History of the Flight
The crew, consisting of a commander and a co-pilot, were scheduled to operate a four-sector duty period starting from Glasgow Airport. The commander had been called in from standby and traveled to Glasgow, where he met the co-pilot for the first time. Both had limited experience at Belfast City Airport. Briefing for the flight, they agreed that the commander would act as Pilot Flying (PF) for the first two sectors due to forecast weather deterioration.
The first sector was uneventful. After landing on Runway 22 at Belfast City, while taxiing clear, the co-pilot performed the ‘after-landing’ checks, which included setting the autopilot selected altitude to zero. The aircraft parked five minutes ahead of schedule.
For the return flight, the commander conducted external turnaround checks while the co-pilot programmed the Flight Management System (FMS). Due to payload changes, a new load sheet was needed. The commander set the autopilot Flight Director (FD) modes to Go-Around (GA), Heading (HDG), and Altitude Select (ALT SEL) without first setting a selected altitude. When Air Traffic Control (ATC) issued the clearance, he set the selected altitude to 3,000 ft. However, neither crew member noticed that the autopilot modes were set to HDG and Altitude Hold (ALT), confirmed by recorded Flight Data Recorder (FDR) parameters. The vertical FD bar moved from the nose-up GA position to the aircraft symbol, corresponding to about 0° pitch.
The crew briefed for departure, including a Delayed Engine Start (DES) taxi, but did not discuss where the second engine would be started. The revised load sheet arrived, and pushback occurred three minutes behind schedule.
During pushback, the right engine was started. After engine start, flap 5 was selected. Taxiing to holding point A1, the aircraft was cleared to enter Runway 22 for backtrack and positioning. After entering the runway, the co-pilot started the left engine, though normal practice would have been to start it before entering the runway.
Approaching the threshold turning circle, the commander confirmed the takeoff clearance as “three thousand with alt sel,” but the selected FD modes were HDG and ALT. Turning 180° to position for takeoff, the commander referred to the Flight Mode Annunciator (FMA) modes, stating “go-around, heading, alt….why isn’t it alt sel....that’s better, yeah,” coinciding with the co-pilot selecting ALT SEL. During the turn, the crew began the line-up checklist, interrupted by ATC providing takeoff clearance. When checking the FMA and Caution and Warning Panel (CWP), the commander stated the FD modes were GA, HDG, and ALT SEL. However, recorded data later showed the modes were ALT, HDG, and ALT SEL.
Takeoff commenced. Shortly after becoming airborne, the landing gear was retracted and lateral navigation (LNAV) mode selected. Pitch initially about 15° nose-up, then reduced to about 10°. After reaching acceleration altitude (1,000 ft above airfield level), flaps were retracted, and the autopilot was engaged at 1,350 ft agl, airspeed 163 KIAS, in Instrument Meteorological Conditions (IMC). Following engagement, the aircraft gradually pitched nose-down at about 1.2° per second from 1,500 ft agl. Over the next 15 seconds, while the crew completed the ‘after takeoff’ checklist, the aircraft descended at an increasing rate, pitch attitude reducing from 10° nose-up to 8° nose-down.
At about 1,300 ft agl, an EGPWS Mode 3 “don’t sink” caution activated. The commander immediately disconnected the autopilot and applied nose-up pitch, arresting the descent which had reached 4,300 ft/min. Engine power was reduced, airspeed increased to 235 KIAS. The aircraft descended further, triggering a “pull up” warning, before transitioning to a climb from a minimum height of 928 ft agl, maximum airspeed 241 KIAS (VMO below 8,000 ft amsl is 245 KIAS). The crew later reported becoming visual with the ground during recovery.
Climbing, the commander stabilized pitch at about 8° nose-up and airspeed at 200 KIAS, with engine power gradually increased to climb setting. At about 1,900 ft agl, the autopilot was re-engaged but again pitched down. The commander disconnected, stating “it’s going to alt hold again, it’s going to pitch down.” He selected indicated airspeed (IAS) mode, deselecting ALT mode, and set target airspeed to 210 kt. The autopilot was re-engaged, and the commander confirmed normal operation. The aircraft levelled at 3,000 ft. ATC inquired if everything was okay, having observed the descent and track deviation. The co-pilot responded that they had experienced an autopilot problem, now resolved. The aircraft continued to Glasgow for an uneventful landing. After landing, the crew discussed the incident but were uncertain why the descent occurred when the autopilot was initially engaged.
Aircraft Information
The DHC-8-402 is equipped with an Automatic Flight Control System (AFCS) including a Flight Director (FD) function providing lateral and vertical guidance, and a single autopilot that couples FD guidance for automatic control. FD guidance is presented on pilots’ Primary Flight Displays (PFD) as vertical and lateral bars.
Autopilot engagement and FD mode selection (HDG, ALT SEL, ALT) are made via push buttons on the Flight Guidance Control Panel (FGCP) on the upper instrument panel. The FD GA mode is selected by buttons on the power levers. Selection and engagement status are indicated on the FMA block at the top of each PFD: active modes in green, armed modes in white.
The ALT SEL mode provides FD commands to acquire and maintain the selected altitude set on the FGCP. As the aircraft approaches the selected altitude, the FD automatically transitions to altitude capture mode (ALT*), then to ALT mode when captured. The ALT mode maintains a target altitude. If activated manually, the target altitude is set to the aircraft’s current altitude, rounded to the nearest 100 ft. The AFCS allows ALT and ALT SEL to be selected simultaneously on the ground or in flight; ALT is active, ALT SEL armed.
If HDG, ALT SEL, and ALT modes are selected on the ground, the target altitude is set to the aircraft’s current altitude (airfield elevation). As the aircraft climbs after takeoff above this target altitude, the FD vertical guidance bar moves to a nose-down pitch attitude. Upon autopilot engagement, the aircraft pitches down to follow the bar to descend to the target altitude. In this incident, the target altitude would have been about 0 ft amsl.
Post-Incident Actions
As a result of this event, the operator has taken several safety actions, including revisions to simulator training and amendments to the taxi checklist.