Air France A320 at Hamburg on Mar 28th 2012, ILS and flight director disagree and result in Alpha Floor Activation

Last Update: February 26, 2015 / 17:14:50 GMT/Zulu time

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Photo of Air France F-GJVA, Airbus A320 — Air France F-GJVA, Airbus A320 (Photo credit: markyharky / Flickr / License: CC by)

Incident Facts

Date of incident
Mar 28, 2012

Airline
Air France

Flight number
AF-2510

Departure
Paris Charles de Gaulle, France

Destination
Hamburg, Germany

Aircraft Registration
F-GJVA

Aircraft Type
Airbus A320

ICAO Type Designator
A320

An Air France Airbus A320-200, registration F-GJVA performing flight AF-2510 from Paris Charles de Gaulle (France) to Hamburg (Germany), was on ILS approach to Hamburg's runway 23 when the crew noticed disagreeing indications of the glidepath indicator and the flight director and aborted the approach. After going around the aircraft positioned for another approach to runway 33 and landed safely about 17 minutes later.

Germany's BFU reported the incident is being investigated by the BEA with the support by the BFU.

On Feb 26th 2015 the French BEA released their final report in French concluding the probable causes of the serious incident were:

- the assignment of a hold short point in the CAT I critical zone of the glidepath transmitter runway 23 contributed to the presence of an aircraft in the critical zone of the glidepath transmitter of runway 23.

- a probable misunderstanding of the risks associated with disturbed ILS signals by the tower controller the controller did not prevent the taxiing aircraft entering the critical zone, the controller also did not inform the approach controller.

- the crew, performing a manual approach and unaware of the disturbed glide slope signal, did not effectively monitor the primary flight parameters to detect that they were not on the expected glide path delaying the decision to discontinue the approach and go around.

- the inadequate implementation of the go-around procedure led to significant discrepancies in speed and pitch and ultimately to the activation of the pitch angle protection.

The BEA reported that the airport had been using single runway 33 configuration for both arrivals and departures until just prior to the arrival of AF-2510. Due to wind changes the aerodrome was reconfigured to use runway 33 for departures and runway 23 for arrivals. AF-2510 was the second aircraft to land on runway 23 and was vectored onto an ILS Cat I approach to runway 23.

In the meantime a Boeing 737, that had landed on runway 33, was taxiing towards the apron and had been cleared to cross runway 23 on taxiway D by tower, the crossing clearance was cancelled however due to the first approach to runway 23. The Boeing 737 thus stopped at the CAT I holding point on taxiway D in the middle of the critical zone for the glideslope transmitter runway 23.

AF-2510 was about 9nm out at that point still in contact with approach control. While approaching the extended center line of the runway, about 40 seconds before the LOC capture activated, the captain, pilot flying, disconnected autopilot and autothrust and continued manually, the flight director indications continued. The first officer, pilot monitoring, advised that there were 50 knots cross wind indicated prompting the captain to use flaps 3 instead of flaps FULL as briefed. The aircraft established on the localizer and glide slope, the pilot monitoring configured master control panel and FMGS for the missed approach procedure.

At that point the Boeing 737 stopped at the CAT I holding point on taxiway D and disturbed the glide slope signal. Although the A320 was slightly above the glidepath the crew received indication of being below the glidepath, in the next 30 seconds the crew reduced the rate of descent without noticing the erroneous indication. Then the glideslope indication fluctuated and indicated the aircraft was above the glidepath, in response the pilot flying reduced thrust and increased the rate of descent up to 2900 fpm, the glideslope capture reengages when the aircraft came close to the 0 deviation indication, the aircraft at that point was 250 feet below the glideslope however. The indication changed to below the glidepath, the crew stabilized the altitude at 1600 feet MSL, selected the gear down and commanded the flaps to position 3, the engines were at idle and the speed was decaying through 151 KIAS, when the glide slope indication suddenly inverted.

The first officer suggested to go around which was accepted by the captain. The captain pitched the aircraft up, however did not advance the thrust levers, the automatic aural alert "SPEED! SPEED!" activates as the IAS decays through 143 KIAS, both thrust levers are gradually advanced into the MCT position, the Alpha Floor protection activated as the pitch increased from 15 degrees to 22 degrees nose up and the airspeed reached a minimum of 121 KIAS. Autothrust automatically engaged and accelerated the engines to TOGA, the captain lowered the nose and the aircraft accelerated. After climbing the aircraft to 4000 feet the crew reported a problem with the ILS runway 23 and queried whether there had been another pilot report receiving a reply in the negative, positioned the aircraft for an approach to runway 33 and completed a safe landing without further incident.

The BEA analysed that the position of the CAT I hold short line within the critical zone of the glide slope transmitter does not comply with ICAO recommendations. Testimony by the controller seeing aircraft frequently at that hold short line suggests that they had become accustomed to the presence of aircraft in that area and were not aware of the risks associated with a disturbed glide slope signal.

The crew had made the choice to manually fly the aircraft, the captain was busy following the flight director indications, and in consideration of high cross wind thus did not have the resources available to identify the inconsistencies in the flight director indications and detect the abnormal vertical speeds resulting while following the flight director indications. At the same time the first officer was busy reprogramming the automatic systems for the go-around and also did not notice the deviations, hence did not call out the deviations.

The incorrect initiation of the go-around with the thrust levers not being moved into the TOGA detent and thus the aircraft systems not activating the "go-around" mode together with a high angle of attack and high angle of pitch and decreasing speed led to the activation of Alpha Floor Protection.

The BEA analysed further that the aircraft's embedded systems do have the information about distance (DME) and altitude available and thus could cross check the aircraft's position in relation to the glide path. As such these systems would be able to detect inconsistencies in the ILS glide slope indications and provide an alert. The BEA also referenced another occurrence, see Report: Air France A343 at Paris on Mar 13th 2012, intercepted mirror glide slope, large pitch oscillations and approach to stall stating that the interception of a mirror glideslope could be detected and prevented by such a system, too.