Methodology for quantifying flying qualities following flight function failures

The structure of Flying Qualities Levels provides the framework for analysing and quantifying the effects in the event of a flight system failure. Failures can be described under three headings – loss, malfunction or degradation – as described below:

(a) Loss of function: for example, when a control becomes locked at a particular value or some default status, hence where the control surface does not respond at all to a control input;

(b) Malfunction: for example, when the control surface does not move consistently with the input, as in a hard-over, slow-over or oscillatory movement;

(c) Degradation offunction: in this case the function is still operating but with degraded performance, e. g., low-voltage power supply or reduced hydraulic pressure.

The first stage in a flying qualities degradation assessment involves drawing up a failure hazard analysis table, whereby every possible control function (e. g., pitch through longitudinal cyclic, yaw through tail rotor collective, trim switch) is examined for the effects of loss, malfunction and degradation. This assessment is normally con­ducted by an experienced team of engineers and pilots to establish the failure effect as minor, major, hazardous or catastrophic. Table 8.4 summarizes the definitions of these hazard categories in terms of the effects of the failure and the associated allowable maximum probability of occurrence per flight hour (Ref. 8.40). The table refers to the system safety requirements for civil aircraft.

In the military standard ADS-33, the approach taken is defined in the following steps (Ref. 8.3) (author’s italics for emphasis):

(a) tabulate all rotorcraft failure states (loss, malfunction, degradation),

(b) determine the degree of handling qualities degradation associated with the transient for each rotorcraft failure state,

(c) determine the degree of handling qualities degradation associated with the subsequent steady rotorcraft failure state,

Table 8.4 Failure classification

Failure severity

Maximum probability of occurrence per flight hour

Failure condition effect


Extremely improbable < 10-9

All failure conditions that prevent continued safe flight and landing.


Extremely remote <10-7

Large reductions in safety margins or functional capabilities.

Higher workload or physical distress such that the crew could not be relied upon to perform tasks accurately or completely.

Adverse effect upon occupants.


Remote <10-5

Significant reductions on safety margins or functional capabilities.

Significant increases in crew workload or in conditions impairing crew efficiency.

Some discomfort to occupants.


Probable < 10-3

Slight reduction in safety margins. Slight increases in workload. Some inconvenience to occupants.

(d) calculate the probability of encountering each identified rotorcraft failure state per flight hour,

(e) compute the total probabilities of encountering Level 2 and Level 3 flying qualities in the Operational and Service Flight Envelopes. This total is the sum of the rate of each failure only if the failures are statistically independent.

Degradation in the handling qualities level, due to a failure, is permitted only if the

probability of encountering the degraded level is sufficiently small. These probabilities shall be less than the values shown in Table 8.5. The probabilities used in ADS-33 are based on the fixed-wing requirements in Ref. 8.41, but converted from the probability per flight to the probability per flight hour, with the premise that a typical fixed – wing mission lasts 4 h. The requirements are not nearly as demanding as the civil requirements of Table 8.4 where the probabilities are typically two orders of magnitude lower.

In contrast, the UK Defence Standard (Ref. 8.42) defines safety criteria for failures of automatic flight control systems (AFCS) according to Table 8.6. The effect is defined

Table 8.5 Levels for rotorcraft failure states

Probability of encountering failure

Within operational flight envelope

Within service flight envelope

Level 2 after failure

<2.5 x 10-3 per flight hour

Level 3 after failure

<2.5 x 10-5 per flight hour

Loss of control

<2.5 x 10-7 per flight hour

<2.5 x 10-3 per flight hour

Table 8.6 AFCS failure criteria (Def Stan 00970, Ref. 8.42)

effect on FQ -> minor




probability of failure <10-2




having ‘effect’ within intervention

time per flying hour

within the so-called intervention time, which is a function of the pilot attentive state. With the pilot flying attentive hands-on, for example, the intervention time is 3 s, but in passive hands-on mode, the time increases to 5 s.

In the following sections, examples are given of failures in the three categories along with results from supporting research.

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