It's very common to see pilots moving a light aircraft by pushing or pulling on the prop, either because there's no tow bar available or simply out of preference or convenience. Is this a reasonable thing to do or can it damage the prop or engine in some way?
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$\begingroup$ does someone want to do the math on how much force needs to be applied to move, say, a 3368 pound airplane (my MTOW) forwards or backwards? $\endgroup$– rbpDec 9, 2014 at 22:50
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2$\begingroup$ My first suggestion would be to read your POH to see what it says! The answer may vary by aircraft model as well (especially geared -vs- non-geared engines). $\endgroup$– LnafzigerDec 10, 2014 at 4:04
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8 Answers
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A couple of my CFIs have asked me to help them park the airplane (in absence of a tow bar) when I was pushing the airplane by putting one hand on the hub and the other on the propeller near the hub, and them (the CFIs) pushing the tail down and nose wheel elevated. But according to AOPA, it is not a good thing:
... never push or pull on the prop blades. Forget what anyone has told you about pulling near the hub or the strength of the propeller. Aluminum blades can bend, and it doesn’t take much to put the blades out of track with one another.
This PDF says the same thing:
Avoid pulling the airplane around by the prop. Yes, this seems the perfect solution to a vexing problem of how to change the airplane’s position without having to walk around and get the tow bar, but it’s worthwhile to make the extra effort. Neither the engine nor the prop particularly benefit from the loads imposed by horsing the whole airplane around.
The FAA says in AC 20-37E
Do not push or pull on propeller blades when moving the aircraft by hand. Tow bars are specifically designed for this operation.
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1$\begingroup$ At my old flight school (Hillsboro Aviation) they also taught me this. However, at Riddle the ramp is large enough that all airplanes can taxi without being towed at all. This is probably to minimize any potential problems towing - such as what you mentioned. $\endgroup$– KeeganDec 9, 2014 at 19:40
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25$\begingroup$ Isn't "horsing the whole airplane around" what the propeller does during powered taxiing? The blades may not like asymmetric loads from handling, but transferring the force from pulling on the axle is exactly what the engine and bearings are made to do, isn't it? $\endgroup$ Dec 9, 2014 at 19:53
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11$\begingroup$ @HenningMakholm They are made to pull the plane straight forward, but not to push it backward or to one side or the other. $\endgroup$– reirabDec 9, 2014 at 20:59
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3$\begingroup$ Are the days of moving a prop and having the engine kick over loooong gone? $\endgroup$– DJohnMDec 10, 2014 at 2:52
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6$\begingroup$ @HenningMakholm during powered movement it's the air pushing on the propeller blades, there's a maximum force that can exert, and it's fairly even on the whole propeller. I'll bet that pushing on a blade (especially on a tip) is more than the air could do. $\endgroup$– Xen2050Dec 10, 2014 at 10:24
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Classic example of one corner case leading to a blanket belief.
Grabbing a propeller by the tip, especially a wooden prop, can cause damage. In use, the tips generate most of the thrust but are also held straight by the rotational forces.
Pulling on a prop right beside the hub will cause no harm - that's where the propellor thrust goes anyway. If you grab the prop on both sides the airframe won't know that the engine isn't running.
So, here's a Rule of Thumb: if you cannot touch the prop hub with your thumb while pulling on the prop, you are holding it too far out.
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$\begingroup$ I would add that I usually only do this when I'm moving the plane straight forward or back. If I have to turn it, I get out the tow bar (although I admit to kicking the tire left or right to make small adjustments). $\endgroup$– rbpDec 11, 2014 at 13:56
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Well, grabbing the propeller at the blade root and pulling from there cannot hurt it. Why? Because the thrust imparted by the propeller to the aircraft is applied at exactly the same place. In other words, the propeller pulls the airplane through the air at the hub. You are so close to the hub that you're not going to create a significant bending moment along the blade.
On the other hand grabbing the propeller at the tips would be questionable at best, and ill-advised!
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I asked around at my gliding club about this, the consensus seems to be that it won't harm the plane as long as I didn't use excessive force.
The reason for doing it is to not have to walk from wing to wing to move the plane. The main counterpoint is that you shouldn't move a plane by yourself unless you're very sure you won't hit anything (another plane, the hangar walls, etc).
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5$\begingroup$ The main counterpoint for most propeller-driven singles I'm familiar with is "There's a towbar, it's designed to move the airplane, and you should use it!" :-) $\endgroup$– voretaq7Dec 9, 2014 at 19:50
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2$\begingroup$ That's true, but at my club all the planes are either taildraggers or private planes with wooden props. Personally, I always get another person (or two) and push/pull by the wing struts. $\endgroup$– KJPDec 9, 2014 at 19:53
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2$\begingroup$ Yeah, taildraggers are a slightly different beast, and I imagine wooden props are less prone to being pulled out of track than metal ones (?). They do make taildragger tow rigs, they look like a lot more effort but I'd try to use one in preference to manhandling the prop :) $\endgroup$– voretaq7Dec 9, 2014 at 19:56
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8$\begingroup$ I can't help but see irony in asking a gliding club about this. $\endgroup$– reirabNov 3, 2015 at 22:47
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$\begingroup$ We have our own hangar that's big enough for the tow planes, the club-owned fiberglass ships, and some members' private airplanes. $\endgroup$– KJPNov 4, 2015 at 15:40
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As always with these questions it seems to be a religious thing. If you ask 2 people you will have at least 3 different answers and most of them will sound somewhat reasonable. I had the situation that my FIs always taught and asked me to move the planes (taildraggers) with me pulling the plane on the prop and them steering using the rudder. A few days ago another guy walked up to me when I moved the plane shouting at me "NEVER do this, you'll destroy the prop" So I called the technical support of the specific airplane (Diamond Aircraft) and asked them.
TL;DR: Manufacturer says that's exactly the way they move the planes in production when they don't have the tow bar around. For reference: The plane has an electric constant speed prop so I guess about the most fragile you can find in terms of prop.
Of course you should always grab as close to the spinner as possible
One can verify plane specifically this from POH or AMM, for example Diamond DA 62 AMM states:
I know this thread is 4 years old but I could not find the questions answered anywhere else and the question comes up on a regular basis esp. in clubs where "knowledge" is passed on from member to member.
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2$\begingroup$ Thank you for the reference. I also looked it up in the DA20 manual and it even states you should rotate the plane towing on the prop: "If no tow-bar is used, during forward traversing the nose-wheel will follow the movement of the airplane. Change in direction is achieved by pulling on the propeller hub" (support.diamond-air.at/fileadmin/uploads/files/…, Section 8.4.1) $\endgroup$– FlorianOct 24, 2019 at 15:55
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By deviating from the standard procedure (using the tow bar) you incur into the following risks:
Mechanical damage: you risk damaging propeller itself if you don't pull from as close as possible to the hub. Furthermore, being more complex, variable pitch blades obviously imply more risk.
Arguably, the forces in play during taxiing are similar to those exerted manually through pulling. Also, in my flight school, during pre-flight inspection we actually perform some push/pulls to assess that the propeller, hub, and drive are ok -- suggesting that there is no mechanical stress derived from doing so.
Safety risks: although arguably rare, you may accidentally start the engine through hand-swinging. I was instructed in my flight school to shut down the engine through the mixture control mainly to prevent this from happening during the pre-flight inspection previously described. This is also why we perform magneto ground checks during shutdown.
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1$\begingroup$ Maybe on an 80HP J-3, but you ain't hand propping my 270 HP high-compression TIO-540cu engine $\endgroup$– rbpDec 11, 2014 at 13:59
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How do you taxi your plane? I use my engine to spin my propeller, which pulls on the hub and pulls my plane hard enough to make it roll on the ground. On takeoff, I do this so hard that the 2000+ lb plane rolls 60 knots or so before it leaves the ground.
Now, with that in mind, do you think pulling on the prop, close to the hub, enough to move the plane 10 feet is doing any damage?
answered Nov 3, 2015 at 21:39
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Regarding "it must be ok to pull the plane by the prop because the prop pulls the plane through the air"... Yes....through the air. The air is much less sticky than the ground. Also, the propeller doesn't pull the plane through the air, it pushes air behind the plane creating a lower pressure area in front of the plane and a high pressure area behind it. This "pulls" the plane into the low pressure area.
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6$\begingroup$ "Yes....through the air. The air is much less sticky than the ground." It also pulls the plane while moving on the ground (ie. taxiing). Also, the depiction of the pulling force as a result of a pressure difference is not very accurate. $\endgroup$– jnovoDec 11, 2014 at 15:05
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$\begingroup$ The cause of the pulling force is not significant - the fact is there is a pulling force applied to the blades which is then transferred to the hub and crankshaft, which is transferred to the engine mounts, then the airframe, etc. What is different is the lack of rigidity in the blades when they're not rotating at high RPM. That lack of rigidity means you could bend them (or their attach points) much more easily by pushing on them. Which is why it's important to apply any force as close to the hub as possible - but not on the spinner itself! $\endgroup$– hempJul 17, 2017 at 1:08