On 10 December 2006, a DHC-8-Q400, operated by Flybe, experienced multiple flight instrument failures in the climb in icing conditions at night which were consistent with icing of the pitot/static system. It was eventually found by the flight crew that they had failed to select either normal or standby pitot/static heaters on and they remained unaware of this error whilst attempting resolution oif their problems by completing various apparently relevant Abnormal/Emergency Checklists which made no difference. The declared a PAN and obtained descent until clear of icing conditions after which all displays returned normal functionality and the heaters were finally seem to have been off and were then correctly selected.
This is an extract from official report by the UK Air Accident Investigation Branch (AAIB):
Precipitation was encountered about 1000 ft after takeoff and propeller anti-ice was selected ON. The autopilot was also engaged. The crew were given a direct routing […], and cleared to climb to FL160 (approximately 16000 ft amsl). The aircraft encountered heavy precipitation during the climb, and a number of visual checks were made for ice. When airframe ice was seen, the crew switched the airframe icing protection system from MANUAL/OFF to FAST. The crew reported that, at FL100, they checked the altimeter indications, which were normal, and carried out a number of other routine check items. As the aircraft continued to climb, the crew received an ICE DETECTED message on the ED [Engine Display], generated by the automatic ice detection system. No action was necessary as airframe, engine and propeller de-ice systems were already on by this time[…].
As the aircraft approached its cruising level, the crew received an “ALT MISMATCH” alert on their Primary Flying Displays (PFDs), warning of a discrepancy in the displayed altitude. A cross-check of the standby flight instrument display showed that the commander’s (left-hand) PFD was showing an erroneous altitude of approximately 150 ft below the co-pilot’s PFD altitude. As the autopilot was selected to receive its inputs from the right hand (co-pilot’s) instrument sources, the crew were content for it to remain engaged.
The aircraft levelled at FL160, just above a cloud layer. Soon after reaching FL160, the crew began to experience further discrepancies between both indicated altitudes and airspeeds, and observed heavy icing on the aircraft structure. The autopilot then disconnected automatically. The commander’s indications of altitude and airspeed decayed rapidly, and were replaced by red failure indications. By selecting the right hand instrument sources to feed his own PFD, the commander was able to restore speed and altitude indications to his display. The Air Traffic Controller handling the flight noticed that the aircraft’s SSR Mode C altitude had disappeared from his radar display, and queried it with the crew. In response, the commander requested an immediate descent, stating that the crew were experiencing instrument problems and that he required a descent to clear the icing layer. The crew were cleared for a descent to FL80.
As the descent began, the co-pilot’s altitude indication (now displayed on both pilots’ PFD as a result of the commander’s source selection) appeared to read correctly, but the airspeed indication began to show a deceleration at a rate which matched the decreasing altitude. The co-pilot kept the power levers at the cruise setting as the indicated airspeed reduced, concerned that the aircraft was approaching a stall (he recalled seeing an IAS of 134 kt). Recognising that this was an erroneous indication, the commander intervened and directed the co-pilot to reduce power and to select an appropriate pitch attitude for the descent. Both the altitude and airspeed indications subsequently reduced rapidly and were replaced by red failure indications. Both pilots reported that several amber caution lights illuminated on the Caution/Warning Panel (CWP), associated with the instrument failure indications.
The commander made a ‘PAN -PAN ’ call to ATC, stating that the crew had lost all pressure instruments, and initiated the Emergency Checklist. The controller assisted by providing the crew with groundspeed readouts from his display, and Mode C altitude information, when it became available in the later stages of the descent. Both pilots reported that the standby IAS display also showed a red FAIL indication during the descent, though it was uncertain whether the standby altitude display remained valid.
As the aircraft approached FL80, the PFD altitude indication returned and the co-pilot used it to level the aircraft. Subsequently, the remaining airspeed and altitude indications from both left and right sources recovered to normal. […]
During discussion between the flight crew immediately after the icing encounter, the co-pilot queried the position of the pitot/static probe heat switches (See Figure 1) with the commander, and said that he thought they may be off. Later, neither pilot could be completely certain whether or not the switches were physically moved at this point, but information from the Flight Data Recorder (FDR) was consistent with the standby pitot/static probe heat switch being moved from OFF to ON, about three minutes after levelling at FL80, having been at OFF since the start of the recording (switches for the left and right pitot systems were not monitored by the FDR).
The aircraft subsequently reached the intended destination without further incident.
Figure 1 Pitot/Static probe heat switches - panel layout and cockpit view
The investigation noted that repeated non-standard checklist procedures, (interrupted checklist prior taxi) and distractions may have created an environment in which the selection of the probe heat switches to ON was missed before takeoff and not detected until after descent out of icing conditions. It was noted that illuminated cautions relating to flight without pitot / static heaters on would have been present on the Master Caution / Warning Lights panel (See Figure 2) throughout the time when they were off.
Figure 2 CWP arrangement and pitot/static caution lights
The report comments that there were undoubtedly distractions and pressures during the taxi and early takeoff phase and continues that it is difficult to say whether this fact may have had a bearing on this incident, but it is important to stress that a correct and disciplined use of the checklist should alert the flight crew to the fact before takeoff.
The following factors are identified as to Contributory factors to the Incident:
- a combination of non-standard use of the checklist, distraction on the flight deck and external pressure contributed to the aircraft taking off with the pitot/static probe heat switches incorrectly selected OFF.
- a high workload during the climb in poor weather and heavy icing conditions probably contributed to further missed checklist actions, such that the aircraft climbed to its cruising level without the omission being noticed.
- the resulting instrument failure indications and subsequent recovery of information were consistent with the probe heat switches being OFF until after the incident had occurred.
- the position of the CWP meant that, under specific circumstances, it may not have been readily obvious to the crew that pitot heat caution lights were illuminated.
No Safety Recommendations were made as a result of the Investigation.
- AT73, en route, Roselawn IN USA, 1994
- AT43, en-route, Folgefonna Norway, 2005
- ATP, en-route, Oxford UK, 1991
- SH36, vicinity East Midlands UK, 1986
See the full Investigation Report published by the UK AAIB.