An articulated (HUD) pitch ladder display
Why current commercial aircraft pitch displays may need to change
Because primary flight (attitude reference) displays have traditionally had a display scale ratio of one to one these displays typically only provide a pitch readout (or ladder) that is limited to the vertical field of view. This can result in the display moving rapidly and making it difficult to use in unusual attitude recoveries.
Now in the military aircraft world of the 1980s the USAF had become concerned about the potential loss of aircraft due to disorientation during unusual attitude recoveries and identified the above as one causal factor. To address this, the USAF’s Wright Patterson Labs conducted a series of studies and trials on the benefits of various enhanced attitude displays in reducing pilot disorientation (1).
This post is part of the Airbus aircraft family and system safety thread.
A fixed (ARI) pitch ladder display
One concept studied was for an ‘articulated’ pitch ladder where the pitch bars were ‘funneled’ by the display towards the horizon with the lines becoming more acute as the aircrafts pitch increased.
The results of these studies (Ward 1990) showed that an articulated pitch ladder displayed in the aircrafts Head Up Display (HUD) improved air crews situational awareness, particularly in distinguishing between nose high and nose low attitudes (2).
While articulated pitch displays have subsequently become fairly standard in the military aircraft environment they have not found their way into the civil transport environment. The most likely reason being that the pitch ladder display evolved out of the HUD technology fitted to fighter aircraft, whereas in the commercial environment the ‘digitized’ cockpit tended simply to emulate traditional analog panel mounted instruments with some minor augmentation, such as ground up/down arrows.
However crew disorientation has been identified as a contributing factor in a number of civil aviation accidents over the years. For example the NTSB’s report on the 1974 Thieles crash of a NW Airlines 727 commented that the unusal nose down and banked attitude of the aircraft would have nearly hidden the horizon line in the attitude reference instrument and that this undoubtedly contributed to crew confusion (NTSB 1976 p18).
Now while I would hesitate to argue that an articulated pitch ladder display would have ‘saved the day’ in the case of Thieles or other unreliable air speed related accidents, I would argue that if aircrew for passenger jets are expected to recover their aircraft from an unusual attitude while dealing with unreliable air speed then available attitude cues should be optimised to support such a crew task.
Of course the ‘cultural’ problem is that airlines and aircraft manufacturers have a safety policy of, and spend a tremendous amount of energy on, preventing aircraft and aircrew from getting into these sort of situations. As a consequence pilot flight displays are optimised to what is needed to fly within the normal envelope, because (a priori) the assumption is that’s the only place they’ll fly (3).
Admitting that unusual attitude cueing is necessary would obviously be perceived as a criticisim of this policy. 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. 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 an essential feature of any fault tolerant system is to have just that sort of ‘never give up strategy’. I believe a static pitch display (even with sky pointers) during unusual attitude recovery does not qualify as such.
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 rept, Jun 89-Jul 90.
NTSB, Aircraft Accident Report, NW Airlines, New York, B727-25, N264, December 1974, PB- 245 581, Washington 1976.
