Description

On 15 November 2018, the nose landing gear of a Bombardier DHC8-300 (C-FPAE) being operated by PAL Airlines on a scheduled domestic passenger flight from Churchill Falls to Deer Lake as PVL1922 did not lock down when selected normally and after alternative methods were also unsuccessful, diverted to Stephenville having declared an emergency on the basis that it had a more suitable runway and better weather. The landing there after an approach in day VMC was on the main gear only, but by the time the aircraft nose made contact with the runway, the speed was low so only minor consequential damage was caused and no injuries were sustained by the 51 occupants.

The aircraft in its final stopping position [Reproduced from the Official Report]

Investigation

An Investigation was carried out by the Canadian Transportation Safety Board (TSB) using data downloaded from the FDR of the aircraft involved but relevant data on the 30 minute CVR had been overwritten and no recording of the exceptional, extensive and relevant use of smartphone communications between the flight crew and their company following their initial discovery of the landing gear extension problem was available.

What Happened

On the day of the investigated event, the aircraft was to operate a flight from Wabush to St. John’s with intermediate stops at Churchill Falls and Deer Lake. The first sector was completed uneventfully as was the second, with the Captain acting as PF, until the aircraft was on final approach for runway 25 at Deer Lake when selection of landing gear down was followed by the illumination of a red ‘gear unsafe’ warning and the absence of a green locked down indication for the NLG accompanied by a number of related cautions including a gear in transit light indication. The Captain responded by re-cycling the landing gear but with the same result and the approach was discontinued. The crew then flew along the runway centreline and asked the FSS (Flight Service Station) to visually check the position of the NLG and were informed that it was “visible but not locked down”.

The aircraft was then climbed to enter a holding pattern to allow the crew to “continue to troubleshoot” the problem. The QRH ‘LDG GEAR INOP’ Checklist was then actioned with awareness that it may be necessary to subsequently resort to the ‘ALTERNATE LANDING GEAR EXTENSION’ Checklist which, once it has been completed, prevents retraction of the gear until after a ground reset by maintenance personnel. The crew then heard an exchange between another company aircraft and the Deer Lake FSS which advised of the need to sweep snow off the runway so it was decided to delay completion of the alternate extension procedure until the availability of the Deer Lake runway had been confirmed since transit to an alternate with the gear extended would reduce the maximum range for diversion.

The crew then noted that their smartphones were displaying a service availability indication. The Captain, whose headset was Bluetooth-capable and paired to his smartphone, then called Company Despatch at St John’s and a conference call was established with the Chief Pilot, the Director of Flight Operations, the Director of Quality, Safety and Training and a St John’s-based Maintenance Shift Supervisor. The First Officer was then given a phone number of one of the St John’s group so that he could make a video call to them using an application on his smart phone which he then positioned so that its camera showed the flight deck. He too had his phone paired to his Bluetooth headset so he could hear the conversation at the St John’s end of the call.

The Captain then made of total of four calls to and received one call from the St. John’s group, the additional calls being required to re-establish communications on the occasions when signal loss terminated the call. This succession of calls was found to have “lasted more than 66 minutes over the next 71 minutes, approximately until the aircraft touched down at Stephenville Airport”.

In the course of this period, the crew briefed the St. John’s group about the problem and the Captain, having changed roles to act as PM, “actioned selected items from ALTERNATE LANDING GEAR EXTENSION checklist” as instructed by members of the group, “particularly the repeated pulling of the nose gear release handle”. Meanwhile, the First Officer acted as PF and maintained radio contact with Gander ACC and Deer Lake FSS.

The crew then heard the company traffic reporting a go around at Deer Lake FSS due to poor visibility caused by blowing snow and were diverting. The Captain “decided to perform manoeuvres in an attempt to release the NLG” and made a PA announcement to alert the passengers before resuming the role of PF, disconnecting the AP and making three “abrupt and progressively larger pitch control inputs” without any change. The aircraft was then climbed to 9,600 feet QNH before beginning a descent and accelerating to 185 KCAS whilst making seven consecutive elevator inputs as an alternative way to increase the g loading as a means to release the jammed gear but without success.

Although the conditions at Deer Lake had improved in the meantime, it was then decided that the flight would divert to Stephenville, an airport situated on the west coast of Newfoundland, on the basis that it had a longer (3050 metres rather than 2440 metres) and wider (61 metres rather than 45 metres) runway as well as better present weather than Deer Lake. Having received ACC approval for this diversion, the Captain, who had continued as PF then declared an emergency. The First Officer’s smartphone then lost connection with the St John’s group but they remained in communication with the Captain and “continued to discuss the position of the NLG and actions required to lower it” with pulling the NLG release handle and how to do it, required actions to be taken at touchdown and (multiple times) what flap position should be set for landing.

The First Officer “attempted to coordinate with the Captain" regarding the descent, approach and landing but the Captain was often communicating with the St John’s group during these attempts which meant that the First Officer had to repeat prompts or questions. The review of the ‘LANDING GEAR FAILS TO EXTEND’ Checklist was carried out primarily between the Captain and the St John’s group with landing considerations such as whether the propellers represented a ground contact hazard and the appropriate flap setting only being “confirmed late in the flight”. It was noted during the Investigation that this information was included in the Checklist anyway as explicit fact or procedure so no discussion was necessary. However, it was agreed that in an effort to free the NLG, the Captain would “intentionally land firmly on the main landing gear and the First Officer would simultaneously pull the NLG release handle”.

An uneventful ILS approach was flown to runway 27 at Stephenville with the Captain deciding on a landing flap setting of 35° degrees. With the St. John’s group still on the phone, touchdown was made as planned but the partially extended NLG remained as such and when it contacted the runway, it was forced into the up position leaving the aircraft nose to slide on the runway surface. Once the aircraft had stopped about halfway down the runway, the engines were powered down and all occupants left the aircraft using the main passenger door and the forward right emergency exit and were bussed to the terminal building.

Damage to the aircraft was limited to crush and abrasion damage to the forward and rear NLG bay doors, abrasion damage to the tyre walls, deformation of the aircraft structure at the NLG door hinge attachment point and abrasion damage to the skin aft of the NLG bay and an adjacent antenna.

Why the Gear Extension Malfunction Happened

The aircraft involved was manufactured in 2001 and had accumulated 36,072 flight hours by the time the investigated event occurred. It was noted that the DHC8-300 NLG assembly consisted of two wheels mounted on a trailing arm attached to a gear leg by a link to the internal piston assembly which moves within a bearing contained within the inner cylinder of the gear leg. This assembly is filled with hydraulic fluid and charged with nitrogen gas which is prevented from leaking out by inner and outer ring seals on the bearing. The assembly requires overhaul in accordance with the CMM whichever of 25,000 cycles or 10 years in service comes first. Any wear or corrosion on the inner surface of the cylinder discovered during overhaul may mean that its internal diameter has to be slightly increased and if this occurs, the outside of the bearing must be marked to show that an oversized bearing must be fitted during reassembly to ensure leakage of hydraulic fluid is prevented.

Examination of the NLG found that there was evidence of hydraulic fluid leakage and disassembly showed that “correction” of the apparent consequences - a low piston extension height - had been achieved by increasing the nitrogen gas charge. Damage and marks on the NLG tyres and on the aft NLG doors showed that when the landing gear had been selected down, the slipstream into which the NLG had been lowered had unduly compressed the NLG leg because of its lower internal static pressure and had led to tyres becoming jammed in that position rather than normally extending.

Further examination of the cylinder found that although it did not have the required external marking to indicate this, it had been given an increased internal diameter during overhaul but still contained a standard-sized bearing. The excessive clearance at the ring seal had reduced its effectiveness and facilitated a fluid leak. The installed NLG leg had completed 6,364 cycles since the last overhaul and 46, 573 cycles since new, having most recently been overhauled by the OEM almost four years earlier. It was noted that no rework or repair to the inner cylinder was recorded during this most recent overhaul and the records from the earlier overhaul were no longer available.

It was noted that five weeks prior to the investigated event, a pilot-reported defect “nosewheel rough on taxi” was rectified by replenishing the NLG leg hydraulic fluid and nitrogen gas pressure in accordance with AMM servicing instructions with no subsequent related defects over 233 cycles.

Discussion

In respect of Daily Checks of the airworthiness of a PAL Airlines aircraft, it was noted that although these are normally carried out by maintenance personnel, when an aircraft night stops at a location where there is no maintenance cover such as Wabush, the only check carried out is an external check by one of the flight crew prior to the first flight of the day. However, Bombardier did not provide a suitable checklist for this purpose and instead “relied on airline operators to determine the specific flight control, fuselage, engine, propeller and landing gear items to be inspected in their pre-flight checklists and what criteria to use when inspecting them”. It was noted that PAL Airlines had an SOP requiring “a thorough pre-flight check during the exterior walk-around inspection” but no corresponding checklist or criteria existed. No NLG anomalies were identified during either of the pre-flight checks conducted by the First Officer on the day of the investigated event nor were any recorded during any other pre-flight checks by pilots or maintenance personnel since the most recent NLG servicing five weeks previously.

In respect of fuel endurance, which did not appear to have been actively considered during the flight, it was found that on arrival at Stephenville, the indicated fuel onboard had been recorded by maintenance as approximately two thirds of the required reserve fuel.

Examination of the FDR data led to the discovery that the improvised and unsuccessful positive ‘g’ manoeuvring carried out by the Captain in an attempt to release the jammed gear had resulted in the aircraft exceeding AFM airspeed limitations including a significant exceedence of both VLE and VLO.

Although the gear extension problem, even when the corresponding QRH procedures could not resolve it, was a relatively minor one, a very considerable time was (unusually) spent by the crew in continuous communication with company management. It was considered that this extended telephone conversation between the Captain and the St. Johns group of PAL Airlines’ managers “had been unusual in that it involved direct contact with company personnel having both operational credentials and management roles, whose opinions would not normally be a part of the crew decision-making process”. It was concluded that insofar as this group of managers “became an extension of the flight crew […] it may have created a permissive environment in which experimentation could replace the following of written checklists”. It certainly disrupted normal flight crew teamwork and added nothing to the applicable QRH response and it was considered that “if crew members are unable to communicate effectively with each other, they are less likely to anticipate and coordinate their actions, which could jeopardise the safety of the flight". The external communication was also likely to have been the indirect cause of both the exceedence of limiting airspeeds during the abrupt ‘experimental’ manoeuvres aimed at un-jamming the nose gear and quite possibly the cause of apparent inattention to fuel endurance prior to commencing and during the diversion.

It was also noted that any use of any PEDs on board PAL Airlines’ aircraft except when taxiing in to the gate after flight was expressly prohibited by the OM and that more generally there was no current regulatory guidance covering smartphone use by flight crew on board Canadian aircraft although exemptions from baseline PED use can be requested and may be approved. It was also noted that whilst this Investigation was in progress, Transport Canada published a NPRM which would permit approved use of PEDs such as smartphones by crews in flight.

The four formally-stated Findings as to Causes and Contributing Factors were as follows:

1. For undetermined reasons, a previous repair to the inner cylinder of the nose landing gear shock strut was not designated with a specific permanent marking as required by the approved component maintenance manual, resulting in the installation of a standard size bearing where an oversize bearing was required.
2. The reduced sealing force caused by the smaller-than-required bearing installation likely allowed the nose landing gear shock strut to leak.
3. The reduced hydraulic fluid volume, in addition to a possible nitrogen leak, reduced the internal static pressure of the nose landing gear shock strut and allowed it to partially compress when it was in the up position.
4. The airflow encountered in flight when the nose landing gear was lowered further compressed the nose landing gear shock strut and allowed the tires to impinge on the aft landing gear doors, thereby jamming the nose landing gear in a partially extended position.

A formally-stated Finding as to Risk was also made as follows:

1. The absence of formal in-flight procedures for flight crews to consult third parties using smartphones increases the risk of distraction, leading to a breakdown in crew resource management during critical phases of flight.
2. If crew members are unable to communicate effectively with each other, they are less likely to anticipate and coordinate their actions, which could jeopardise the safety of the flight.
3. If pilots delay making a decision to divert, there is a risk that the fuel remaining will be insufficient to provide the flight endurance required to mitigate unforeseen circumstances at the diversion airport.
4. If aircraft are operated beyond airspeed limitations, there is a risk of compromising flight safety, resulting in injury to the occupants or damage to the aircraft.
5. If manufacturers require an aircraft inspection of items critical to the safety of flight without providing a checklist of items and inspection criteria, there is a risk that operators will not identify unserviceable items or conditions.
6. If voice recordings and cockpit sounds are not available to an investigation, the identification and communication of safety deficiencies to advance transportation safety may be precluded.

One ‘Other Finding’ was also made:

• Because the nose landing gear had already moved from the uplocked position and was jammed in a partially extended position, pulling the nose gear release handle had no effect.

Safety Action taken as a result of the investigated event prior to the completion of the Investigation was noted as having included the following:

• Safran Landing Systems amended the Nose Landing Gear CMM to include a specific instruction to check the inner cylinder dimensions to ensure that the correct components are installed. They also issued a SIL which emphasised the importance of following all instructions provided in the CMM including those related to the methods of identifying components after repair and highlighted the CMM update requiring a check of the dimensions of the specified components before assembly to ensure that the mating parts are being correctly installed.
• PAL Airlines amended the content of its company-formulated ‘Dash 8 Layover Check’ to emphasise the NLG shock strut minimum and maximum extension measurements and to require its maintenance personnel to record the observed strut extension measurement NLG after each such Check. The company also introduced a formal External Pre-Flight Checklist for its pilots.

The Final Report of the Investigation was authorised for release on 3 June 2020 and it was officially released on 20 July 2020. No new Safety Recommendations were made.

Related Articles

• Quick Reference Handbook (QRH)
• Emergency and Abnormal Checklist
• Crew Resource Management (CRM)
• Fuel - In-Flight Management (Abnormal Operations)