DISCLAIMER:
Reviewing accidents and incidents is an essential part of becoming a better pilot or engineer. The intention should never be to blame anyone, but rather to understand WHAT happened, WHY it happened and HOW to PREVENT it from happening again.
The following information is based on the accident report [1], unless otherwise stated. The "lessons to learn" at the end are my contribution to enhance flight safety.
Registration: SE-DUX
Aircraft type: CL-600-2B19
Date: 12.12.2016
Fatalities: 2
Operation: Cargo
Flight phase: Enroute
Location: Oajevágge, Sweden
Category: Equipment failure, inflight upset
Summary
The mishap aircraft was a cargo version of the Bombardier CRJ series on a routine cargo flight from Oslo/Gardermoen Airport (ENGM) to Tromsø/Langnes Airport (ENTC). The captain was Pilot Flying (PF). Enroute, during the approach briefing, the crew was presented with a misleading attitude indication and subsequently lost control of the aircraft.
Aircraft description
Flight control system
The aircraft had hydraulically actuated flight control surfaces, commanded via cables. An artificial feel system provided force-feedback for the pilots. Trim was achieved using the horizontal stabilizer, there was also a stall-warning and -prevention system available.
Flight instruments and systems
The instrument panel consisted of an Electronic Flight Instrument System (EFIS) and a set of standby instruments. There were two Inertial Reference Units (IRUs) installed. In normal configuration, the left PFD would display attitude information from IRU 1 and the right PFD would display data from IRU 2. The standby attitude served as an independent third attitude source. The PFD’s attitude source could be changed from “normal” to “both on IRU 1” or “both on IRU 2” using a selector panel.
There was also an automatic comparison between the two PFD’s that would display a specific alert on the PFD’s in case of a disagreement.
The sequence of events
The pre-flight, departure and first part of the cruise were uneventful. While conducting the approach briefing and in Instrument Meteorological Conditions (IMC) at 33000 ft, the autopilot disconnected and the crew was presented a misleading attitude indication on the captain’s Primary Flight Display (PFD) as well as a “pitch miscompare alert”. The attitude indication on the captain’s PFD drifted rapidly away from the real value, as seen in Figure 4.
Description of Figure 4:
- First row: Two seconds after the attitude deviation started. Note the “pitch miscompare” caution.
- Second row: 13 s into the event, both PFDs went into “declutter” mode and the “pitch miscompare” caution was removed as soon as the indication exceeded +30°/-20° pitch or +/- 65° roll angle.
- Third row, extreme attitudes on both PFD’s, left side misleading, right side authentic.
The captain started to recover from the upset that he believed was taking place, based on the attitude indication on his PFD. As the indication was “nose-high”, he mainly applied “nose-down inputs”, creating negative load factors reaching -1 g. Figure 5 shows the indicated pitch on the captain side (blue) and the calculated real pitch (green) as a function of time.
The F/O’s PFD and the standby indication remained valid and authentic during the entire event. The aircraft descended rapidly towards terrain, exceeding Vd/Md and was destroyed upon impact. Important: During the event, the pilots agreed that they were experiencing an upset, but they apparently never compared the indications of their instruments.
The impact was extremely violent, pitch value approaching -90° and indicated airspeed in the region of 500 kt (!)
Almost the entire aircraft was shattered within a circle of about 20 m diameter, leaving the crew no chance of survival.
Main cause and contributing factors
- IRU 1 started to output invalid pitch, roll and heading data
- The crew was unable to identify the misleading data source and lost control
- The crew did not communicate effectively with regard to the malfunction
- The crew was possibly impaired by the strong negative load factors induced by their own control inputs
Lessons to learn for pilots:
- A misleading attitude indication is one of the most challenging malfunctions a flight crew can be faced with
- If an instrument shows an unexpected value, it is of vital importance to cross-check (as far as possible) to ensure authenticity, BEFORE ANY ACTION IS TAKEN
- For a PFD attitude indication, the standby attitude indication provides a very powerful crosscheck, as it usually has a different data source
- When initiating an upset recovery procedure, it is crucial that both pilots agree on the nature of upset (i.e.: “nose-high” or “nose-low” etc.)
Lessons to learn for engineers:
- Critical parameter validation is absolutely vital. Today, there are many powerful “voter algorithms” available, that automatically detect and reject hazardously misleading information
- When using a “declutter” function on a display, extreme care should be taken when a decision is made to remove a warning or caution
- Software configuration in a simulator should be the same as in the real aircraft
- During a safety assessment, never forget that the flight crew will not immediately know what the problem is and that it will take time to establish the facts
Revision/20200503
References
[1] Swedish Accident Investigation Authority, Final report RL 2016:11e, “Accident in Oajevágge, Norrbotten County, Sweden on 8 January 2016 involving the aeroplane SE-DUX of the model CL-600-2B19, operated by West Atlantic Sweden AB”, 2016