Reacting to the failure

There are a number of factors that will affect the time it takes a pilot to react to a failure. These include the characteristics of any warning system installed, the reaction of the aircraft itself, the arousal state of the pilot and the proximity of his or her hands to the relevant control. When testing the effect of failures, the establishment of realistic pilot intervention times is crucial and most assessing authorities have developed a set of rules regarding the way in which these times are constructed. The following definitions are taken from Ministry of Defence Standard 00-970 [7.2] which divides the intervention time into rotorcraft response time and pilot response time.

7.6.3.1 Rotorcraft response time

The rotorcraft response time covers the time between the failure occurring and the pilot becoming aware of it. This time will depend on the characteristics of the rotorcraft, on the nature of the failure and in particular on the characteristics of any associated warning. For example, in the case of an audio tone indicating engine failure the response time is equal to the time it takes this to activate. If there is no warning then the pilot may only become alerted to the failure by a change in the flight path. This is usually the case for runaways of the AFCS where the rotorcraft response time is determined by the time it takes for the aircraft to achieve an angular rate of 3 degrees per second about any axis or a change in acceleration of 0.2g along any axis [7.2]. The rate is increased to 5 degrees per second and the acceleration to 0.25g for passive flight phases where the pilot is less involved in controlling the aircraft. Although the pilot may be alerted by other changes, in noise or vibration levels for example, they are usually harder to define and are not often used as the basis for specification compliance. In situations where the failure cues are not easily defined it is often necessary to conduct no notice failures during simulated mission tasks to determine the rotorcraft response time.

7.6.3.2 Pilot response time

The pilot response time is defined as the time between the pilot being made aware that there has been a failure and recovery action being initiated via the controls. The pilot response time is further divided into the decision time and the reaction time. The decision time varies depending on the level of involvement in the flying task while the reaction time is dependent purely on whether or not the pilot’s hands are on or off the flight controls. Table 7.1 shows the pilot response time criteria taken from 00-970 [7.2].

The flight phases associated with the decision time have precise definitions:

• Active flight. The pilot is using the flight controls continuously to maintain or change the flight path of the aircraft.

• Attentive flight. The pilot has to pay particular attention to flight control for short periods. This may involve making occasional adjustments to the flight path using the flight controls, for example, during instrument flight. Alternatively it may involve monitoring the actions of the flight control system closely, such as during an automatic approach to the hover.

• Passive flight. This covers long periods of flight during which the pilot need only give a minimal amount of attention to controlling the flight path or monitoring the AFCS; for example, during instrument flight with autopilot holds engaged.

The attentive and passive phases of flight may then be further divided into hands-on and hands-off depending on whether or not the pilot would normally be grasping the flight controls. Deciding which flight phase should be used when conducting failure testing is clearly very important, as it will directly affect the decision time and therefore the entire intervention time. From the point of view of the manufacturer it is preferable to have the specification compliance testing conducted against the shortest possible decision time, as the aircraft is more likely to be satisfactory. For the test pilot, however, the only consideration is to ensure that the decision time which is used accurately reflects the way that operational pilots would fly the aircraft. Thus if he knows from his operational experience that pilots normally engage the radar altimeter height hold when flying low level over the sea, leaving the flight controls unattended for substantial periods, then clearly the passive hands-off criterion should be used.

Once the appropriate flight phase has been chosen, testing is conducted to determine if the aircraft can be recovered safely at the end of the total intervention time.