Question to the Ministry of Defence:

To ask the Secretary of State for Defence, how his Department calculates fatigue index for Typhoon aircraft; and at what accumulation of fatigue index are aircraft rendered unserviceable.

The Fatigue Index (FI) for the Royal Air Force's Typhoon aircraft is calculated using strain gauges fixed to 16 common locations on every airframe. It is correlated against a static test airframe to which loads have been applied to represent expected design usage. The total amount of fatigue damage applied to the test airframe is classed as 100 FI.

The values of fatigue for each of the monitored locations on the airframe are calculated by the aircraft software using proven and validated algorithms for each flight and expressed as a percentage of 100FI. A cumulative record is kept for each aircraft and flight. Individual aircraft will become unserviceable as they approach the safe-life (earliest FI value at which failure is predicted) proven by test for each location. Aircraft serviceability is continually monitored to ensure the capability of the fleet.