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I saw the following photo of a Honeywell Aviation Services Boeing 757-225:

Honeywell Aviation Services Boeing 757-225

If it's not a fake, the airplane has a propeller mounted on a pylon on one side.

What's the purpose of such strange design? Why does it need a propeller engine, if it already has two jet engines? Or is it a permanently deployed RAT?

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    $\begingroup$ The plane doesn't need the propeller engine, the propeller engine needs the plane to bring it into the appropriate test conditions. $\endgroup$
    – DeltaLima ♦
    Commented Sep 11, 2018 at 15:32
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    $\begingroup$ The livery should be a bit of a hint here. Honeywell is a major manufacturer of avionics and also engines and components. $\endgroup$
    – Jan Hudec
    Commented Sep 11, 2018 at 17:17
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    $\begingroup$ @JanHudec never mind the company name, the fact that it says “Flight Test” was the clue. $\endgroup$
    – Notts90
    Commented Sep 12, 2018 at 14:49
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    $\begingroup$ Now that would be a really big RAT... $\endgroup$
    – reirab
    Commented Sep 12, 2018 at 20:18
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    $\begingroup$ That is one cluttered livery. It shows off Honeywell Wi-Fi, wheels and brakes, Wi-Fi, a new avionics suite, Wi-Fi, their APU, and did I mention Wi-Fi? $\endgroup$
    – user71659
    Commented Sep 13, 2018 at 17:10

3 Answers 3

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From the description below the image you linked:

The Honeywell, Boeing 757 test bed appeared on static display for a few days before departing half way through the event.

A testbed aircraft is an aircraft used for flight research or testing new equipment such as engines.

You can see other photos without engine or with a jet engine instead of a propeller one.

The engine is not needed to fly, but is being tested, and apparently that position has been deemed a good solution to the problem "where do we attach an extra engine we need to do tests on?".

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The airplane doesn't "need" that engine.

It is a test platform for testing the engine during flight.

Obviously, you don't want to rely on the engine you're testing, in case something goes wrong! So the plane flies with 2 large turbofan engines, while the engineers are testing the smaller engine mounted up front.

If you look closely, you can even see the airplane is labeled "Flight Test".

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Testing a new engine design can only go so far on the ground. At some point, it has to be tested in conditions representative of actual use - in other words, in flight. You don't want to be dependent on an untested engine to get you airborne and back on the ground safely. The solution to this problem is to mount the test article on an aircraft that is entirely airworthy regardless of the operational state of the engine under test. That way the test engine can be put through its paces without compromising the safety of the test crew.

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  • $\begingroup$ I guess this is the reason why the test bed is fairly massive B757, a smaller airplane would be in much bigger trouble in case the tested engine misbehaves, like goes in reverse. $\endgroup$
    – Tero Lahtinen
    Commented Sep 13, 2018 at 11:42

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