ALTITUDE BUST

DEFINATION

Eurocontrol definition of an altitude bust is “any unauthorised vertical deviation of more than 300 ft from an ATC clearance”. In RVSM airspace this is reduced to 200 ft.

It occurs when an aircraft fails to fly the level it has been cleared to. It can lead to a loss a separation from terrain, manmade objects and other aircraft.

The result of altitude bust can result in the following hazards:

collision with another aircraft due to loss of separation
collision with ground or manmade object (CFIT)
injuries to passengers and cabin crew due to abrupt avoidance manoeuvres by the flight crew.

TYPES OF ALTITIDE BUST


Aircraft climbs/ descends through cleared level (picture (picture embedded from Skybrary on 09 October 2010)	Aircraft levels off at correct altitude but with the wrong altimeter setting (picture embedded from Skybrary on 09 October 2010)	Aircraft in level flight climbs/ descends witout clearance (picture embedded from Skybrary on 09 October 2010)

The types of altitude deviation are:

The controller assigns an incorrect altitude or reassigns a flight level after the flight crew have been cleared to an altitude.

Breakdown the pilot-controller communication:
controller transmits an incorrect altitude that is not read back by the flight crew. The absence of read back is then not challenged by the controller- the flight crew reads back an incorrect altitude which is not challenged and read back by the controller
the flight crew accepts an altitude clearance meant for another aircraft (call sign confusion)

The flight crew accepts, understands and reads back the cleared altitude but set the wrong altitude due to :
distractions/ interruptions
failure by the other pilot to cross check the set altitude
expectation bias – the flight crew were expecting another altitude which was set
confusion of numbers- the flight crew confused the altitude instruction with the heading instruction given.

The autopilot fails to capture the selected altitude. This normally happens because of the late selection of the autopilot coupled with the high rate of climb.

The tunnelling effect where the flight crew fails to respond to altitude alert aural and visual warning when hand flying.

The flight crew conducts an incorrect go around procedure.

Low transition altitude , with low/high QNH

Technical fault of the aircraft which fails to alert the flight crew of an altitude bust.

The misunderstanding of altitude clearances due to the speed and use of non standard phraseology.

Complex and long instructions with multiple clearances issued on the same transmission.

Factor that contribute to altitude bust:

distraction/ interruption during the transmission and selection of altitude
high workload –ATCO and flight crew
poor design of airspace procedures- misunderstanding of the information presented
holding patterns
high traffic volume
rate of climb/descend which are high

EXAMPLE OF ALTITUDE BUST:


Saudi 763, Embedded fromAirline Safety.Net on 09 October 2010	Saudi 762, Embedded fromAirline Safety.Net on 09 October 2010

This accident happened on the 11th of November, 1996. An IL76 bound for Delhi, level at FL 150, was advised that a Saudi Arabian B747 would be crossing them at FL 140. The radio operator acknowledged the transmission. The co pilot only heard the last part of the transmission regarding the altitude of the Saudi B747 and misinterpreted it as a clearance to descent to FL 140. They then initiated a descent to FL 140. The radio operator onboard the IL 76, realising the mistake urged the flight crew to climb back to FL 150. A climb was initiated but was too late leading to a mid air collision.
The cause of the accident resulted from poor communication coordination in the flight deck leading to an altitude bust and mid air collision.

MITIGATING FACTORS:

MITIGATING FACTOR:

adherence to SOP and flight deck discipline
the use of standard RT phraseology and RT discipline
reduction in similar sounding callsigns
better design of SID/STARS and chart design
training to focus on the importance of crosschecking a, read back hear back procedures
the use of implementation, training and usage of TCAS.