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I was just reading through the POH of a DHC-6-100 Twin Otter and was surprised to read the following:

3.8.2 LOW PRESSURE - OTHER SYSTEMS. The airframe deicing system and the autopilot are operated by bleed pressure. In the event of a low pressure condition, a caution light on the caution lights panel will illuminate. Under these conditions, the pressure may be restored as outlined in paragraph 3.8.1. However, if increasing engine speed does not rectify the condition, the use of the autopilot and de-icer system should be discontinued.

(emphasis mine)

I understand why the deicing system is using bleed air, but why is the autopilot powered by bleed air? How does this autopilot work?

The following question is somewhat related:

but it never mentions anything powered by bleed air.

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The reason (back in the day) was an envisioned low-cost autopilot ownership in smaller planes,$^1$ but it was a troublesome system as it aged.

The first production [Twin Otter] aircraft left the Downsview factory with an option for the Honeywell H-14 autopilot. This system used large pneumatic servos to control the aircraft. This was a popular option at the time however as the aircraft and autopilot aged the maintenance involved in keeping the system serviceable resulted in many operators disabling or removing the system (rockymountainaircraft.ca).

It works via pneumatic servos; pistons that are actuated by pressurized air.

enter image description here
Source: NASA


$^1$: NASA contractor report (prepared by Honeywell) on "fluidic" flight control systems (also covers the aforementioned Honeywell H-14 autopilot).

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    $\begingroup$ in the 1960's and '70's, production line assembly tools were run off of compressed air and the switches and error detection devices were too. in fact, it was possible to build binary logic systems based on compressed air (called "air logic") which at the time was less expensive than electronic logic. But no longer! and it's just as well; anyone who designed an automatic assembly tool around air actuators and air logic switched over to servomotors, etc. after doing it with air ONCE. $\endgroup$ – niels nielsen Apr 5 at 0:59

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