CL60, Calgary Canada, 2020

Summary: 

On 23 February 2020, a Bombardier Challenger 605 making a flapless landing after a system failure during descent briefly lost control when reverse thrust selection after a normal touchdown was followed by the aircraft becoming airborne before stick push activation was followed by a hard second touchdown. Structural and abrasion fuselage damage was caused. The Investigation attributed the flap failure to corrosion within the specified maintenance inspection interval and the brief loss of control during landing to a combination of inadequate crew preparation for the landing and the fact that simulator replication of aircraft handling did not correspond to reality. 

Event Details
When: 
23/02/2020
Event Type: 
Day/Night: 
Day
Flight Conditions: 
VMC

32478

Flight Details
Type of Flight: 
Private
Intended Destination: 
Take-off Commenced: 
Yes
Flight Airborne: 
Yes
Flight Completed: 
Yes
Phase of Flight: 
Landing
Location - Airport
Airport: 
General
Tag(s): 
Copilot less than 500 hours on Type, Deficient Crew Knowledge-handling
HF
Tag(s): 
Inappropriate crew response (technical fault), Procedural non compliance
LOC
Tag(s): 
Significant Systems or Systems Control Failure, Flight Management Error, Extreme Pitch
AW
System(s): 
Airframe
Contributor(s): 
Inadequate Maintenance Schedule, Corrosion/Disbonding/Fatigue
Outcome
Damage or injury: 
Yes
Aircraft damage: 
Major
Non-aircraft damage: 
No
Non-occupant Casualties: 
No
Occupant Injuries: 
None
Occupant Fatalities: 
None
Off Airport Landing: 
No
Ditching: 
No
Causal Factor Group(s)
Group(s): 
Aircraft Operation, Aircraft Technical
Safety Recommendation(s)
Group(s): 
None Made
Investigation Type
Type: 
Independent

Description

On 23 February 2020, the crew of a Bombardier Challenger 605 (C-GKCP) being operated by the Canadian Pacific Railway Company on a private international passenger flight from Palm Beach to Calgary received and actioned the drill for a flap system failure during descent to destination. However, during the subsequent flapless landing after a day VMC approach, selection of maximum reverse thrust was followed by a pitch up to become briefly airborne and then a hard nose gear first second touchdown. There was significant structural and some abrasion damage to the fuselage but no injuries to the 13 occupants.

Investigation

An Investigation was carried out by the Canadian Transportation Safety Board (TSB) assisted by data downloaded from the FDR and CVR of the aircraft involved. 

It was noted that the Captain who was acting as PM for the flight involved and had been working for the operator for 15 years had a total of 10,186 hours flying experience including 5,024 hours on type, the latter recorded as having been accumulated over 4½ years. The First Officer had joined the operator in 2017 and had a total of 1,055 hours flying experience including 82 hours on type having gaining his type rating two months earlier. He had completed line training on type whilst paired with the same Captain as on the investigated flight 2½ weeks earlier and had been specifically assessed as proficient in respect of the flap configuration exercise during his training.  

What Happened

When flaps 20 was selected during the initial approach into the Calgary area, a ‘FLAPS FAIL’ caution was immediately annunciated on the EICAS with FDR data indicating deployment to a recorded 1° before failing. The corresponding QRH procedure was actioned although the PM did not read out the content of supporting ‘CAUTION’ notes integral to the procedure. ATC were advised of the flap system status and requested and received delaying radar vectors. A brief for a flapless approach to the assigned runway 17R which at the ELW required a VREF of 155 knots

A visual approach to the runway was subsequently flown with the crew confirming to ATC on request that no assistance from the RFFS was required. The AP was disconnected at 1000 feet agl followed by disconnection of the A/T (required by the QRH procedure but not mentioned by the First Officer). The runway threshold was crossed at VREF + 10 KCAS and main gear touchdown occurred approximately 900 metres after this at a recorded 154 KCAS followed by the nose gear 2½ seconds later. 

Two seconds later, with the control column approximately 2° forward of neutral, the thrust reversers were selected to maximum. About 4.5 seconds after this deployment with reverse thrust increasing through approximately 60% N1, the aircraft nose began to pitch up and the nose landing gear lifted off the runway. About 12 seconds later, as the recorded pitch attitude and angle of attack were increasing through 13° and 22° respectively, reverse thrust was deselected and a forward input applied to the control column. The pitch attitude and angle of attack briefly continued to increase as the aircraft simultaneously banked approximately 3.5° to the right and the left main landing gear briefly left the runway surface.  As the pitch attitude increased to a recorded maximum of almost 17° and the angle of attack peaked at almost 29°, the aircraft’s rear fuselage struck the runway and the aft fuselage fuel dump drain mast broke off the aircraft and fell onto the runway at its intersection with Taxiway U. Less than a second later, the SPS (Stall Protection System) activated the stick pusher function and after a second or so, the aircraft pitch attitude was reducing at 19° per second. The Captain responded by making “significant nose-up input on the control” in opposition to the stick pusher but despite this, the nose gear then hit the runway with a negative pitch rate of at least 15°/second. Both engines were at approximately 55% N1 and decelerating at this time and the engines were continuing to decelerate, before reaching flight idle 7½ seconds later with the aircraft at 65 KCAS. 

Once at taxi speed, instructions to clear the runway and taxi to their allocated parking position were given. The crew reported being unaware that the rear fuselage had contacted the runway during the post touchdown pitch up and that as a consequence the 33 cm long fuel drain mast had broken off. There were five further aircraft movements on Runway 17R before the presence of FOD on the runway was reported to ATC by the crew of a landing aircraft some 17 minutes after it had broken off. The crews of two aircraft on approach were advised of this report and both continued to a landing after which an airport vehicle recovered the carbon fibre composite drain mast 23 minutes after it had been left on the runway.

Damage to the aircraft in addition to loss of the drain mast included significant abrasion damage to the lower skin of the tail fairing and significant structural damage to the lower forward fuselage. The latter included deformation of the right lower fuselage skin adjacent to the nose landing gear and damage to several frames in the lower forward fuselage as well as deformation of both the left and right nose landing gear upper torque box structure. 

Why it happened

The initiating event was the flap system failure but the landing accident only happened after the landing was not conducted in accordance with the guidance contained in the applicable QRH drill.

  • The Flap System failure
    • It was established than when flap 20 was selected on the initial approach, the flap flexible drive shaft which connects the two flap actuators for the left hand inboard flap panel, had failed. A detailed examination of this shaft found two puncture holes in the protective outer casing under the data plate which, by allowing the cavity to “breathe” with repeated changes in altitude, had allowed contaminants and condensation to enter which had, over time, led to corrosion. It was also suspected that moisture ingress into the drive shaft casing from the drive shaft to flap actuator interface had also contributed to significant corrosion of the ends of the flexible drive shaft core. 
    • Bombardier maintenance recommendations for the Challenger 605 were found to be based on an average annual utilisation of 500 hours, much higher than the 275 hours average utilisation of the operator involved.  The consequence of this was that routine maintenance predicated on accumulated flight hours or flight cycles were typically being completed with a calendar time that was approximately twice as long as the intervals that would have occurred if the aircraft was being used as anticipated by Bombardier. The failed flap flexible drive shaft was subject to a detailed inspection every 2,400 flight hours which meant that instead of an inspection every 4.8 years based on the assumed typical utilisation, it would only occur every 8.7 years.
    • It was also noted that although Bombardier had identified that corrosion of flap flexible drive shafts and actuators due to internal contamination was a significant in-service issue on the Challenger-derived Canadair Regional Jet series aircraft and in 2007 had issued a 2007 SB to address this, they had not issued similar SBs for the Challenger 600 series aircraft. 
  • Aircraft handling during landing  
    • Despite the cautionary remarks included in the flap failure QRH procedure (reproduced in the panel below), the operator’s pilots had not received any explicit training or guidance on the important difference between aircraft handling during a normal touchdown and a flapless one. On the contrary, most procedural advice implied that handling during a flapless landing was similar to that for a normal landing.
      • Improper landing technique during a flaps failure can generate nose gear loads sufficient to cause structural damage. To prevent this after main gear touchdown, gently lower the nose to the runway and apply brakes only after nose wheel touchdown.
      • Before selecting reverse thrust, ensure that [the] nose wheel has      touched down and nose down elevator has been applied.
  • Whilst cautions in the AFM and FCOM clearly explain the hazard of the nose pitching up and that it can be controlled with the elevators and specify the order in which reverse thrust should be selected, they provide very little pressure information. The FAA Operational Evaluation Report was noted to describe the amount of pressure required as “significant” whereas the AFM and FCOM say only to “ensure nose down elevator is applied”. The caution in the QRH procedure also provides order advice, but very little pressure information and unlike the AFM and FCOM text, this caution does not explicitly explain the hazard of the nose pitching up.
  • The pilots involved had coincidentally recently undergone training in a Level ‘D simulator which included a flapless landing exercise and it was considered that they may have remembered this as a “benign event” which it would have appeared to be because such simulators do not accurately replicate the behaviour of the Challenger 605 after the nose wheel touches down on the runway nor are they required to. Such a recent training experience was considered very likely to reinforce the expectation that a successful flapless landing would be not much different from a normal one based on the “knowledge reinforcement” principle.  

Six Findings as to Causes and Contributing Factors were formally recorded as follows:

  1. Moisture entered the flap flexible drive shaft casing, likely through the undetected puncture holes and the drive shaft casing to flap actuator interface, which led to the subsequent corrosion and failure of the inner drive shaft.  
  2. As the occurrence aircraft’s actual annual utilization was approximately half of that expected by the manufacturer, the calendar time interval between maintenance inspections increased. As a result, the corrosion that developed was not detected because the shaft had not yet reached the 2400 flight-hour maintenance interval.
  3. The Challenger 605 flight simulator used for recurrent training did not accurately represent the zero-flap handling characteristics of the aircraft when the thrust reversers are deployed, nor was it required for certification. As a result, the flight crew was inadequately prepared to prevent, or recover from, the nose pitch-up that occurred when the thrust reversers were deployed.
  4. The pilot monitoring did not read the cautions included on the flaps fail checklist out loud to the pilot flying during the completion of the Quick Reference Handbook procedure. As a result, information critical to the safe operation of the aircraft was not brought to the attention of the pilot flying.
  5. Insufficient pressure was applied to the control column to maintain the nose on the runway while using maximum reverse thrust; as a result, the nose pitched up, the aft fuselage contacted the runway, and the aircraft transitioned to a partial weight-off wheels state.
  6. When the left main wheel left the runway surface and the angle of attack reached 28.9°, the stick pusher activated and commanded a rapid nose-down pitch input. As a result of this rapid nose-down pitch, the forward section of the aircraft was damaged when the nose wheel contacted the runway.

Five Findings as to Risk were also formally recorded as follows: 

  1. If manufacturers implement product improvements on one fleet of aircraft but do not implement them on all similar fleets, there is a risk that some aircraft will not benefit from product improvements, potentially affecting their reliability.
  2. If aircraft procedures do not provide explicit information detailing unusual flight control inputs that are required during abnormal procedures, flight crews might apply insufficient or incorrect flight control inputs, thereby increasing the risk of an adverse outcome.
  3. If aircraft rescue and fire fighting is not put on standby when a flight crew is performing an abnormal procedure, it increases the risk of a delayed response should aircraft rescue and fire fighting assistance be required.
  4. If companies do not have policies in place that define the roles and responsibilities of all crew members on board an aircraft, and if passengers and flight attendants are not briefed on an abnormal aircraft condition, there is an increased risk that the passengers and cabin will not be adequately prepared in the event of an emergency situation.
  5. If airports rely only on physical inspections of aircraft movement areas, there is a risk that foreign object debris will not be identified and removed in a timely manner, which could lead to damage to equipment and aircraft, and injuries to persons.  

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

  • The Canadian Pacific Railway Company conducted a comparison of  reduced flap landing procedures in the AFM and QRH and as a result amended some company procedures to better address differences between the caution / warning notes published in the two documents.
  • The TSB issued an Air Safety Information Letter to Bombardier to highlight the Challenger 605 series flap system inspection interval and the applicable cross-fleet product improvement observation.
  • Bombardier issued a temporary revision to the QRH flap fail procedure to expand on the already-provided guidance on aircraft handling during a flapless touchdown.

The Final Report of the Investigation was authorised for release on 21 April 2021 and it was officially released on 12 May 2021. No Safety Recommendations were made.

Related Articles

SKYbrary Partners:

Safety knowledge contributed by: