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I went flying yesterday with a friend in my 1972 Grumman AA-5. She's a good copilot for someone who doesn't know much about flying. I showed her how you can pitch up and down, cause 0G, and float, which is fun. I did that when she was stretching in an awkward position to surprise her, but she took revenge and did it quite hard and abruptly while in cruise around 105 MPH without turbulence, which made us hit our heads on the canopy.

We probably didn't have our seatbelts tight enough. But could this have structurally damaged the plane? Should I tell my mechanic? It behaved normally afterward.

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    $\begingroup$ Welcome to aviation.SE! I think the more important question is, did you notice anything wrong with your head? If not, then it's very unlikely there was enough force to damage the aircraft. You could discuss it with your mechanic just in case, of course. $\endgroup$
    – Pondlife
    Commented Jun 3, 2020 at 2:47
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    $\begingroup$ We've all let a non-pilot passenger have a little go at the controls, but honestly if someone did this to me, pilot or not, it would be the last time they got in an aircraft with me. $\endgroup$
    – Jamiec ♦
    Commented Jun 3, 2020 at 8:28
  • $\begingroup$ I don't know @Jamiec, if the pilot is playing tricks why would the passenger not have a go? I let passengers handle the controls often, but I mandate gentle inputs, I doubt that was the case here. $\endgroup$
    – GdD
    Commented Jun 3, 2020 at 9:01
  • $\begingroup$ Thanks a lot for your replies guys! My head was fine afterwards haha, I definitely told her (in a civilized manner) not to put abrupt inputs on the controls anymore, even though I should have made it clear before. I feel confident that the aircraft is fine, but I'll look around for any damage next time, I'll also ask my mechanic at the airport if he can have a quick look at it if I see him! $\endgroup$
    – Charles D.
    Commented Jun 3, 2020 at 16:06

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Now you know what a terrible idea it is to do "fool around" stuff with a passenger.

The AA-5's maneuvering speed is 122 MPH so you can't overstress it at 105; it will stall first (and you'd have likely snap rolled it) so you should be fine.

When in doubt, the rule of thumb is, when it's metal, if it's not permanently bent, it's good to go. The metal structure is a big spring (elastic deformation under load and returns to its original shape below the "yield" threshold) and if you don't go past its elastic limit into the plastic range (the yield limit - permanent set to a new shape after bending) it just springs back and Bob's your uncle. A single application of a near overload to a metal structure has no significant effect fatigue wise if you stayed in the elastic range.

If you think you might have over stressed it past the metal's yield point so there is a permanent set, there will be telltale wrinkles in the skins of the wings or tail that weren't there before. If you know what to look for, I wouldn't call for a mechanic, but if not sure what you're looking at, you should have an expert take a look.

But, like I said, a pull or push on the AA-5 at 105 mph can't overstress the airframe in the first place, so if you were really going that speed, I wouldn't worry about it. I would just resolve to be more careful about passenger briefings and "horseplay".

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  • $\begingroup$ I would think that stress fractures wouldn't show up as a bend, yet wouldn't be considered "good to go". $\endgroup$
    – FreeMan
    Commented Jun 3, 2020 at 14:53
  • $\begingroup$ Stress fractures are from exceeding the yield limit. To get stress cracks without exceeding the yield limit, you need repeated cycles. Many many cycles. There was a CRJ900 that had an engine bird ingestion incident on takeoff in Scandinavia that made the entire back end of the airplane vibrate so bad that the the Flight Attendant strapped in at the back had arm and leg injuries from the several inches of amplitude up and down flinging her up and down like a rag doll. One of the pilots quit the business.The airplane got a structural inspection; nothing was bent (or cracked) so back in service. $\endgroup$
    – John K
    Commented Jun 3, 2020 at 18:11
  • $\begingroup$ I agree that you won't bend the metal spar on the AA-5 -- it's a single cylinder that runs through the fuselage and wing. The only issue might be the wing skins which are bonded composite. Wing flex puts shear loads along the bonding areas which can cause delamination of the skin, especially along the trailing edge. It's a common issue and is something that should be checked in every annual inspection according to the IA that did the inspections on the AA-5 that belonged to a club where I was a member. $\endgroup$
    – Gerry
    Commented Jun 3, 2020 at 21:46
  • $\begingroup$ I think you mean bonded aluminum. Yeah the bonds have very low peel strength as well which required rivets to be added to skin corners under an AD I believe. The Tiger was an amazing airplane in its day, but personally, I wouldn't touch an aluminum bonded aircraft, although transport airplanes use bonded skin laminations in fuselages sometimes. Riveted airplane pretty much last forever if you don't park them outside. $\endgroup$
    – John K
    Commented Jun 4, 2020 at 0:11
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What is the Va (Maneuvering Speed) of your aircraft at that weight? I would not think that an abrupt deflection of one set of control surfaces would cause structural damage at less than 100 knots. Especially if it were not a full deflection. A trainer type aircraft like that should be able to withstand up to 1.52 negative Gs. Maybe an A&P on here can give more insight. But, when in doubt, whip it out (that credit card for your A&P bill).

Here is a fun and very useful/informative little exercise. Get to a cruise altitude of at least 3000 feet AGL or higher. Advance the throttle to full and establish yourself in Vy (or even better, a Vx) climb. Once you are fully established in the climb, retard the throttle back to idle and count to 3 seconds before doing or changing anything. You will be surprised how hard you will have to nose the aircraft over to prevent a stall and establish Vg. You may even experience negative Gs similar to the ones you experienced earlier. If you don’t have spin training, only do this with a current and proficient CFI.

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  • $\begingroup$ Sounds like a strong lack of angle-of-attack stability? Would, say, a Cessna 172 flown with two people in the front seat give this result? $\endgroup$
    – quiet flyer
    Commented Jun 3, 2020 at 10:44
  • $\begingroup$ @quietflyer - I am not quite sure what your question is. Could you restate it? If you are asking if you could experience negative G-loading in a Cessna 172, the answer is yes. The exercise I described simulates an engine loss on takeoff including a startle response. Unlike a power-on or a power-off stall, you do not have the option of using thrust from engine power to recover. Airspeed bleeds off quickly. And, your only option is to pitch down aggressively to avoid a stall. $\endgroup$
    – Dean F.
    Commented Jun 3, 2020 at 13:17
  • $\begingroup$ @quietflyer Less likely.Grumman singles characteristically have lighter control forces in all axes than similar Cessnas and Pipers. It was part of their appeal when they came out, besides speed for the power.There is a still a strong positive stick force gradient when pulling G, but it's a bit shallower than something like a 172, and a 20lb yank on the column could be expected to make a harder vertical acceleration in the Grumman than the Cessna. Also a new pilot might interpret something less than 2Gs applied suddenly as 3 or 4. Less than a G negative will result in a sharp rap on the head. $\endgroup$
    – John K
    Commented Jun 3, 2020 at 13:50
  • $\begingroup$ +1 for "when in doubt, contact your A&P" not some (very knowledgeable) internet strangers. $\endgroup$
    – FreeMan
    Commented Jun 3, 2020 at 14:55

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