In any sector, incidents and accidents rarely have simple explanations as to their cause, with many factors combining to bring them about. Investigator Andrew Blackie describes one aviation incident that led to loss of control and a rapid rolling descent.
On the 31st December 2013, a 69-year-old pilot, his passenger and three dogs boarded their private jet, a Cessna 525A CitationJet CJ2+, and set off for southern Europe from Leeds-Bradford airport in the north of England. Twenty minutes later, as the aircraft approached 43,000 feet over Daventry, the aircraft departed from controlled flight by rolling to the right through 360 degrees five times in ten seconds. Over the next two minutes, while the pilot struggled to regain control, the aircraft lurched from steep climbs to precipitous descents, at times pointing vertically down and descending at over 20,000 feet per minute.
The pilot regained control and the aircraft made a safe landing but not before the wing structure had been stressed beyond its operating limit resulting in significant damage to the aircraft.
Investigating these events is dependent on access to a wide range of information. Key to accessing these sources is a combination of trust and supporting legislation. The Air Accidents Investigation Branch (AAIB), the UK government aviation safety investigation body, operates as the no-blame part of the UK’s aviation system. Enabled by secondary legislation and decades of detailed technical investigation, the AAIB holds a position of trust within the UK aviation industry.
For any investigation, finding out what happened is the easy bit and is usually determined quickly. However, reflecting on the state of investigation reporting globally, it is not uncommon for the complexity and interactions of the socio-technical system to be lost.
With access to some recorded data and an initial account from the pilot, the investigation of this incident quickly came up with a simple statement of what happened: the aircraft was damaged because, during a loss of control, it exceeded its structural limits. Control was lost because the aircraft decelerated to below its safe airspeed apparently without the pilot being aware.
The investigation revealed three key areas and removing any of these would have likely prevented the accident:
- The aircraft was flying too slowly.
- The pilot was unaware of the critically slow speed.
- The aircraft was designed such that it departed from controlled flight when operated too slowly.
The investigation looked at the operating technique used by the pilot. In this case, the pilot had previously noted that his aircraft hunted (wobbled) slightly in pitch while climbing in ‘Flight Level Change’ mode - an autopilot mode which keeps a fixed airspeed. This hunting, while not desirable, is not unheard of in various combinations of aircraft and autopilot. It can be uncomfortable for passengers and because of this, the pilot had decided to operate the autopilot in a different mode called ‘Vertical Speed’ mode. This mode sets a fixed vertical rate and allows the airspeed to vary. Critically there is no limit to how much the airspeed can vary and as an aircraft climbs, the changing atmosphere reduces its performance. Eventually either the rate of climb must be reduced or the aircraft airspeed will decrease. ‘Vertical Speed’ mode does not protect against the autopilot demanding more performance than the aircraft can provide. Pilot vigilance and intervention is required to avoid hazardously low airspeed and for this reason many airlines prohibit their crews from using this mode while climbing.
To stay around his target of 140 knots, the Cessna pilot had made periodic interventions to select a reduced rate of climb. Fifty seconds before the loss of control the airspeed was 128 knots (already below target by 12 knots) and the pilot reduced the climb rate to 500 feet per minute. Based on his experience, he believed the reduced rate would allow the aircraft to reach his planned height without slowing more.
This is an extract of an article published in the Nov/Dec issue of The Ergonomist. To read more on further contributing factors - aircraft design, safety net failure and attentional capture - please subscribe to The Ergonomist.
Andrew Blackie was an Air Accidents Investigation Branch (AAIB) Operations Inspector from 2007 to 2017. He is now an independent consultant at Abris Consulting Ltd.