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Bank Angle Awareness

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Category: Loss of Control Loss of Control
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Loss of Control accidents and incidents can happen as a result of a particular form of flight crew ’loss of situational awareness’, namely a temporary loss of awareness of the aircraft bank angle.

There are three typical ways in which an unusual roll attitude can develop with delayed flight crew awareness:

  • When an aircraft is being flown manually and being controlled solely by external visual reference in conditions of poor flight visibility and/or visual discrimination, sometimes at night.
  • When a flight crew temporarily cease flight instrument scan to undertake other duties in the false belief that the AP is engaged and that their aircraft will therefore follow the flight trajectory defined by FMS/FD inputs.

In all cases, loss of awareness is possible because there is an absence of tactile sensory feedback. Any sensations that might be readily detected and identified are often masked by a simultaneous lowering of the aircraft nose - resulting in altitude loss or a reduced rate of climb.

However, in all cases, loss of bank angle awareness should be confined to just one pilot and the consequence of this should be promptly observed by the monitoring pilot (PNF) and corrected by Pilot Flying (PF). Nevertheless, reports of accidents and serious incidents, in which a loss of control occurs, show that even with three crew members on the flight deck, this scenario can still occur.

Examples of documented occurrences of this sort, where recovery is successful, are relatively scarce. However, two current trends are providing increasing evidence that there are many more incidents than are documented. These are a gradually increasing habit of self reporting flight crew errors under 'just culture' regimes and the increasing prevalence of routine Flight Data Monitoring (FDM) programmes.

The ideal source of data is a combination of comprehensive flight crew reports/interviews and the flight data record to go with it. Relevant parameters which should be monitored for the particular risk being considered here are bank angle, rate of roll, flight control inputs, and angle of attack. The altitude of occurrence is also of interest in relation to the recovery opportunity. If, for whatever reason, flight crew initial reports are not received promptly, then it becomes even more important that the capture and identification of the incident flight data is both reliable and prompt. Flight crew recollections of all the circumstances are less easily recalled as time passes.

In terms of accident prevention, knowledge of a potential risk scenario provides the reason to re-visit crew procedures and training. Unfortunately, whilst all Operators can act on evidence from others (“learning from other peoples mistakes”), it is likely that data on this type of incident found as a result of FDM analysis will not be sufficiently serious to qualify as a ‘Serious Incident' under the ICAO definition, and will not be widely or openly shared.

Finally, there is one partial technical safety net which has become quite widespread. This is the simple device of a voice call-out to alert pilots to abnormal bank angle. This comes as an option with various forms of TAWS as an inclusion in GPWS Mode 6. As a result of TAWS becoming ICAO-mandated equipment, most multi crew aircraft operated have the ‘enhanced Mode 6’ option of ‘Bank Angle Alert’. If linked with other GPWS Modes this may come with the usual cut off above 2500 feet agl (and below 30 feet agl). The Alert call is activated at a bank angle which increases with height in this range - at 10 degrees bank angle at 30 feet agl increasing linearly with increasing height to trigger at 40 degrees at 150 feet and then remaining at 40 degrees at greater heights. There is also a version of TAWS equipment available which provides bank angle alerting at heights above 2500 ft agl without limit and aircraft fitted with this have an important additional protection for the cruise where many documented losses of control en route have begun.

Accidents and Incidents

The following events involved lack of awareness of aircraft bank angle:

  • B737, Adam Air, HF LOC, Sulawesi Indonesia, 2007 - gives an account of the loss of an Adam Air Boeing 737-400 after both pilots became distracted from awareness of the AP status (not engaged) during the cruise whilst the bank angle gradually reached an extreme from which they could not effect a recovery.
  • B767, LOC, New York USA, 2000 - which gives an account of the transient loss of control of a Delta Airlines Boeing 767-300 during a night initial climb being made solely by visual reference without timely intervention by either of the other two pilots on the flight deck during which bank angle reached nearly 66 degrees.
  • B747, vicinity Stansted UK, 1999 - which gives an account of how control of a Boeing 747-200 Freighter was lost when the pilot followed the erroneous indications of a single ADI without effective intervention from either of the other two crew members.

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Further Reading