Direct and indirect attitude indicator

What does “indirect” attitude indication mean?

direct and indirect attitude indicator

“Direct” attitude indicator displays a moving horizon while the aircraft is fixed (as shown in the left parts of both pictures).
“Indirect” (or “outside-in”) attitude indicator displays a moving aircraft while the ground is fixed (as shown on the right parts of both pictures).

What is safer to use: “direct” or “indirect” attitude indicator?

In the 1970s – 90s the Soviet Union conducted large-scale study [PDF, 100+ pages in Russian] what flight image civil and military pilots have, as well as people who never piloted a plane.
In different groups tested persons defined their flight image the following way:

Tested group

Aircraft fixed, horizon tilts

  • • Pilots who use “indirect” attitude indicator: 35%
  • • Non-pilots: 70%
  • • Pilots who use “direct” attitude indicator: 49%
  • • Present-day civil pilots: 65%

Aircraft tilts, horizon fixed

  • • Pilots who use “indirect” attitude indicator: 65%
  • • Non-pilots: 30%
  • • Pilots who use “direct” attitude indicator^ 51%
  • • Present-day civil pilots: 35%

The results of the test on defining unusual attitudes and recovering from them were the following:

Mistakes when defining spatial orientation

using “direct” attitude indicator

  • • Roll depiction: 19%
  • • Pitch depiction: 15%
  • • Roll defining: 13%
  • • Pitch defining: 9%
  • • Roll-out: 16%
  • • Pitch recovery: 9%

using “indirect” attitude indicator

  • • Roll depiction: 0%
  • • Pitch depiction: 0%
  • • Roll defining: 3%
  • • Pitch defining: 2%
  • • Roll-out: 3%
  • • Pitch recovery: 2%

The overall conclusion of the study stated that from the point of view of safety it is advisable to use “indirect” attitude indicator for maneuverable aircrafts.

The studies conducted in the USA also demonstrated the superiority of using “indirect” attitude indicator when recovering an aircraft from unusual attitudes. According to the studies this superiority is based on the physiological patterns of human brain (see Previc. The neuropsychology 3-D space, American Psychological Bulletin 124, 123-164. 1998).

William R. Ercoline, D.Eng., a famous American research-pilot, performed analysis of the US Air Force aircrafts crashes during 1971 – 2000 related to spatial disorientation. The study stated that 82 US Air Force pilots crashed during last 15 years, which is 20% of all A-class accidents. The cost of the crashed aircrafts was equal to 1,9 billion dollars. In Russia during 1989 – 2008 10 aircrafts crashed due to spatial disorientation when using “direct” attitude indicator. The loss of aircrafts equaled to 3 transport helicopters and 7 civil airplanes.
No plane crashes due to spatial disorientation when using “indirect” indication were registered in the USSR and Russia for the entire period of operation.

Is “indirect” indication good for me?

Yes! “Indirect” indication is natural for human perception. When training using “indirect” indication 100% of students – Air Force pilots cope with instrument flying, and when using a “direct” indication 35% of the students are unable to cope with it at all.

I always use “direct” indication. Is it safe to switch to “indirect” one?

Yes! The results of the abovementioned research revealed that the pilots who changed spatial orientation from “direct” to “indirect” showed no errors in aircraft roll and pitch depicting and defining.

Is it legal to use “indirect” attitude indicator?

Yes! In the US certification documentation in 2007 a standard was introduced which allows installing an “indirect” attitude indicator and using it for aircraft recovering from roll and pitch limits and positions out of limits (Advisory Circular AС No:25-11A, adopted by U.S. Department of Transportation Federal Aviation Administration on 21.06.2007). This standard is preserved in the renewed documentation and valid up to date (Advisory Circular AС No:25-11B dated 07.10.2014).

Get the comfort of “indirect” indication with Glance EFIS device by Glance Avionics!

Glance EFIS device is a full-featured flight, navigation, and engine monitoring instruments system for experimental, UL and LSA General Aviation aircrafts and helicopters.
Glance EFIS has a unique option – you can switch from “direct” to “indirect” style of attitude indicator!
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