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I suspect most pilots have done it at least once: briefly experience zero g when flying a parabolic path. It's quite an experience (if your stomach can handle it).

Question is: are there any risks involved in doing something like that? (I know, getting in an airplane by itself is a risk, but that's not the point)

I could think of a few potential risks, but I'm not sure if they are real or not:

  • Engine lubrication in single engine piston airplanes

And if not properly executed:

  • Risk of a stall, both in the pull up phase and in the "arc" phase
  • Overstressing the airplane when pulling out of the dive to get up to speed

Follow-up questions: Is it considered to be an acrobatic maneuver? Are there any regulations that would prohibit it?

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    $\begingroup$ Fuel starvation also comes to mind.... Also, the aerobatic question is answered here: aviation.stackexchange.com/questions/307/… $\endgroup$ – Lnafziger Jan 21 '14 at 22:53
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    $\begingroup$ any mechanism that is gravity powered or relies on gravity to work properly will not work anymore, one of those is bubbles in hydraulics because the reservoir has some air in it and that may get sucked up during the 0G time $\endgroup$ – ratchet freak Jan 22 '14 at 10:54
  • $\begingroup$ Why does the engine lubrication argument specifically applies to single engine piston airplanes? Isn't gravity based lubrication used on multi engine airplanes / turbine engines? $\endgroup$ – DeltaLima Jan 22 '14 at 15:04
  • $\begingroup$ @DeltaLima I don't know, hence the question :-) $\endgroup$ – Philippe Leybaert Jan 22 '14 at 15:36
  • $\begingroup$ @delta lima: You are correct that the lubrication argument does NOT apply exclusively to single engine aircraft. Acrobatic aircraft have inverted fuel and lubrication systems. This allows the engines to run inverted. Typically larger aircraft (twins, etc) are not built for acro flight and therefore the engines don't have these systems. I presume the same is true for the turbine engines, but I have no experience with those. $\endgroup$ – Skip Miller Jan 22 '14 at 17:55
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The biggest risk is improper execution. If you build up too much airspeed and pull out of the dive at 4-G and break something, that will be your primary issue.

Your secondary risks will be associated with the fuel and oil system starvation if they depend on gravity for operation. This risk is balanced by how long you can sustain 0-g flight. Oil issues are bigger risk than fuel, as even if you did sustain 0-g long enough to starve the engine, you should get it back as soon as you are in positive G. This particular risk will vary by airplane and I probably can't say something as general as "not typically a problem for short duration 0-g excursions".

An additional secondary risk is unsecured objects in the cockpit. This presents a number of possible complications:

  • floating objects may distract you from aviating
  • floating objects may break something (the example of a window in the cockpit would cause a major distraction for the remainder of flight)
  • Most importantly, this can change your CG which can have dire consequences if something heavy enough gets moved far enough.

I wouldn't put risk of stall too high on the list, because you should be doing this with enough altitude and in coordinated flight so a stall would just be a simple recovery.

As for whether this is an aerobatic maneuver, it is a lot like a departure stall but a botched recovery (pushing over to 0-g before the stall breaks) and only a passenger could say for sure what you were actually doing.

That leads into the last risk I can think of, and that is when your passenger deposits his/her lunch all over the airplane. In 0-g that could get on the windscreen and obscure your vision. Be careful!

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    $\begingroup$ Floating/flying objects could also be a hazard. (A flight instructor friend of mine told me about a window that was broken out this way.) $\endgroup$ – Lnafziger Jan 22 '14 at 16:20
  • $\begingroup$ @Lnafziger good point, I'll add that to the answer. $\endgroup$ – casey Jan 22 '14 at 16:45
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    $\begingroup$ @Lnafziger not to forget dirt etc. Sometimes that makes me sneeze... oh, and another danger would be an improperly secured crew (both to to others and themselves). $\endgroup$ – yankeekilo Jan 22 '14 at 17:32
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I did quite a number of zero gravity manoeuvres on a Cessna Citation II, not as a pilot but as an experiment coordinator.

The engine lubrication was gravity based on that aircraft, and zero-g caused a Master Caution / oil pressure warning. Since the zero gravity manoeuvres lasted only for about 12 seconds, this wasn't a problem according to the engine manufacturer.

Entry and recovery caused extra g-forces on the airframe, I remember we reached 2.5g during pull up. Especially the recovery out of the parabolic flight was tricky, balancing between duration of the zero-g segment, g-force during pull up and overspeed. Sometimes spoilers were deployed at the end of the parabolic section to keep the speed down.

There isn't much risk of stall, since the higher wing loading occurs at quite a high speed, keeping the required lift coefficient low. The low speed segment on top of the parabola is a zero g, so no stalling there.

Another serious risk is getting hit by things flying around the cockpit and cabin when negative G is accidentally achieved. Flight manuals crashing into your head at 2.5g during the pull up are no fun.

Negative G's are also a risk for the fuel system when it is purely gravity based. If air bubbles get trapped inside the fuel line it can cause engine stall. I think this is less of a problem in jet / turbine engines which seem to have fuel pumps in any case.

On the Citation II negative G s caused a spray of fuel from the vents in the wing tip; spectacular but harmless.

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    $\begingroup$ Even airplanes with fuel pumps can have their fuel pick up port uncovered during negative g flight and air can get into the system that way. $\endgroup$ – Lnafziger Jan 22 '14 at 16:42

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