Alitalia A320 and Ryanair B738 at Madrid on Sep 7th 2019, incorrect descent regime causes loss of separation

Last Update: September 29, 2020 / 20:26:32 GMT/Zulu time

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Incident Facts

Date of incident
Sep 7, 2019

Classification
Incident

Airline
Alitalia

Flight number
AZ-60

Destination
Madrid, Spain

Aircraft Registration
EI-IKL

Aircraft Type
Airbus A320

ICAO Type Designator
A320

An Alitalia Airbus A320-200, registration EI-IKL performing flight AZ-60 from Rome Fiumicino (Italy) to Madrid,SP (Spain), was descending towards Madrid.

A Ryanair Boeing 737-800, registration EI-ENN performing flight FR-3059 from Menorca,SP to Madrid,SP (Spain), was descending towards Madrid.

The Airbus had been above the Boeing for several minutes while tracking the same airway, both aircraft had been instructed to track to waypoint ASBIN and descend afterwards. While tracking towards ASBIN both aircraft maintained their descent profile and thus kept the vertical separation, following ASBIN when descending to their pre-cleared altitudes the descent regimes became incorrect causing the loss of separation. ATC subsequently turned the Boeing out of the way resolving the conflict. Both aircraft continued for safe landings in Madrid.

Spain's CIAIAC reported the separation between the two aircraft reduced to 300 feet vertically and 1.4nm horizontally when the descent instructions previously transmitted were not carried out at the correct descent regime. The incident is being investigated.

On Sep 29th 2020 the CIAIAC released their final report concluding the probable cause of the incident was:

The loss of separation between the two aircraft was due to the failure of both crews to comply with the respective instructions received involving the rates of descent that both aircraft were required to follow.

Contributing to this is the fact that the controller in sector RNN:

- did not actively monitor and enforce the instructions that both crews had received related to their rates of descent, and

- did not provide a more forceful instruction (such as diverting one of the two aircraft from its heading or leveling off the higher aircraft), given the lack of results provided by the instructions involving the rates of descent.

The CIAIAC analysed:

Both aircraft were in sector RNN, with AZA060 flying to ASBIN point (IAF) and RYR4UX to PRADO point. Initially, AZA060 was above RYR4UX, and both of them were descending, although their vertical separation was sufficient. RYR4UX was instructed to descend to 6000 ft as it was descending through FL289, and was then cleared to proceed to ASBIN point at a high speed.

AZA060 was then instructed to descend to FL230 as it was descending through FL260.

At 13:58:27, the controller in sector RNN instructed RYR4UX to follow a rate of descent of -2000 fpm or higher, and immediately afterward instructed AZA060 to descend to FL140 and follow a rate of descent of -2000 fpm. Both crews readback correctly. One second later, the aircraft were separated by 1.2 NM horizontally and 1700 ft vertically.

At 13:59:19, the controller in sector RNN instructed AZA060 to maintain a high speed, but its crew did not readback. As a result, the controller repeated the instruction eight seconds later, again receiving no reply. The images obtained from Palestra show that this aircraft started to increase its rate of descent above the authorized -2000 fpm, and at 13:59:27, its rate of descent was -2400 fpm.

At 13:59:33, AZA060 requested that the instruction be repeated. At that time, its rate of descent was -2769 fpm and the aircraft were separated by 1.3 NM and 1900 ft. Two seconds later, the controller again instructed it to maintain a high speed. At that time, the rate of descent of AZA060 was -3313 fpm and of RYR4UX was -1969 fpm. Three seconds later, at 13:59:36, the STCA-PAC alert was activated.

At 13:59:39, the situation was no better: AZA060 was descending at -3825 fpm and RYR4UX at -1950 fpm, and the aircraft were 1.3 NM and 1400 ft apart.

At 13:59:55, the prescribed separation was lost as the aircraft were 1.3 NM and 800 ft apart, resulting in the STCA-VAC alert activation. The rate of descent of AZA060 was -4281 fpm (its cleared rate of descent was -2000 fpm), and the rate of descent of RYR4UX was -1613 fpm (it had been cleared to maintain a rate of -2000 fpm or higher).

The controller had instructed the higher aircraft to maintain a rate of descent of -2000 fpm, and to more than -2000 fpm to the lower aircraft. Neither aircraft observed these limits, which resulted in a gradual reduction of the vertical separation between them. Both instructions are deemed correct, but the controller’s monitoring of the crews to enforce them cannot be considered correct, and this oversight failure led to the loss of separation.

It is similarly deemed incorrect for the controller not to have issued a more forceful instruction (such as diverting one aircraft from its course or halting the descent of the higher aircraft) in order to limit the loss of separation and speed up the adequate separation recovery, in light of the little success that the instructions pertaining to the rates of descent were having. At 13:59:36, the STCA-PAC alert was activated. The controller did not intervene until 39 seconds later (at 14:00:15, before the point of closest approach was reached).

At 14:00:12, according to the radar data, the minimum separation between the aircraft – 1.4 NM and 300 ft – occurred as AZA060 was crossing FL188 at -3656 fpm and RYR4UX was crossing FL185 at -1638 fpm.

At 14:00:15, the controller reminded RYR4UX to follow a rate of descent of -2000 fpm or higher. The distance between the two aircraft was 1.5 NM and 300 ft, RYR4UX was descending at -1781 fpm and AZA060 at -3331 fpm.

The controller in sector RNN later instructed AZA060 to maintain FL180 as the aircraft was nearing this altitude, and so its crew requested confirmation. The controller again instructed the crew to maintain FL180, reiterating that they were to maintain a rate of -2000 fpm and not higher. The crew stated that they were below said level, and they were instructed to descend to FL170.

Later, the controller instructed RYR4UX to maintain 250 kt and he reminded AZA060 to maintain FL170, since it had climbed to FL179.

At 14:01:22, the controller informed RYR4UX that it had been instructed to maintain a rate of descent of -2000 fpm or higher, to which the crew replied that its rate of descent was now -2500 fpm.

Finally, the controller in sector RNN readback the message from RYR4UX and instructed it to maintain 220 kt, and AZA060 to descend to FL140 and subsequently to 6000 ft.

Neither crew reported receiving any advisories in their respective TCAS, and neither crew filed a report detailing this loss of separation. They had been unaware of the airprox until they were asked by the CIAIAC to provide information on the incident.

Both aircraft continued their respective approaches and completed their flights uneventfully.

Relevant facts

The following facts are deemed as relevant and decisive in leading to the loss of separation between the two aircraft:

1) RYR4UX was instructed to follow a rate of descent of -2000 fpm or higher in order to establish a difference in vertical speed with respect to the other aircraft involved in the incident, but it did not follow this instruction.

2) By contrast, AZA060 was instructed to follow a rate of descent of -2000 fpm, which it failed to do for approximately 1 minute 20 seconds, on occasion exceeding this assigned rate by more than double.
Incident Facts

Date of incident
Sep 7, 2019

Classification
Incident

Airline
Alitalia

Flight number
AZ-60

Destination
Madrid, Spain

Aircraft Registration
EI-IKL

Aircraft Type
Airbus A320

ICAO Type Designator
A320

This article is published under license from Avherald.com. © of text by Avherald.com.
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