How a invention that flew on the SR-71 could help commercial aviation today
In a previous post on unusual attitude I talked about the use of pitch ladders as a means of providing greater attensity to aircraft attitude as well as a better indication of what the aircraft is dong, having entered into it. There are, of course, still disadvantages to this because such data in a commercial aircraft is usually presented ‘eyes down’, and in high stress, high workload situations it can be difficult to maintain an instrument scan pattern. There is however an alternative, and one that has a number of allied advantages.
What I’m talking about is a neat little device called the Peripheral Vision Horizon Display (PVHD) which was invented by Dr. Richard Malcolm who gave it it’s alternate name, the Malcom Horizon. Dr Malcom was concerned with the number of military aircraft losses due to aircrew failing to recognise they’d entered into an unusual attitude flight regime and subsequently recover from it.
The PVHD works like this. A low level light beam is projected onto the instrument panel of the cockpit, as the aircraft rolls and pitches attitude data is fed to the PVHD which rotates and raises/lowers the PVHD beam. The beauty of the system is that the pilot or pilots set the display so that it just disappears when looked at with the central part of their vision. The display is still visible to the peripheral vision (which is very good at detecting motion changes) thereby providing what is in effect a subliminal cue to the crew of attitude changes. A very, very neat concept.
The system has been flown on a number of platforms, most notably the SR-71 which had one fitted to assist pilot’s in maintaining situational awareness during air to air refuelling manoeuvres. The SR-71 version also included a heading indicator by varying the intensity of the horizon line (1). The system has also been fitted to the A10 and F4 aircraft and flight tested extensively, receiving positive aircrew responses.
The advantages of such a system are two-fold. First it provides an additional redundant back channel of information to the crew as to the aircraft’s attitude, without additional cognitives workload. Second it’s subliminal affect, can also assist in circumstances where the pilot’s other senses are tricked by somatic illusions, which can be extremely strong. Both these factors can be important in dealing with unusual attitude recovery. The first because one way air crew can get themselves into such situations is when they’re presented with high workload/high stress events with all the attentional channeling and loss of situational awareness that such events can bring. Having a source that you can pay attention to, without having to consciously pay attention to it is clearly invaluable in such situations. The second is that having entered into an unusual attitude somatic illusions can catch even the most experienced pilot’s so a subliminal source that is whispering ‘no, this is the real attitude’ improves the chances of an aircrew or pilot recovering successfully (2).
I discovered the Malcom’s PVHD when I was working on the problem of how to (cheaply) demonstrate the independence of flight data given that the flight data was coming through a common redundant processing architecture. The PVHD was a great ‘end run’ around the problem as we could deploy it as an independent display which could take it’s data straight from the EGI via the aircraft’s data bus (3). The other advantage it had was that we could install it without the ensuing, and somewhat horrendous prospect, of putting another display in the main instrument panel. Finally when considered as purely an adjunct to the traditional meatball style attitude indicators it did not require a huge number of certification hoops to jump through.
As commercial aviation has suffered a series of accidents over the years that involved aircraft entering into unusual attitudes, perhaps it would be worthwhile looking at the PVHD. Especially as it would require neither major hardware changes to current flight instruments, nor increase pilot workload. I mean, just looking at it couldn’t hurt, surely? Who knows we might actually save some lives.
1. Remembering that when you flew the SR-71 you had to wear a pressure suit helmet and that visibility out the window was somewhat restricted you can see the obvious advantages of this style of display.
2. Training on unusual attitude has definitely improved over the years, but the harsh reality is that a simulator cannot simulate the onset of such somatic illusions. So what do we do about them?
3. The idea would have been to then implement the whole device in hardware and firmware for diversity of design. Not that I don’t love software, but…
NASA Conference Publication 2306, Peripheral Vision Horizon Display (PVHD), (Mar. 15–16, 1983).