the history of rudder stiffness reports on G-NPTF. Initial troubleshooting by the operator’s maintenance organisation following the 9 February 2023 report of rudder stiffness was perceived to have had reduced the stiffness in the rudder system.

Further troubleshooting performed in response to guidance provided by the manufacturer, did not result in the identification of any findings which explained the rudder stiffness.

This guidance was in-part informed by the operator’s feedback from the troubleshooting, which did not include information about the overall maintenance condition of the rear bay, as observed post-incident. The guidance did not specifically direct the operator or its maintenance organisation to look at the rear quadrant shaft bearings. While corrosion/ degradation of the bearings was an issue historically known to ATR and addressed by SB 72-27-1020, the absence of numerous or recent in-service reports of difficulty with these bearings together with the lack of findings from the troubleshooting, meant that it was not included as a consideration in the ATR troubleshooting process for reports of stiffness within the rudder system. The operator was not aware of SB 72-27-1020 and therefore did not consider it in the troubleshooting for G-NPTF.

The manufacturer considered that the visual inspections of rudder mechanical control/rudder cable circuit (either performed routinely or during troubleshooting) should have identified the friction at the quadrant shaft. The manufacturer indicated its expectation that the maintenance condition of the rear bay, in combination with the reports of rudder stiffness should have prompted further examination.

The investigation noted that neither of the visual inspections directly referred to the rear quadrant shaft or its support bearings. They did not require the rear quadrant shaft to be rotated by hand, but rather operated by moving the rudder surface or the pedals.

The investigation considered that even a detailed visual inspection, without further examination, may not identify any problems with the bearings. The sealed nature of the bearings and their installed location on the rear quadrant shaft precludes visual inspection of their condition without some level of disassembly. It’s likely that friction or degradation in the bearings may therefore only be reliably detected by rotating the rear quadrant shaft by hand, after isolating it from the rest of the rudder control system and confirmed by removal/ inspection of the bearings.

...