Description

On 23 December 2008, a DHC8-400 being operated by Flybe on a scheduled passenger flight from Southampton to Edinburgh continued descent below its cleared altitude of 2100ft in day Visual Meteorological Conditions (VMC) prior to and then whilst tracking the ILS LLZ for Runway 23 at destination. It remained below the ILS GS until the ATC GND Controller, who had no formal responsibility for this phase of flight but was positioned alongside the TWR Controller, observed that aircraft had descended to within 800 ft of local terrain approximately 5 nm from the runway threshold. The flight crew appeared unaware of this when making a ‘Finals’ call to TWR at 5.5 nm and so the Controller queried the descent. The aircraft was then levelled to achieve 600ft agl at 4nm from the threshold and an uneventful landing subsequently followed.

Investigation

An Investigation was carried out by the UK AAIB and established that with the aircraft commander as PF and the AP engaged, FD mode selection upon leaving 3000ft amsl for 2100 ft amsl with clearance to establish on the ILS LLZ, had been VS and LOC modes but not ALT SEL. The ILS LLZ had then been captured in descent through 1250 ft amsl with almost full scale fly up showing on the flight deck ILS displays. ATC APP transferred the aircraft to TWR without noticing the breach of clearance or continued descent well below the ILS GS. As the aircraft passed through 1,000 ft amsl the co pilot checked in with ATC TWR with the premature descent still un-noticed by either pilot despite flight deck indications confirming a position lower than full scale fly up on the ILS and a PF declaration of ‘Visual’ (the runway) and Visual Approach Slope Indicator Systems indications confirming the below GS status.

Once ATC TWR had prompted awareness of the abnormally low altitude, there was a delayed response attributed by the aircraft commander to difficulty disconnecting the Autopilot. At 700 ft agl, landing gear down was selected followed by land flap selection and receipt of land flap at 570 ft amsl. . An uneventful landing followed. It was noted that the absence of any Terrain Avoidance and Warning System (TAWS) Mode 5 ‘Glideslope’ Alert was attributable to the fact that the landing gear was not selected down until the premature descent had been arrested. Both the relevant aircraft systems and ground installations were subsequently confirmed to have been fully serviceable.

On 8 May 2009, whilst the investigation was in progress, a similar event occurred to another of the same Operator’s DHC8-400 aircraft whilst it was making an ILS approach to Runway 23 at Glasgow, UK albeit with a different flight crew involved. Because of the circumstantial similarity, an investigation into this second event was also carried out as part of the principal investigation.

In this second case, which also occurred to a scheduled passenger flight in day VMC with the aircraft commander as PF, as the aircraft passed through 2,600 ft amsl with the AP engaged, the Mode was switched from HGD to LOC. Again, the aircraft continued descending below the cleared altitude whilst tracking the ILS LLZ in the absence of a GS Mode selection. By 1,600 feet amsl the landing gear was down and locked and land flap was then selected. The first of five EGPWS “Glideslope” warnings was recorded at 5.4 nm DME as the aircraft descended through 975 ft agl and initially the aircraft continued to descend with the same vertical speed. After a second “Glideslope” warning at 920 ft agl the autopilot was disconnected and the rate of descent was reduced. A third “Glideslope” warning was recorded at approximately 730 ft agl after which descent was converted to level flight over slightly rising ground. At 630 ft agl and 4.1nm DME, a fourth Glideslope warning was recorded and the aircraft remained in level flight with a full “fly up” indication on the glideslope indicator until a fifth “Glideslope” warning was recorded at 4.0 nm DME. The aircraft intercepted the ILS GS shortly afterwards and continued to an uneventful landing.

The Investigation noted that in both cases, although it was ATC who had alerted the flight crews to their premature descent, APP to TWR handoff had been made with the aircraft on the LIS LLZ but in descent below the ILS GS. Appropriate ATC action was noted to have been taken in respect of procedures.

The Investigation also noted that an Minimum Safe Altitude Warning (MSAW) system had been on trial at Edinburgh had not been operational at the time.

Finally, the Investigation note that the Aircraft Operator, Flybe, had an SOPs in place at the time of both events to the effect that:

Provided PF has called "visual", no further reference to altitude is required and if the visual profile is normal, no reference to speed and sink.

The Final Report of the Investigation was published on 4 March 2010 and may be seen in full at SKYbrary bookshelf:AAIB Bulletin: 3/2010 ref. EW/C2008/12/05

It contained two Safety Recommendations. The first was to the aircraft manufacturer and arose from the mode selection difficulties which characterised both events:

• Safety Recommendation 2009-005

It is recommended that Bombardier Aerospace enable automatic arming of the altitude select mode of the flight director fitted to Dash-8-400 series aircraft upon selection of a new altitude and vertical mode.

The second was made to the Aircraft Operator, Flybe, after they declined during the investigation to modify their Standard Operating Procedures to require that a “final fix” check should be made on all approaches, even ones where the PF has declared that they are “visual” with the runway:

• Safety Recommendation 2009-006

It is recommended that Flybe consider amending its standard operating procedures to require an altitude check whilst on final approach even when the pilots are in visual contact with the runway.

Further Reading

• Approach and Landing Risks
• Terrain Awareness
• Level Bust Prevention - Aircraft Technical Equipment