Egypt Cargo A306 near Brussels on Jan 1st 2017, climb above cleared level due to misunderstood TCAS RA results in near collision

Last Update: December 25, 2017 / 15:18:16 GMT/Zulu time

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Photo of Egypt Air SU-GAY, Airbus A300 — Egypt Air SU-GAY, Airbus A300 (Photo credit: Alec BHX/KKC / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Jan 1, 2017

Classification
Incident

Airline
Egypt Air

Flight number
MSX-541

Departure
Ostend, Belgium

Destination
Cairo, Egypt

Aircraft Registration
SU-GAY

Aircraft Type
Airbus A300

ICAO Type Designator
A306

An Egypt Air Airbus A300-600 freighter, registration SU-GAY performing flight MSX-541 from Ostend (Belgium) to Cairo (Egypt), was cleared to climb to FL210 out of Ostend.

An Air France Airbus A320-200, registration F-GKXN performing flight AF-1640 from Paris Charles de Gaulle (France) to Amsterdam (Netherlands), was enroute at FL220 on a converging trajectory.

The instruction to climb to FL210 was re-issued to MSX-541 about 5 minutes before the trajectories were about to cross and was read back correctly again. 2 minutes prior to the crossing point ATC instructed MSX-541 to maintain FL210 and provided traffic information of the crossing A320 above.

MSX-541 however continued the climb above FL210, both aircraft received TCAS resolution advisories, AF-1640 was instructed to climb and complied, MSX-541 however continued their climb opposed to the TCAS RA.

Belgium's AAIU reported that the separation between the two aircraft reduced to 300 feet vertical and 0.74nm horizontal.

MSX-541 continued their climb and to Cairo for a safe landing, AF-1640 returned to FL220 after being clear of conflict and continued to Amsterdam for a safe landing.

Belgium's AAIU rated the occurrence a serious incdent. The AAIU reported that MSX-541 was instructed three times to climb/maintain FL210 and was provided with traffic information in the last of these instructions. However, the aircraft continued to climb above FL210 resulting in TCAS resolution advisories on both aircraft, MSX-541 received a "LVL" RA, AF-1640 received a "CLB" RA.

On Dec 22nd 2017 Belgium's AAIU released their interim report stating that AF-1640 (AFR640F) was cleared to maintain FL220, while MSX-541 was cleared to climb to FL210. The AAIU wrote: "ATC was monitoring both aircraft. The ATC controller’s intention was, as he was familiar with the situation, to have the MSX541 passing behind the Air France aircraft. However, MSX541 was climbing faster than usual, due to a light load. AFR640F was maintaining FL220. The crew called ATC when reaching the Belgian border. The ATC controller instructed AFR640F to fly towards Hamstede before reaching the FERDI waypoint, in order to speed up the crossing." ATC did provide traffic information about AF-1640 to MSX-541.

MSX-541 was climbing at about 2500 fpm towards FL210 when TCAS issued a resolution advisory (RA) to level off. The AAIU annotated, had the crew followed that RA they would have levelled off at FL207 and no resolution advisory would have been generated at AF-1640, even if the RA was complied with with a delay of 10 seconds the aircraft would have levelled at FL209 with no RA at AF-1640. However, the crew mistook the RA to increase their climb, the climb rate increased to 3500 fpm causing a RA to be generated at AF1640 to climb. AF-1640 climbed at 1500 fpm as required. At the same time a RA to descend was issued at MSX-541, which at that point was climbing through FL213 and reduced rate of climb, however, did not initiate a descent at 1500 fpm as required.

The AAIU stated the separation between the two aircraft reduced to 427 feet vertically and 0.69nm horizontally at 11:47:11Z. The AAIU also stated that had AF-1640 not reacted to their RA and MSX-541 climbed as during the occurrence, the vertical separation would have reduced to 215 feet. A faster response than the nominal delay of 5 seconds by AF-1640 could have increased the vertical separation by 71 feet to 497 feet.

About 36 seconds after the first RA was generated both flight crews received the "clear of conflict" announcements by their TCAS systems.

The AAIU wrote:

Safety issue identified: Misinterpretation of TCAS RA instructions

Misinterpretations of TCAS RA instructions were already identified in the past. In particular, the instruction to stop climbing was, in the past, announced by the message “ADJUST VERTICAL SPEED”. In several cases, this announcement was wrongly understood by crews and led to Airprox events.

The TCAS system was therefore modified in order to change the “ADJUST VERTICAL SPEED” announcement to a clearer “LEVEL OFF” announcement.