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Why does every kind of propeller-powered plane lack an auto-throttle?

No single-props no twin-props, not even turboprops like the Dash-8 Q400. To be honest, I don't know many turboprops, but if the Dash-8 doesn't have one, why shouldn't others? I mean, even a mini Cherokee 180 has an autopilot, whats so hard implementing an auto-throttle? And the Dash-8 is an airliner... without auto-throttle!

Thanks for your answers, I would really appreciate your answers.

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  • $\begingroup$ Ask yourself how expensive an autothrottle is vs how much safety/convenience benefit there is and I think you'll answer your own question. There are many airliners without autothrottle that have flown safely for decades, and autothrottles have directly and indirectly contributed to accidents as well. $\endgroup$ – nexus_2006 Jun 8 '16 at 14:54
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    $\begingroup$ Turboprops like Dash-8's are flown by "entry-level" pilots who are looking to move up to the bigger jets. Probably part of the reason is the same logic behind not handing your teenager the keys to your Ferrari, they need to learn hands-on with something not-so-complicated. Also, usually turboprops are short-haul regional carriers that don't really need the hands-off that the bigger jets do. $\endgroup$ – Ron Beyer Jun 8 '16 at 15:09
  • $\begingroup$ @nexus_2006 may you post some links to the accidents caused my autothrottle? Would be interested. Thanks $\endgroup$ – Fabrizio Mazzoni Jun 8 '16 at 15:24
  • $\begingroup$ God, I would be happy just to get fuel injection. $\endgroup$ – Tyler Durden Jun 8 '16 at 19:25
  • $\begingroup$ The words "auto" and "flying" are not good words to use together. $\endgroup$ – Tyler Durden Jun 8 '16 at 19:25
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Because each jet engine only has one lever; each piston engine has three.

Jet engine's throttle is pretty straight forward - move forward, more power. The computer automatically manages fuel flow to prevent surge, flameout and N1 overshoot.

On a piston engine you have three - a mixture lever, a propeller lever and a throttle lever. Granted, I don't think auto-mixture would be too hard to implement. Now, do I want a high-speed climb or high-power climb? And remember, the prop and throttle must be managed together. You cannot go beyond a certain power range with the props set for low rpm, otherwise you'll damage the engine.

It's like a car's automatic transmission, except it has one more dimension.

Power affects pitch

When you change power you must change the aircraft's pitch altogether. Autopilots on modern airlines have quite a number of vertical modes: ALT, V/S, G/S, VNAV, FLCH, TO/GA, CLB etc. None of those have the concept of gear ratio, which is required for propeller management.

Can you design an autopilot system such that it functions on propeller engines? Absolutely. But it is going to be complicated. Not only the usual operations, but the failure modes, have to be engineered and learnt by the pilots. How do you deal with an auto-mixture failure? RPM mismatch? Such training would add unnecessary cost to an otherwise simple aircraft operation. If your airplane does not have a variable pitch propeller, it is probably too simplistic for this kind of autopilot anyway.

Jets have another property which makes auto throttle a logical choice - the faster you fly, the more powerful the engines. Thus, on a jet, you would have to make continuous throttle adjustment if you change your airspeed. So an auto throttle would be helpful. Pistons do not behave like that.

Turboprops

One of the main reasons for choosing piston aircraft is - it is simple. It just fly. No fancy stuff. No lengthy preflight checks. No computers. I just want to get flying. This thinking also extends to turboprops.

It's worthy to note that the ATR does not have auto throttle as well. The ATR 42 has power management (but not speed management), where activating it will simply move a bug on the torque gauge; the pilots have to manually move the levers to follow it. The ATR 72 has a notch for the pilots to place the throttle levers, but still, no motor to move the levers automatically. I can't think of any reason behind this design philosophy besides simplicity and cost.

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  • $\begingroup$ It's also a major safety factor on jets at commercial cruise altitude where coffin corner is small. Leaving it to the pilot would be problematic on long cruises. $\endgroup$ – Simon Jun 8 '16 at 20:12
  • $\begingroup$ Some aircraft diesels have FADEC and a single power lever, so in theory wouldn't autothrottle be as simple for them as for a jet? $\endgroup$ – StephenS Oct 4 '19 at 22:51
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Autothrottles are a wonderful piece of automation to have, but they are expensive to install and maintain, and the technology behind them is complicated and can be easily misunderstood by a pilot who isn't fully up-to-speed with the automation and "what's it doing now."

An autopilot is pretty easy to grasp, conceptually. Turn on altitude hold, it does. Turn on heading hold, it does. Couple the autopilot to a VOR or GPS course, and it holds that. There's no energy management, no interactions between the left/right and the airspeed. Navigating vertically changes that, because you have a lot of coupling between the vertical modes (i.e. things driving the elevator) and the throttle modes (i.e. things driving the throttle). This translates into more complicated software, more controls, and more required pilot understanding of what's going on in the FMC and autopilot.

Beyond this, when you have an autothrottle, you need to make sure that it will disengage fully when you tell it to, that it can be overridden with pilot input, and that it won't drive the engine into an out-of-limits condition (RPM, Manifold Pressure, Torque, EGT, etc). Jet engines are generally pretty simple in this regard (particularly when they're already protected with a FADEC) compared to turboprop or (especially) reciprocating engines. Do you really want your autothrottle trying to lean out the engine in the climb?

The end result of all of this is the the economics tend not to be worth it to put autothrottles in the various prop-driven aircraft.

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  • $\begingroup$ Are turboprops really that much more complex than a jet? $\endgroup$ – fooot Jun 8 '16 at 19:22

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