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Report: Paris on Mar 13th 2012, intercepted mirror glide slope, large pitch oscillations and approach to stall
By Simon Hradecky, created Tuesday, Mar 12th 2013 16:42Z, last updated Tuesday, Mar 12th 2013 16:42Z
An Airbus A340-300, performing a flight from Bamako (Mali) to Paris Charles de Gaulle (France), was descending towards Paris for an ILS Category III approach to runway 08R but went significantly above the approach profile despite descending with speed brakes deployed. The approach was continued. When the aircraft was 9nm before the runway threshold, the aircraft had been established on the localizer however was at 4950 feet MSL about 1750 feet above glideslope, target speed selected at the FCU was 183 knots, the flaps were selected to position 1. The crew slowed the aircraft and selected flaps to position 2, at 4nm before touchdown the aircraft was at 3700 feet MSL and 2100 feet above glideslope, the glideslope indications in the cockpit had already reverted to the side band of the glideslope transmitter (mirror glideslope at 9 degrees), about 30 seconds later the crew selected the gear down. At about 2nm out, the aircraft descended through 2850 feet MSL 1600 feet above glide, the vertical channel of the autopilot mode change glideslope capture and autothrust changed to speed mode. The aircraft pitched up from about 1 degrees nose up to 26 degrees nose up, the vertical speed changed from -1600 fpm to +3300 fpm, the airspeed dropped from 163 KIAS to 130 KIAS. When the aircraft rotated through 26 degrees nose up, the crew disconnected the autopilot and pushed the side stick forward to near the mechanical stop, the pitch attitude and the rate of climb reduces, the aircraft began to accelerate to 143 KIAS again, the autothrust system disconnected. About 30 seconds later the crew re-engages autopilot 1 and autothrust in climb thrust with the intention to perform an automatic go-around, the autopilot's lateral channel engages in LOC, the vertical channel in G/S, the autothrust in SPEED, the aircraft crossed the runway threshold at 2700 feet MSL (2370 feet AGL), the pitch attitude reduces to 5 degrees nose down and the aircraft rapidly descended. The pilot monitoring recognized the modes announciated were not suitable, disengaged the autopilot, placed the thrust levers into the TOGA detent and the aircraft entered a normal go-around at 2000 feet MSL. The crew subsequently positioned the aircraft for a second approach and landed without further incident.
The French BEA released their final report in French concluding the probable causes of the serious incident were:
- the inadequate monitoring of the aircraft's trajectory by both air traffic control and the crew while conducting a CATIII approach under radar guidance
- the decision of the crew to continue the approach although the aircraft had crossed the final approach point above vertical profile
Contributing factors were:
- the absence of visual cues on the radar screen of air traffic control when aircraft intercept the vertical profile below 5000 feet MSL
- the inappropriate methode of intercepting the glideslope from above used by the crew
- the capture of a side band/mirror glideslope by the autopilot which resulted in an excessive pitch increase
- fatigue by both controllers and crew which may have contributed to the serious incident
The BEA complained that the recordings of the cockpit voice recorder had already been overwritten when the BEA was notified of the incident. The investigation such had to rely on data off the quick access data recorder, radar data, the ATC recordings and testimonies by controllers and crew.
The aircraft was maintaining FL090 about 30nm before the runway threshold with the autopilot's lateral channel in HDG mode, the vertical mode in ALT and autothrust in speed mode maintaining 250 KIAS, the crew contacted approach at that time and was cleared to intercept the localizer 08R followed by the instruction to descend to FL080. At about that time the aircraft crossed the vertical profile from below to above the profile. The aircraft was subsequently cleared to descend to FL060, the autopilot's vertical channel and autothrust engage in OP DES mode, the lateral channel captures and tracks the localizer. The aircraft was at 7220 feet MSL about 1275 feet above vertical profile at 17.5nm before the runway threshold, at that point the controller instructed to maintain a speed of 200 IAS or greater, the aircraft is still doing 250 KIAS. The crew inquires for further descent, the controller apologizes for forgetting and clears the aircraft to descend to 3000 feet and the ILS approach runway 08R. The crew selected 220 KIAS and 3000 feet into the flight control unit (FCU), the autoflight systems maintain the open descent mode. The speed as well as the rate of descent reduce (tracking selecting speed has priority) resulting in an ever increased deviation above the vertical profile, the crew applied speed brakes. After the aircraft had slowed to 220 KIAS the rate of descent increased to 1840 feet per minute reducing the deviation (to follow the 3 degrees glidepath a rate of 1100 feet per minute would be needed).
When the aircraft was 10nm out descending through 5500 feet the controller instructed to maintain a speed of 160 KIAS or greater and handed the aircraft off to tower, but did not inform tower that the aircraft was above the vertical profile. The crew selected 183 KIAS target speed and selected the flaps to position 1, once again the rate of descent is reduced by the autopilot and the deviation from profile increased again resulting in the sequence of events already described in the initial paragraph.
The BEA reported depending on the altitude assigned by ATC to intercept the ILS the final approach point is 14nm out at 5000 feet MSL, 11nm at 4000 feet MSL, 8nm at 3000 feet and 5nm out at 2000 feet MSL. The radar screen of the controllers provide a visual cue only for intercepting the final approach at 5000 feet although controllers are permitted to assign other altitudes to permit separation of simultaneous parallel approaches. The ATC procedures require, with low visibility procedures in use, that the aircraft should reach a speed of 180 KIAS or less at 15nm before touchdown and a speed of 160 KIAS or less before intercepting the glidepath.
When the autopilot activated the capture glideslope mode, the aircraft would already have needed to descend at an angle of 10 degrees to reach the runway and was consequently already above the 9 degrees side band of the glideslope transmitter, in which the deflections would be reversed by design, above 9 degrees angle the indications would demand an upward correction causing the autopilot to command the large pitch increase until the crew disconnected the autopilot.
5 safety recommendations were released as result of the investigation.
