767-300ER strut compression on landing

dasleben

dasleben

That's just, like, your opinion, man
I know there are some other 767 guys hanging around here who might be able to answer a question!

I'm doing a little light reading on 767 tail strikes (basically, how I can avoid the chief pilot's office!), and there's something the manuals aren't making clear. The book says that with struts fully extended, tailskid contact occurs at about 9.6 degrees of pitch. However, with struts fully compressed, it occurs at slightly less than 8 degrees. Fairly big difference.

So, here's my question: When exactly are the struts "fully compressed"? Are they only fully compressed momentarily if you slam the airplane onto the ground, or are they considered "fully compressed" (as far as the manuals are concerned) with normal weight on the mains after landing? The books really give no guidance on this.

Thanks!
 
dasleben said:
When exactly are the struts "fully compressed"?
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"3 fingers" is fully compressed (close to full dead weight on the landing gear) :)
763_Tailskid.png

This is probably a good depiction of "fully extended," but with the trucks not indicating they are tilted.
​
 
Thanks for the info! I have a similar chart in my manuals. Just curious: Where did the "3 fingers" approximation come from? I'll have to have a look at that during my next walkaround.
 
Minuteman said:
"3 fingers" is fully compressed (close to full dead weight on the landing gear) :)
...
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I would have thought that "fully compressed" means that they have reached the mechanical limit of travel, not their normal condition at max ramp weight.
 
SteveC said:
I would have thought that "fully compressed" means that they have reached the mechanical limit of travel, not their normal condition at max ramp weight.
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That was my initial thought as well, but there's no real way to tell from these manuals. Seems like what you're saying should be the case, but that just opens up even more questions. For example: If the struts aren't "fully compressed" with normal aircraft weight on them, what's the pitch attitude that will cause tailskid contact after a normal, smooth landing (i.e. you allow the nose to rise right after touchdown)?

Oh, Boeing. :insane:

By the way, the reason behind this post is that I had an autobrake fault right at touchdown the other day. Without those to create a pitch-down moment (and the ground spoilers creating a slight pitch-up moment), the nose got really floaty after touchdown. I didn't get close to a strike, but it was a little alarming at first. Actually had to push the nose onto the runway to sit it down. It'd be nice to know where that tailskid was during the time I was screwing around with the nose.
 
I assume (there's that word again!) that the definition of "fully compressed" would be as I described because it is, at least the way I visualize it, the worst case scenario. In other words it is the situation that allows a tail strike at the lowest possible nose up attitude. And yes, the only way to fully compress the struts in my scenario would take a pretty "solid" landing indeed, but isn't that exactly when the tail strike is most likely to happen? At the very last second the pilot realizes that his sink rate is much too high so he pulls back on the yoke just as he's banging it on - struts compress fully while the nose is rising unusually high = scrape!

edit to add:
If the struts aren't "fully compressed" with normal aircraft weight on them, what's the pitch attitude that will cause tailskid contact after a normal, smooth landing (i.e. you allow the nose to rise right after touchdown)?
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Somewhere between 8 and 9.6 degrees, depending entirely upon how smooth your landing is and how much the aircraft weighs at the time. That's my guess.
 
I got "three fingers" from an AFM (I don't fly 'em), but it did not explicitly describe that as "fully compressed" (see, I can assume as well, thinking in terms of an air/ground switch :)).

The other day I encountered a similar pitch limit chart for an MD-80 that had two curves as well: one for struts fully extended, and the other was struts fully compressed and tires deflated, which does imply two extremes.
 
SteveC said:
I assume (there's that word again!) that the definition of "fully compressed" would be as I described because it is, at least the way I visualize it, the worst case scenario. In other words it is the situation that allows a tail strike at the lowest possible nose up attitude. And yes, the only way to fully compress the struts in my scenario would take a pretty "solid" landing indeed, but isn't that exactly when the tail strike is most likely to happen? At the very last second the pilot realizes that his sink rate is much too high so he pulls back on the yoke just as he's banging it on - struts compress fully while the nose is rising unusually high = scrape!

edit to add:

Somewhere between 8 and 9.6 degrees, depending entirely upon how smooth your landing is and how much the aircraft weighs at the time. That's my guess.
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Pretty much my take on it as well, then. I suppose the safest course of action would be to assume the worst (i.e. that the struts are fully compressed) and make sure I never have to test that assumption... :)

Minuteman said:
I got "three fingers" from an AFM (I don't fly 'em), but it did not explicitly describe that as "fully compressed" (see, I can assume as well, thinking in terms of an air/ground switch :)).

The other day I encountered a similar pitch limit chart for an MD-80 that had two curves as well: one for struts fully extended, and the other was struts fully compressed and tires deflated, which does imply two extremes.
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"Three fingers" on a 767 seems like an awfully small amount while sitting on the ramp, but I'll have to look the next time I do my walk. Was this a -300ER maintenance manual you were looking at, by the way? My FCOM (Flight Crew Operating Manual) doesn't even address the strut in that much detail. As minor trivia by the way (since you mentioned it), the air/ground sensing is done by truck tilt angle, not strut compression. :)

Thanks guys for your help!
 
Fully compressed means just that, it is just a geometric number and all transports have similar data. Don't read more into it. You have hit the stops, it is fully compressed. A hard landing can do that.

Maintain a SAFE pitch attitude and use POWER to control vertical path in the landing phase in ANY swept wing jet. That, all by itself, would have eliminated the majority of accidents that went bad between flare and rollout.
 
seagull said:
Maintain a SAFE pitch attitude and use POWER to control vertical path in the landing phase in ANY swept wing jet. That, all by itself, would have eliminated the majority of accidents that went bad between flare and rollout.
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So you're saying Pitch for Airspeed/Attitude and Power for Altitude??? (I keed, I keed...this doesn't need to turn into one of those discussions)
 
seagull said:
When near the ground in a large jet, it would be prudent to fly it that way!
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Indeed, I have a friend that works for Boeing on the 787 as an instructor, he's retired Delta and flew the MD-11 as a Captain/Check Airman for 3 years. He said airplane was one plane that he never felt completely comfortable flying. The one thing he said that you never, ever had a thought of doing was raising the pitch to try and slow a sink rate/always add power to arrest it...it's one of those things that's always stuck in my head...

Do you have that same impression being a Captain on it? I've always been curious about the MD-11's "quirks".
 
seagull said:
Fully compressed means just that, it is just a geometric number and all transports have similar data. Don't read more into it. You have hit the stops, it is fully compressed. A hard landing can do that.

Maintain a SAFE pitch attitude and use POWER to control vertical path in the landing phase in ANY swept wing jet. That, all by itself, would have eliminated the majority of accidents that went bad between flare and rollout.
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Yeah, that seems to work really well with this thing. Very little drama compared to, well, everything else I've flown. Seems to be very well behaved (and I found, surprisingly roll-happy with the flaps down). I'm just paranoid about dragging that skid. I suppose not so much during landing (skid is a good 4-5 feet from the runway if you're on speed), but minimum tail clearance for takeoff of 24 inches? Yowza!
 
The MD-11 is the same as any other large swept wing transport. Some features exacerbate the issue slightly, but not significantly. The same principles apply to all large swept wing aircraft, and there have been landing incidents/accidents in virtually all of them due to not following those principles.
 
Minuteman said:
"3 fingers" is fully compressed (close to full dead weight on the landing gear) :)
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From an AMT's side I've heard of the 3 fingers before but it depends on the chrome showing. I always wondered about pressure and temperature being another factor. If there was enough nitrogen and fluid in the strut(depending on temperature and weight), it should absorb the impact without bottoming out. I was worried when you said that you meant it would bottom out.
 
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