The Tumble Mode - Where Test Pilots Fear To Tread

Guy Gratton, Chief Technical Officer and Test Pilot, British Microlight Aircraft Association, UK
Simon Newman, Senior Lecturer, Aerospace Engineering, University of Southampton, UK

Abstract

Following a fatal accident in 1997, and identification of common patterns in several (usually fatal) previous accidents the AAIB (United Kingdom Air Accidents Investigations Branch) asked the BMAA (British Microlight Aircraft Association) to pursue a course of investigation into the tumble mode, which had been attributed as the primary cause of that fatal accident. Through an ongoing collaboration, the University of also Southampton became involved in this investigation.

The tumble mode is a peculiarity of weightshift controlled aircraft – that is flexwing microlights and hang-gliders. It is a departure from controlled flight leading to a nose-down pitch autorotation: pitch rates of 400°/s are known. When a tumble occurs in a microlight aeroplane, it is rare for the crew to survive and loss of the aircraft is universal.

Prior to work starting in the UK to try and eliminate the tumble, one type had suffered 7 fatal tumble accidents in a service history of less than 200,000 hours, and the total number of fatal tumble accidents was somewhat greater.