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MD-80 In-Flight Bobbing?  
User currently offline757-200 From , joined Dec 1969, posts, RR:
Posted (15 years 7 months 3 weeks 6 days 19 hours ago) and read 1624 times:

Hello,

I have heard of MD-80's having bobbing problems in-flight. By this I mean that when the autopilot is switched to ALT hold, when peak altitude is reached, the plane will level off, then descent 3-20 feet below desired altitude, then nose up again and drift 3-20 feet above altitude. And this continues through the entire cruise. It sounds like an autopilot problem to me. Or could be altimeter, too. Now, is this true? You would think that that would make a lot of passengers very sick. Can anyone confirm this for me? Now, don't bash, this was just a rumor I heard!

757-200

5 replies: All unread, jump to last
 
User currently offlineMartin24 From , joined Dec 1969, posts, RR:
Reply 1, posted (15 years 7 months 3 weeks 6 days 18 hours ago) and read 1624 times:

You´re right, the MD-80 "bobs". It depends greatly on how much climb rate you have just before altitude capture. For example, if you let her climb with 2500 fpm or more, you need not complain if she´s "overshooting" about 50 feet, then overcorrecting in the other direction (means down). But if you´re nice and gentle and you´re climbing with about 1000 fpm, she´s doing the level-off much smoother.
But the passengers don´t feel the bobbing. They just sense the initial capture phase (as in any other aircraft, i suspect) when the autopilot gives a nose-down-input to change the pitch from about 5-15 degrees to about 0-5 degrees nose-up (depending on aircraft trim). If you have a high climbrate, the plane starts to capture the altitude/level much earlier than at a low rate (which is only logical) . Nevertheless this nose-down-input is much stronger then, and for passenger comfort its better to approach your cleared level with low climbrates.
Before anyone yells at me now and tells me that no MD-80 can make 2500 fpm climb at high altitude: I´m not talking only about level-off at cruising level, but about level-offs in general. ("stop climb at FL90 due to traffic", etc.)
For the bobbing in cruise: again you´re right, sometimes the needle of the altimeter is oscillating +/- 20 feet (or even more) around the altitude/level. Reasons for that are (a): a "wrong" sensing of the barometric altimeter (3-20 feet are not really much, don´t you think?) and (b): a not-state-of-the art autopilot which is simply too slow to hold the plane perfectly at "zero". By the way, i´d like to know if a "modern" aircraft can maintain altitude/level that exactly (+/- 0, i mean). I doubt it. But again, the passengers don´t feel it, because if there are changes in the attitude (pitch) of the aircraft, they are too small to be sensed.

hope this helps martin


User currently offline757-200 From , joined Dec 1969, posts, RR:
Reply 2, posted (15 years 7 months 3 weeks 6 days 17 hours ago) and read 1624 times:

I can tell you that the 727, 757, and 767 DO NOT bob. I do not know about the rest.

User currently offlineMD-11 From , joined Dec 1969, posts, RR:
Reply 3, posted (15 years 7 months 3 weeks 6 days 14 hours ago) and read 1624 times:

Are you sure about that 757-200.. Say a plane is cruising at FL370, I don't think ANY plane can maintain a steady reading of exactly 37000 feet in the altimiter for the duration of the cruise, even if there is no turbulence. I guess I could be wrong though. But I think a much bigger problem is the side-to-side yawing of the plane in cruise, which is something that the 777 for example has suffered from, and that truly can make passengers sick unlike a 10 feet change in the altitude.

User currently offlineDavid L From United Kingdom, joined May 1999, 9546 posts, RR: 42
Reply 4, posted (15 years 7 months 3 weeks 6 days 13 hours ago) and read 1624 times:

I noticed yaw oscillation in an F-28 (Sabena). The cycle was about 2 or 3 seconds and it was so subtle that I only noticed when I was looking down out of the extreme edge of the window. It's the only F-28 flight I've ever made so I've no idea if it's normal.

User currently offline757-200 From , joined Dec 1969, posts, RR:
Reply 5, posted (15 years 7 months 3 weeks 6 days 10 hours ago) and read 1626 times:

You are right, but what I meant was the do not even come close to bobbing like the MD-80. Yes, no plane can hold zero climb or descent for very long, but those seem to bob only withing the area af about10 feet at the most. How do I know this? First off, I have the Cital Guide to Commerial Aircrat and Airliners, as I mentioned earlier. Wonderful book, it has everything you need to know, second, I have downloaded dynamics for every airliner from Project Freeware!!(TM), which ar supposed to make the airliners handle like the real thing in FS98. That is NOT, however, where I learned that the MD-80 bobs. So, yes your'e right, but those planes are moderately better at holding altitude longer. But thank you for your reply!

757-200


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