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Flight Instrument Presentation of Aircraft Attitude

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Whether part of an EFIS display or a mechanical instrument, an Attitude Indicator (AI), Attitude Director Indicator (ADI) or Artificial Horizon (AH) provides flight crew with essential information about aircraft attitude relative to the real horizon when the latter cannot be determined because of poor forward visibility or dark night conditions. As such, the correct interpretation of what the instrument shows at any given time is critical to maintaining control of an aircraft in the absence of a clearly discernible horizon. During basic instrument flying training, pilots quickly become familiar with the principles of such a display. However, this familiarity is often mainly focused on the appearance and interpretation of the display within or near the ‘normal flight envelope’. For civil transport pilots, the interpretation of such displays at more extreme attitudes is usually only addressed during periodic practice in recovery from ‘unusual attitudes’. Consequently their training rarely replicates the typical loss of control scenario where an unusual attitude develops slowly and unnoticed and has often become quite extreme by the time it is suddenly recognised and recovery is attempted. This is especially true now that such a large proportion of everyday flying is done with the autopilot engaged rather than the pilot manually manipulating the controls.

An artificial horizon instrument presents pitch and roll against a circularly-shaped, vertical, cross-sectional representation of earth and sky which is aligned perpendicular to the direction in which the aircraft nose is pointing. The display of both pitch and roll is thus relative to the artificial "horizon" which is defined as the line of transition between a blue ‘sky’ and a light brown ‘ground’. When an aircraft is flying in a horizontal attitude longitudinally and laterally, a symbol which represents the aircraft will be aligned along the horizon line with a semicircular blue ‘sky’ above the line and a semicircular light brown ‘ground’ below it.

The principle used in the instruments fitted to most aircraft - including all western-built ones - is that the horizon line on the instrument - the division of the displayed circle between the semicircle of blue ‘sky’ and the semicircle of brown ‘ground’ - follows the real horizon whilst the aircraft symbol, which is fixed to the instrument, stays horizontal. In respect of pitch, the fixed aircraft symbol will then effectively ascend into the blue sky if the aircraft nose is raised from the horizontal and will effectively descend into the light brown ground if the aircraft nose is lowered. In respect of bank, the horizon line will deviate from the horizontal relative to the instrument if the aircraft banks left or right whilst the aircraft symbol will remain horizontal relative to the instrument.

Superficially, the bank case may seem slightly odd since, as displayed, the horizon has moved rather than the aircraft. However, the relative movement is correct and the observed position of the horizon bar will be replicating the real horizon as it would be seen if it was visible ahead.

Exceptionally, some Russian-built aircraft have these instruments designed to function in a completely different way whilst still maintaining the correct relativity between the aircraft and the horizon. In this alternative type of display, the horizon line is fixed and aircraft symbol moves relative to it. Since correct relativity between aircraft symbol and the horizon line must be maintained, the angle between the horizon line and that symbol must still equal bank angle. So when the aircraft is banked, the horizon line will align with that bank angle and the aircraft symbol will move beyond it by the same amount. Whilst it might seem intuitively more reasonable to ‘fly’ the aircraft symbol, it requires that the horizon line stays fixed to the instrument i.e. that it remains horizontal as seen by the pilot. This doesn’t raise any issues unless the real horizon is discernible at which point, the real and artificial horizons will then not be aligned. It may even be considered an advantage to be able to see the instrument horizon line at all times since the indicated direction for recovery will always be obvious, whereas in the more common alternative it may move out of sight at extreme attitudes.

Artificial Horizon Formats

Although the psychology of the two displays differs fundamentally in the portrayal of bank angle, neither presentation has historically been shown or widely claimed to be superior to the other, provided that the pilots have received their basic instrument flying training and proceeded through a flying career on aircraft equipped with similar instrumentation. However effective aircraft control has sometimes been lost using both types because of confusion in attempting to suddenly interpret aircraft attitude from a depiction which is beyond the normally-experienced range. This is most often found in the commercial air transport context when an abnormal bank angle has developed slowly and without pilot awareness during instrument flying conditions. In general aviation, a similar confusion may arise when a VFR-rated pilot ends up attempting to maintain control of their aircraft after entering IMC; however, in this case, awareness of abnormal attitude is more likely to be progressive than sudden, since the loss of a previous visual horizon is likely to have been noted as a significant precursor to at least potential control difficulty.

A ‘special case’ of the commercial air transport scenario has occasionally been postulated during accident investigations where flight crew who have received their initial instrument flying training, and often considerable subsequent operational experience, on the Russian-type horizon display have then been rated on an aircraft type which has the ‘western’ type of horizon display, especially when they are relatively inexperienced using the ‘new’ presentation. The significance of the display transition is that if a pilot suddenly becomes aware of an abnormal roll attitude for which the response to initiate recovery needs to be ‘intuitive’ rather than ‘considered’, then any attempt to ‘fly’ the horizon line on a banked western-built aircraft back onto the (fixed) aircraft symbol will actually increase rather than reduce the bank angle.

The only defence against the erroneous reading of aircraft attitude from an artificial horizon display when dependent on such an instrument for sole reference, is adequate initial and recurrent training on recognition of and recovery from such situations. For this training to be effective for professional air transport pilots, who spend nearly all their time managing aircraft which are flying within a ‘normal’ attitude regime on autopilot, training must be carefully structured. The aim must be that the element of surprise at the sight of an artificial horizon which is displaying an unexpected abnormal attitude is correctly dealt with.

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