Pitch ladders and unusual attitude recovery

05/07/2011 — 1 Comment

F/A-18 HUD Display with articulated pitch ladder (Image Source: Wiki Comons)Why current commercial aircraft displays may need to change in the wake of AF447

This post is part of the Airbus aircraft family and system safety thread.

During the 1980s the United States Air Force (USAF) had become concerned about the loss of aircraft due to pilot disorientation during unusual attitude recoveries. Studying the problem USAF researchers started looking at how attitude data was provided to aircrew, traditionally primary flight displays in aircraft have a display scale ratio of one to one, due to the limitations of analog instruments, and only a fixed ratio pitch readout constrained to the vertical field of view.

As it turned out USAF researchers found that the fixed ratio display design might actually be a significant cause of crew disorientation during an unusual attitude recovery because of their rapid and disorienting movement. To address this problem, the USAF then started exploring various enhanced attitude display concepts to see if they could reduce pilot disorientation (1). One of these concepts was for what was called an articulated pitch ladder in the Head Up Display or HUD, in which the pitch bars were funnelled by the display towards the horizon so that the bars angles became more acute as the aircraft’s pitch increased. Experiments showed that an articulated display improved air crews situational awareness, particularly in distinguishing between nose high and nose low attitudes (Ward 1990) (2). As a result articulated pitch ladders were subsequently and rapidly adopted by the USAF and the wider military fast jet community.

But while pitch ladders have become standard in military fast jets they have not become so in civil air transportation, where digital displays have emulated traditional analog instruments (3).

…the stall warning began again and continued while the First Officer commented, ‘There’s that Mach buffet, I guess we’ll have to pull it up.’

Extract from N264 CVR transcript (NTSB 1976 p6)

Yet even in the civilian transport sector crew disorientation has been a contributing factor in a number of accidents. See for example the NTSB’s report on the 1974 Thieles crash, in which the unusual nose down and banked attitude of the aircraft obscured the horizon line in the ARI thereby delaying crew recognition of their stalled condition (NTSB 1976 p18) or more recently Air France AF447 crash where the crew did not identify that the aircraft was stalled at all (4).

“…Do you understand what’s happening or not? …We’re losing control of the airplane here.”

Extract from AF447 CVR transcript (BEA Report)

Now while an articulated pitch ladder wouldn’t necessarily have ‘saved the day’ in either of these accidents, if civilian aircrew are expected to recover their aircraft from an unusual attitude whilst simultaneously dealing with a disorienting environment then perhaps all available cues should be optimised to support the crew.

Unfortunately the problem is that regulators, airlines and aircraft manufacturers share a safety trope that the best way to prevent accidents is by reducing their likelihood rather than mitigating the consequences. Having reduced the likelihood of unusual attitude, so the  rationale goes, we don’t need to expend additional resources in preparing for them (5). Of course such a conclusion is based on the assumption that the risk assessments that underpin them can be relied upon.

In the case of recovering from an unusual attitude the industry response has been to prevent aircraft and aircrew from getting into these sort of situations in the first place through training and flight control systems, and in the case of Airbus implemented as their flight protection software laws. But an unfortunate consequence of this avoidance strategy is that displays in civil aviation are designed for flight within the normal envelope, because as the safety myth goes, that’s the only place they’ll need to be used (6).

Yet another trope within the aviation industry is that a safe design equates to dealing with single points of failure which results in an implicit assumption that one only has to deal with one hazard at a time, which is fine if these hazards are truly independent. But unreliable airspeed is also a reason that an aircrew may fly the aircraft into an unusual attitude, so clearly the ‘one hazard at a time’ is not a valid assumption to make in this case (7).

Admitting that better unusual attitude cueing is necessary and that if there is unreliable airspeed event it is likely to lead to an unusual attitude event would obviously be perceived as a criticisim of these shared safety myths. As to whether the industry has sufficient maturity to deal with that, time will tell I guess.

Notes

1. An articulated pitch ladder funnels the pitch lines toward the horizon as chevrons, so that in a nose high attitude the pitch ladder chevrons point in the down direction and similarly as the nose goes below the horizon the chevrons point up. As the pitch increases the slope of the chevrons becomes more severe providing a strong visual indication of increasing pitch.

2. The full conclusions of the study were that a partially (below the horizon line only) articulated pitch ladder combined with a full width horizon line and retention of existing pitch numbers was the solution that provided the most improvement and aircrew satisfaction.

3. For example the inclusion of digital ground up/down arrows.

4. Which introduces the issue of how closely allied unusual attitudes are to a loss of situational awareness. One can expand the traditional definitions of unusual attitudes to define it as anything that a pilot can’t immediately recognise. That is, whenever there’s a loss of correspondence between what the aircraft is doing and what the pilot perceives. This de-correlation may be due to a cognitive mismatch where the pilot can’t explain what’s going on, or may be physiological, e.g. due to spatial disorientation caused by vestibular confusion. From this perspective we probably shouldn’t be surprised then that unreliable airspeed events which are inherently confusing have a causal relationship with subsequent crew induced physical upsets of the aircraft.

5. The following incidents involve both a unusual attitude and unreliable airspeed as a causal factor:

  • Northwest Airlines B727, Thieles crash 1974,
  • Turkish Airlines B737, Ceyhan crash, 1999 (suspected),
  • Midwest Airlines B717, Union Start incident 2005,
  • Air France A330, Atlantic crash, 2009.

6. To put it another way, the traditional meatball style attitude reference stops providing useful cues because the aircraft is being flown outside the intended envelope, but this is not perceived as a problem because you ‘shouldn’t’ be flying there. The counter argument is that an essential feature of any fault tolerant system is to have just that sort of never give up strategy in the face of out of specification but not unexpected conditions. A static pitch display (even with sky pointers) during unusual attitude recovery does not qualify as such because of the reasons noted above.

7. For example current training regimes train for unreliable airspeed and unusual attitude events as separate training exercises. Similarly high altitude unreliable airspeed training requires explicit crew intervention and flight control inputs which may actually initiate departure from the flight envelope if improperly executed.

Reference

Ward, G.F., Effects of Head-Up Display (HUD) Pitch Ladder Articulation, Pitch Number Location and Horizon Line Length on Unusual Attitude Recoveries for the F-16, Final report, June 1989 to July 1990.

NTSB, Aircraft Accident Report, NW Airlines, New York, B727-25, N264, December 1974, PB- 245 581, Washington 1976.

One response to Pitch ladders and unusual attitude recovery

  1. 
    David Vogel 28/02/2013 at 6:17 pm

    Hi Matthew, Good argument. Another factor that might be usefully considered are the “coding differences” inherent in the graphic examples given. It might be argued that the colour presentation on contemporary EFIS &/or electronic PFDs supersedes the need for the additional visual cues (such as articulated pitch ladders) that are desirable in a monochrome HUD. Or that such visual cues are (highly) desirable for monochrome attitude displays (including conventional electromechanical AHs), although not absolutely necessary for colour PFDs with sky/ground colour coding, however …. for consistency in display symbology, such visual differentiation of up/down coding (such as articulated pitch ladders below the horizon, and straight above) might then be beneficially carried across to be incorporated in colour PFDs. David.

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