Right on

I watched the return of Amanda Knox on local TV this evening and they had beautiful aerial coverage of the British Airways 747-400 landing at KSEA. The pilot not only made a silky smooth landing but the nose gear tires straddled the centerline when they contacted the runway. Couldn't have been any better.

 

It seems that the longer a type has been in service, the better the landings are, all other things being equal.

During its first few years of service, I was appalled at how lousy the landings were in all of the 767s I was flying in, and I think I know why:

The wing on the 767 was so efficient (relative to what had come before) that the airplane didn't want to stop flying! The pilots would flare and the airplane would float down the runway, and aggressive measures (like, perhaps, raising the spoilers while the airplane was still airborne) were needed to get it to land. Eventually pilots adopted new techniques (like probably waiting longer to flare) and the landings got better.

Am I full of hot air or does this premise seem reasonable?

 

I think that you are correct. The 727 did the same thing but I do think that ground effect had a lot to do with it on the 727 having such a short main gear. Hitting that magic spot of barely enough lift to ease the rate of descent at minimum altitude and speed is a rarity most of the time. I used to visit an old pilot who flew Curtiss Condors for American and he said that if you didn't slow that airplane down before your final you would find yourself floating to the end of the runway. I have heard pilot comments lately that Boeings are hard to slow down after a let down.

 

What would be the difference between waiting longer to flare, or flaring sooner and floating down the runway? (given the same airspeed over the threshold) - wouldn't making final at a lower speed be a better solution?

 

I believe the 727, and maybe 747, trick is not to flare, but rather to push forward on the yoke just before touchdown. This has the effect of pushing the nose down and lifting the main gear up just as they make contact.

Anyway, this is what an old 727 guy told me.

Incidentally, I'm planning to be on that BA flight tomorrow. The landing a few days ago was pretty good.

 

FNC was showing them on flightaware. "They're now over Manitoba"

 

Lots of theories why that works in the 727. I have heard claims that for an instant it unloads the tail which effectively makes the airplane lighter and arrests the sink. Whatever it is, I have seen some impending crunch landings that were miraculously saved by pushing forward on the wheel at the last second. However, to get a super greaser, you hold it off just like any other airplane. By the way, Boeing flight manuals recommend a firm touchdown for good wheel spin-up. But, what fun is that?

Modern Boeings also float due to higher flight idles which allow instant engine spool-up. After touchdown, the engines drop to a lesser ground idle.

Dave

 

I had a good friend who was an old UAL pilot and who flew DC-8's. When he transitioned to 727's he had a terrible problem with landing the airplane due to the unnatural forward yoke application after flare. I discussed this with one of our stability and control guys and it had something to do with the initial response of up elevator producing a momentary tail down moment before the increased angle of attack produced any lift and you could pound the mains into the deck.

 

I agree with this-- I'm pretty sure it works because the airplane has so much momentum that the fuselage rotates before it has any effect on the actual flight path. As it rotates, it lifts the main gear above the runway, or at least slows their descent relative to the runway.

I think this is somewhat common to heavy swept wing jets-- I'm a bit surprised that it wouldn't work on the DC-8.

Incidentally, BA made a great landing yesterday afternoon as well. Better than the London landing (which was pretty good as well)- my guess is the Captain flies outbound and the FO flies home.

 

I remember that there was a rash of early 727 accidents on landing - this may have been a factor.

 

The DC-8 had a much longer main oleo than the 707 or 727 and I assume, less ground effect and cushioning when landing. One of the hardest landing airplanes I ever flew in was an L-1011. Stiff and hard contact with the runway.

 

I think that had to do w/ stalling... it was recommended to keep the power up when landing, unlike the 707.

If I recall correctly... been awhile (not a pilot, just a groupie.)

 

Most of the 727 landing incidents were due to not maintaining enough thrust to power the high lift system. With all the high lift devices deployed (Kruger flaps, slats,and triple slotted TE flaps) it took something like 80% thrust to push them through the air so that they worked. The UAL Salt lake incident was due to a steep approach angle and minimum power. When it was time to flare there wasn't enough thrust to produce the required lift to arrest the descent. The airplane assumed the angle but continued to descend at too high a rate. This happened several times until everybody caught on to maintaining plenty of power on approach. If I remember correctly, the 727 high lift stuff doubled the lift coefficient of the wing but it took a lot of push to make it work.

 

Thanks for the explanation, Bob...

 

You are welcome. I used to teach this stuff in the early 60's before the 727 flew and you wouldn't believe the number of doubting Thomas's who predicted horrible outcomes for the airplane. I was good friends with Lew Wallick, the test pilot who made the first flight, and he had some marvelous stories about testing the 727. Really a nice guy, too.

 

I remembered that I had this in my file. With everything totally dirty it makes the airflow do things that it really doesn't want to do but as you can see that the four slots and high camber can produce a lot of lift.
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The last few 727s that I flew on seemed to have longer takeoff and landing rolls than I remember -- one takeoff at Buffalo was so long as to be downright scary -- and on one of those occasions I could see the flaps and it appeared that they were not using max flap for landing. Was there a technical reason why they would refrain from using max flap?

 

Here is a photo that I have taken of a 727 on short final showing the flaps and slats extended.

http://www.jetphotos.net/viewphoto.php?id=6569354

 

Great shot. Thanks for sharing. I have always had a soft spot for the 727 for it was a great accomplishment
and a good airplane.

 

Maybe the pilot didn't like to use everything.hard one to answer.