Friday, October 16, 2009

Road Bumps

A while ago -- a long while ago, I just found some old notes -- a fellow pilot out at Oakland in his Cessna Skyhawk had a closer-than-comfortable encounter with an MD-80 that was transporting federal prisoners.

We were e-mailing about the juicy non-blogged details, and then I said, "Can you imagine if there had been a collision and they had to evacuate it on the runway? It would have been like a scene out of The Fugitive!" (I love that movie. I probably rewatch it a couple times a year). But then a moment's thought made me realize, "Mind you, running over you probably wouldn't have damaged it at all."

He produced this speculative cockpit conversation aboard the MD-80:

"What was that noise?"

"I dunno... probably just the new embedded hold short lights. Oh look, there's that weirdo Piaggio parked next to Execjet again! Now where the hell did that 172 in the runup area go?"

Ouch. He continues to be vigilant so that scenario does not come to pass.

My aircraft is only big enough that I have to worry about where my prop wash goes more for courtesy than endangering smaller aircraft, but I suppose there are airplanes small enough that I could run one over in the dark. I think I'd notice, though. I know right away as I add power if some helpful FBO employee has chocked a wheel without me noticing.

9 comments:

Angus said...

My pet peeve re chocks is when they're left lying out in a dark unlit area for the unwary pilot to taxi over. We call them ramp grenades.

Aviatrix said...

What I think is funny is that every company or FBO develops its own culture as to which is the correct wheel to chock. You can have people tell you you're doing it wrong because you chocked the left main, and it's supposed to be the nosewheel. And you can have someone so blind to the possibility that anything other than the nosewheel could be chocked, that they won't see the one on the left main. And there are overkill people who want to chock all three.

Jeremy said...

Wouldn't it make most sense to chock a main wheel, since it has much more weight on it than the nose gear? Based on my rusty high school physics, it would seem that rolling over the chock would require the chock to be able to exert significant upward force on the gear. This would be effectively a torque around the axis defined by the two other wheels that remain on the ground. If the chock is on the nose gear, the plane would pitch around the axis of the main gear wheels - an axis along which the plane is relatively balanced, and is designed to pitch along during take-off rotation. But the force required would be much more if applied at the main gear, where the downward force (weight) is greater, and the lever arm between the chock site and the axis of movement (a line between the other main gear and nose gear), is smaller. (A shorter lever arm means more force to get the same overall torque.)

I guess another consideration is the static friction between the chock and the tarmac - the horizontal force required before the chock (and therefore aircraft) slides along the ground, which then invalidates the above discussion. If the chock is roughly wedge shaped, I think the higher weight of the main gears would cause more downward force on the chock resulting in more static friction, but I'm not sure about this. Not to mention if the plane is parked on grass/dirt, the possibility of the chock sinking into the ground.

Overall I'd argue for the main gear, but it'd be interesting to know what official tests and more informed people think about this...

GPS_Direct said...

Ah, yes - those "helpful" FBO chocks. It only took me twice to learn that lesson... We'll, three if you count the time it dawned on my why my just departed pax was standing on the ramp calling my cell phone...

Haven't hit a "grenade" yet. Though I'd suspect its a bit like having an empty nitrogen-filled strut suddenly drop to fully compressed on rollout... Surprise!

Verification word: Expoin - what you are doing when attend a trade-show.

Aviatrix said...

Jeremy, I won't question your physics. My choice (nosewheel) stems from the facts that I keep the chocks in the forward baggage compartment, that I don't have to climb under the nacelle to get to the nose gear, and that I may have to do up or undo the nose gear scissors anyway, so it's less effort.

On a high wing single it's easier to chock a main, because there's no propeller in the way and you don't have to crawl under the wing.

nec Timide said...

There is also the of the nose wheel caster. If you chock only one main it is possible for the plane to pivot around the chocked wheel. Especially with impolite sources of prop/jet/rotor wash.

SwL_Wildcat said...

Speaking of pivoting around a main wheel. Sorry if I posted this link before, but I thought it was a good example of everything you should not do. Passenger brief anyone?

Hamish said...

Ay! That MD80 still haunts the ramps at Oakland (as I discover almost every time I fly in the evenings). It's sort of become my nemesis and running in-joke… my pet peeve isn't so much with chocks as with tie-down chains that are just slightly too short to reach to the wing tiedown points on a 172 or 182. Argh! I once went to a certain unnamed airport on the California Central Coast where they were even too short for an Arrow. Humph.

Aviatrix said...

I love the word "nemesis."