Indigo A320 near Mumbai on Mar 21st 2022, uncleared descent causes loss of separation

Last Update: October 6, 2023 / 16:25:22 GMT/Zulu time

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Incident Facts

Date of incident
Mar 21, 2022

Classification
Report

Airline
Indigo Airlines

Flight number
6E-6261

Departure
Delhi, India

Destination
Mumbai, India

Aircraft Registration
VT-IAY

Aircraft Type
Airbus A320

ICAO Type Designator
A320

An Indigo Airbus A320-200, registration VT-IAY performing flight 6E-6261 from Delhi to Mumbai (India), was nearing Mumbai at FL380 and had just contacted Mumbai Radar, when the crew requested to descend. ATC cleared them to descend to FL370, the crew read back they were cleared down to FL310, the controller missed the incorrect readback. The crew initiated the descent to FL310.

An Air Asia India Airbus A320-200, registration VT-HYD performing flight I5-773 from Delhi to Goa (India), was enroute at FL360, when a predictive conflict warning activated at the controller's desk. On checking the data the controller recognized 6E-6261 had selected FL310 as their target altitude and radioed the crew to stop the descent at FL370, however, there was no response. The controller again tried to radio the crew, again no response, and a third call also remained without response.

In the meantime the co-pilot, who had left the cockpit of VT-IAY, returned to the cockpit and the captain handed control to him to leave the cockpit for a toilet break, upon the 4th call to stop the descent at FL370 the copilot responded they had already descended below FL370 and stopped the descent at FL360. The controller had already instructed I5-773 to descend to FL350, the Air Asia crew complied. The conflict warning ceased, and both aircraft were cleared for further descent and landed safely at their destinations.

India's AAIB released their final report concluding the probable causes of the occurrence rated a serious incident were:

The breach of separation occurred due to non-adherence of SOP on the part of flight crew of IGO6261, wherein they were not maintaining listening watch when the area controller transmitted multiple times to maintain FL370 after PCW warning was generated.

Contributory Factors

- Readback error made by the flight crew of IGO6261 to the cleared descent level of FL370 given by the controller.

- Controller not correcting the readback error made by the flight crew of IGO6261 and confirming the cleared descent level as FL310 instead of FL370.

- Crew of IGO6261 were busy in handing over/taking over controls as PIC immediately left the cockpit after co-pilot re-entered the cockpit, which probably led to crew not maintaining listening watch at critical situation.

- Loss of situational awareness on the part of controller who kept on calling IGO6261 which was not responding instead of shifting focus and instructing IAD773 to avoid potential conflict as PCW warning has already generated with aircraft IGO6261 descending at a faster rate.

- High Rate of Descent of IGO6261 which was higher than the prescribed limit for descending to FL370 due to the readback/hearback error.

The AAIB analysed:

The aircraft IGO6261 was maintaining FL380, when it came in contact with Mumbai Area Control. Before top of descent the co-pilot who was performing the duties of PM left the cockpit to use LAV. During this time PIC was communicating with ATC. PIC requested for descent for which the controller gave descent to FL370, however, the PIC made a readback error and readback the descent clearance as “Descend level 310 IFLY6261 confirm” to which the ATC controller also confirmed as “IFLY6261 Affirm”. There was no aircraft at that time under the jurisdiction of area controller with similar call sign and no aircraft was given descent clearance to FL310 which could have resulted in confusion for descent clearance or read back/hear back error. The readback error made by the PIC could be because of confirmation bias on the part of PIC, as she thought that the descent clearance would have been more as she did not observe any traffic below. This also indicates that PIC was probably not monitoring the navigation display properly, else the aircraft IAD773 which was maintaining FL360 could have been identified by the PIC on the NAV display. The PIC then set the descent level as FL310 & accordingly set the rate of descent (ROD) as 2624 fpm (feet per minute) on FMS and the aircraft started descending at a faster rate. The ROD went up to a maximum value of 2648 fpm which was way above the maximum ceiling of 1500 fpm as per the SOP. The controller then gave heading instructions to IGO6261 and informed about the wrong altitude (310) selected by the PIC instead of 370 after Predicted Conflict Warning (PCW) was generated in the automation system. However, there was no response from the flight crew of IGO6261 to this transmission. Thereafter, the controller gave another transmission to IGO6261 to maintain FL370, but there was still no response from the aircraft IGO6261. This implies that the crew were not maintaining the listening watch to the transmission made by the controller. This probably happened when the co-pilot re-entered the cockpit and immediately thereafter, the PIC handed over the controls to co-pilot and left the cockpit to use LAV. Meanwhile, the controller gave another call to IGO6261 which was acknowledged by the co-pilot. The controller asked IGO6261 to maintain FL370, however, the co-pilot informed that the aircraft (IGO6261) has already crossed FL370 and are at FL360 now. The aircraft IGO6261 thereafter levelled off and was maintaining FL360. Had the crew of IGO6261 maintained the listening watch to the transmission given by the controller after the read back error (by the crew) and hear back error (by the controller) the breach of separation, i.e., the incident could have been avoided. The rate of descent set by the crew of IGO 6261 was also way higher than the maximum allowed considering the cleared level of 370. This further aggravated the situation as the aircraft started descending at a faster rate which could have been arrested in time if the crew had maintained listening watch to the transmission given by the controller after readback/hearback error.

After the incident, no FSR was raised by the crew as no TCAS was generated during the incident and also crew were not informed by the controller about the breach of separation.

ATC Area Controller

The controller gave IAD773 which was maintaining FL360, direct routing to MBATA (waypoint) in order to facilitate IGO6261’s descent which was bound for Mumbai. On the request made by the crew of IGO 6261, the controller gave descent to FL370, however, the crew of IGO6261 made a readback error and readback the cleared level as FL310. The controller did not correct the readback error made by the crew and confirmed the read back error made by crew to descent to FL310 instead of FL370. The hear back error made by the controller could be because there was an intermediate call by another aircraft (AIC774) after the controller gave the descent clearance to IGO6261 and the readback by the crew of IGO6261 to the descent transmission given by the controller. There was no aircraft at that time under the jurisdiction of involved area controller with similar call sign and no aircraft was given descent clearance to FL310 which could have resulted in confusion for descent clearance or read back/hear back error. However, when the controller was giving heading instructions to IGO6261 during which PCW warning was generated, he immediately checked the data block of IGO6261 and observed that the pilot selected altitude was FL310 instead of FL370. The controller continued the transmission to IGO6261 and informed the same to the crew by transmitting “IFLY6261 Mumbai now turn right fly heading 217 and maintain flight level 370 on reaching sir and check your selected altitude 370 was the level cleared”. The controller intended to give heading 210 instead he gave 217 inadvertently. However, there was no response from the flight crew of IGO6261 to this transmission. Thereafter, the controller gave another transmission to IGO6261 to maintain FL370 but there was still no response from the aircraft IGO6261. Meanwhile, the controller gave another call to IGO6261 which was acknowledged by the co-pilot. The controller asked IGO6261 to maintain FL370, however, the co-pilot informed that the aircraft (IGO6261) has already crossed FL370 and are at FL360 now. Thereafter, CCW warning was generated. To resolve the conflict, the controller then gave transmission to Air Asia aircraft IAD773 which responded immediately and instructed it “IAD773 turn left fly heading 120” which was immediately readback by the aircraft. The controller then gave descent to IAD773 as “IAD773 Mumbai descend to flight level 350”. As per the instruction, the aircraft IAD773 descended to FL350. By this time the CCW warning went off and the aircraft were clear of conflict when IGO6261 was maintaining FL360 and IAD773 was descending passing FL351 with heading 120. However, the controller did not inform the aircraft IGO6261 about the level breach done by it after it responded to his transmission and also thereafter. No TCAS was generated on either of the aircraft, hence, no FSR was raised by the crew of both the aircraft.

Hearback error made by the controller contributed to the occurrence with the fact that it further confirmed the readback error made by the crew of IGO6261 to descent to FL310 instead of FL370. However, after the hear back error, when the PCW warning was generated, the controller continued to transmit to IGO6261 to maintain FL370 when there was no response from IGO6261 by the time the aircraft IGO6261 responded the CCW warning was generated and the separation was breached. To resolve the potential conflict, the controller could have instructed IAD773 instead of calling IGO6261 repeatedly when there was no response from them aircraft as PCW warning was already generated. Further, the controller did not inform the aircraft IGO6261 about the level breach, which resulted in non-reporting of event/raising a FSR by the crew of IGO6261/IAD773 as no TCAS was generated. This further resulted in non-removal of CVR for the purpose of investigation.