B737-500 - Unstable?
Thread Starter
Joined: Mar 2003
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B737-500 - Unstable?
Hey,
On a flight last week a Captain of a B735 told me that he thought the aircraft is unstable compared with other 737s. I was quite suprised to hear this as I have always thought of the 735 as quite a stable aircraft. Any thoughts on this?
Kind Regards
BAe 146
On a flight last week a Captain of a B735 told me that he thought the aircraft is unstable compared with other 737s. I was quite suprised to hear this as I have always thought of the 735 as quite a stable aircraft. Any thoughts on this?
Kind Regards
BAe 146
Joined: Nov 2003
Posts: 496
Likes: 12
From: UK
It is not more unstable, but it is lighter which makes it feel more responsive.
If you are comparing it to the -300 or particularly the -400 they are more speed stable but that again is due to their higher momentum.
I agree about the elevator on landing at low speeds, but that is really only a factor if you are holding off the touchdown ie trying to squeak it onto a long runway, for a normal flare you should only be increasing the pitch attitude by a couple of degrees.
If you are comparing it to the -300 or particularly the -400 they are more speed stable but that again is due to their higher momentum.
I agree about the elevator on landing at low speeds, but that is really only a factor if you are holding off the touchdown ie trying to squeak it onto a long runway, for a normal flare you should only be increasing the pitch attitude by a couple of degrees.
Joined: Sep 2001
Posts: 764
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From: UK
The word "unstable" on its own is ambiguous. To be meaningful it is necessary to say which stability mode is unstable and/or to describe what your control inputs and the aircraft's response are. Beware of just using "unstable" as a word when you find something more difficult or unpleasant.
Moderator
Joined: Feb 2000
Aviation Qualifications: CPL
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From: UK
Roger "Dodge" Bailey gave a talk a year or so ago in Oxford where he recounted how low the lateral and directional stability of the Sea Hurricane where in his opinion. He then described an argument with a former wartime Hurricane pilot who maintained that the aircraft was an incredibly stable gun platform and Roger didn't know what he was talking about.
Dodge's conclusion was of-course that neutral static stability in certain axes (particularly lateral and directional) can imply less gust response - particularly to lateral gusts. And therefore what is technically a less stable aeroplane, can appear more stable to the operator.
All down to terminology and how you use it really, as Lomcevak says, the term "stability" on it's own is incredibly amibuous and can mean any of about a dozen things.
Similarly, I once made myself unpopular by declining to allow somebody to do a mod to an aircraft with high directional static stability, that was intended to increase that stability. My argument was that it was likely to make the aircraft directionally uncontrollable in side-gusts. Well, since when he ignored me and did it anyway, he went off the side of the runway and wrecked the aircraft, I was probably right. But, his perception was that the aircraft was directionally unstable, because of lateral excursions when flying in turbulence - the reverse was actually true, he had too much directional stability. Some people (especially light aircraft owners I think sometimes) just won't listen, and as a result an aircraft was written off, although thankfully nobody hurt.
G
Dodge's conclusion was of-course that neutral static stability in certain axes (particularly lateral and directional) can imply less gust response - particularly to lateral gusts. And therefore what is technically a less stable aeroplane, can appear more stable to the operator.
All down to terminology and how you use it really, as Lomcevak says, the term "stability" on it's own is incredibly amibuous and can mean any of about a dozen things.
Similarly, I once made myself unpopular by declining to allow somebody to do a mod to an aircraft with high directional static stability, that was intended to increase that stability. My argument was that it was likely to make the aircraft directionally uncontrollable in side-gusts. Well, since when he ignored me and did it anyway, he went off the side of the runway and wrecked the aircraft, I was probably right. But, his perception was that the aircraft was directionally unstable, because of lateral excursions when flying in turbulence - the reverse was actually true, he had too much directional stability. Some people (especially light aircraft owners I think sometimes) just won't listen, and as a result an aircraft was written off, although thankfully nobody hurt.
G
Last edited by Genghis the Engineer; 25th February 2005 at 09:55.
Joined: Jul 2000
Posts: 1,061
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From: My views - Not my employer!
Gotta say that I find no such probs with the -500. The biggest pain when light is the engine response...Using so little thrust anyhow that when a little speed creeps on it takes some working to get back to Vref 5. Flap 40 is all I can suggest... (and then the Vref can fall below 110kt!)
Joined: Aug 1999
Posts: 1,050
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From: England
Non rigourous observations from a non test pilot with a couple of years on 3/4/5s: I would say that:
- during automatic flight.
- the b737-500.
- is not optimally damped in pitch.
i.e. it takes a lot more than one oscillation to damp out pitch disturbances. This seems most apparent during climb and descent with pitch mode controlling speed. Long time period oscillations of +/- 10 kts seem to be typical. However I have also seen +/- 700 fpm in Alt Hold or VNAV PATH during level flight.
I am told that the aircraft uses the same control law as the -400, though I stress I have no idea if that is true or not.
On a similar vein, having the ap and fd disagreeing in pitch for quite long periods during climb and descent seems to be a 'feature of the model'.
CPB
- during automatic flight.
- the b737-500.
- is not optimally damped in pitch.
i.e. it takes a lot more than one oscillation to damp out pitch disturbances. This seems most apparent during climb and descent with pitch mode controlling speed. Long time period oscillations of +/- 10 kts seem to be typical. However I have also seen +/- 700 fpm in Alt Hold or VNAV PATH during level flight.
I am told that the aircraft uses the same control law as the -400, though I stress I have no idea if that is true or not.
On a similar vein, having the ap and fd disagreeing in pitch for quite long periods during climb and descent seems to be a 'feature of the model'.
CPB
Joined: Oct 2002
Posts: 822
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From: In my seat
Our new company SOP's state that minimum Vref. for Flaps30 is 124kts.
We currently only fly the -500's, but the NG's are coming really soon!
Oscillations do happen in level cruise with autopilot engaged. I take them out when the needle hits the 500'/min. mark. Also, the autothrottle is sometimes lagging quite seriously...
Is this also the case with the NG?
We currently only fly the -500's, but the NG's are coming really soon!
Oscillations do happen in level cruise with autopilot engaged. I take them out when the needle hits the 500'/min. mark. Also, the autothrottle is sometimes lagging quite seriously...
Is this also the case with the NG?
