The Anatomy of a Pitchup!
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Personal experience.
We were in a Boeing 707 in VFR conditions on top of active thunder-storms. At the first jolt of a strong updraft we disconnected the autopilot. The second updraft pitched the aircraft into a 20-25 nose high attitude. There was little or no increase in g, or zoom in altitude! The aircraft's momentum carried it along on its projected flight path, in this attitude. Because we had a visual horizon we were able to ease the nose back down to the horizon and continue on course, without further incident.
Northwest 705 accident.
The crew were flying a Boeing 720B, 8 feet shorter than a 707. (A shorter moment arm to oppose a pitchup). They were climbing up to their altitude, trying to avoid the heavy turbulence in an active thunderstorm area. The aircraft was pitched up into a 30-35 degree nose high attitude by a strong updraft. Due to the strong vertical updraft, the application of forward pitch control had little effect in changing the aircaft's attitude. The crew then applied full nose down horizontal stabilizer trim, with minimum effect. Upon exiting the pitchup and returning to a normal relative wind condition the position of the flight controls pitched the aircraft over into a steep dive, and it broke up in the air.
NASA says that an updraft can extend up to 15 miles in diameter.
The members of the CAB investigation team could not understand why the crew held the flight controls in the nose down position for such a long time period.
Both gyro's nose down stops showed severe impact damage.
We were in a Boeing 707 in VFR conditions on top of active thunder-storms. At the first jolt of a strong updraft we disconnected the autopilot. The second updraft pitched the aircraft into a 20-25 nose high attitude. There was little or no increase in g, or zoom in altitude! The aircraft's momentum carried it along on its projected flight path, in this attitude. Because we had a visual horizon we were able to ease the nose back down to the horizon and continue on course, without further incident.
Northwest 705 accident.
The crew were flying a Boeing 720B, 8 feet shorter than a 707. (A shorter moment arm to oppose a pitchup). They were climbing up to their altitude, trying to avoid the heavy turbulence in an active thunderstorm area. The aircraft was pitched up into a 30-35 degree nose high attitude by a strong updraft. Due to the strong vertical updraft, the application of forward pitch control had little effect in changing the aircaft's attitude. The crew then applied full nose down horizontal stabilizer trim, with minimum effect. Upon exiting the pitchup and returning to a normal relative wind condition the position of the flight controls pitched the aircraft over into a steep dive, and it broke up in the air.
NASA says that an updraft can extend up to 15 miles in diameter.
The members of the CAB investigation team could not understand why the crew held the flight controls in the nose down position for such a long time period.
Both gyro's nose down stops showed severe impact damage.
Last edited by wsherif1; 7th Jan 2005 at 06:33.
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A strong updraft will pitch an aeroplane nose down. To confirm this, if the aeroplane is in a strong nose UP attitude and maintaining level flight, it is obviously in a very powerful down draft. We are talking powerful downdrafts here.
I would have thought any damage to pitch down stops in the VGs would not be caused inflight, but either in wear and tear during rocking on start up or during impact damage or damge post break-up.
What is the date of this accident, and what prompts this to be raised at the moment?
I would have thought any damage to pitch down stops in the VGs would not be caused inflight, but either in wear and tear during rocking on start up or during impact damage or damge post break-up.
What is the date of this accident, and what prompts this to be raised at the moment?
I encountered a unique situation climbing out of Sondrestrom Fiord, Greenland. At approx FL130, on a clear moonlight night, when approaching a large lenticular cloud, but 3-4000 ft above it, the aircraft increased climb very rapidly, +5000 ft/min; the aircraft pitched up peaking at 30 deg, even with full forward stick. Control was regained after approx 1 min at FL180.
I don’t know which side of the cloud I was on, up-draught or down-draught, but the aircraft transited the full extent of the cloud and did not encounter any reciprocal effects.
I have seen similar, but not the same effect, very close to large thunderstorms (close-up for flight tests; too close for passengers). The overall levels of turbulence were high and even though the aircraft would climb/descend like a lift (elevator) in the very severe up/down-draughts there was no sustained change in pitch. However, any pitching tendency could have been masked by the requirement to maintain constant pitch attitude, and thus with a tight control loop, any stick force could have been masked by a high adrenalin level.
NsF, I don’t follow your logic about pitch up/down. If an aircraft encounters an updraft the relative wind increases the airspeed vector and AoA; this gives more lift and a change in pitch trim, generally nose up (aircraft flying faster requires forward stick). What’s the alternative point of view?
I don’t know which side of the cloud I was on, up-draught or down-draught, but the aircraft transited the full extent of the cloud and did not encounter any reciprocal effects.
I have seen similar, but not the same effect, very close to large thunderstorms (close-up for flight tests; too close for passengers). The overall levels of turbulence were high and even though the aircraft would climb/descend like a lift (elevator) in the very severe up/down-draughts there was no sustained change in pitch. However, any pitching tendency could have been masked by the requirement to maintain constant pitch attitude, and thus with a tight control loop, any stick force could have been masked by a high adrenalin level.
NsF, I don’t follow your logic about pitch up/down. If an aircraft encounters an updraft the relative wind increases the airspeed vector and AoA; this gives more lift and a change in pitch trim, generally nose up (aircraft flying faster requires forward stick). What’s the alternative point of view?
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1- The tailplane causes the aircraft to weathercock nose down in an updraft.
2- The autopilot will attempt to regain altitude and cause the nose to come down to maintain altitude if you are 'blown' upwards.
Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?
2- The autopilot will attempt to regain altitude and cause the nose to come down to maintain altitude if you are 'blown' upwards.
Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?
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I'm with NSF on this;
1.) An aircraft will always want to weathervane into the relative wind (ie pitch down into an updraft)
2.) It would seem logical that the only circumstances under which an aircraft could be pitched up and yet remain in level flight would be;
a.) in a down draft or,
b.) in slow flight,
the latter of which would also be symptomatic of an aircraft trying to stay level in a downdraft.
Perhaps there is some other phenomenon taking place in these pitch-up events that does not fall into the updraft / down draft category.
1.) An aircraft will always want to weathervane into the relative wind (ie pitch down into an updraft)
2.) It would seem logical that the only circumstances under which an aircraft could be pitched up and yet remain in level flight would be;
a.) in a down draft or,
b.) in slow flight,
the latter of which would also be symptomatic of an aircraft trying to stay level in a downdraft.
Perhaps there is some other phenomenon taking place in these pitch-up events that does not fall into the updraft / down draft category.
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What does it do in an updraft? AoA increases. The response to this is designed into the aeroplane- it will pitch down to restore- this is called positive stability- when disturbed, it will try and regain its former state. Negative stability is when you disturb the steady state, and the aeroplane diverges from this- ie in a pitch up state, it wants to pitch further up. This is only allowed in computer controlled aeroplanes as a human cannot fly such a machine.
If an aircraft encounters an updraft the relative wind increases the airspeed vector and AoA; this gives more lift and a change in pitch trim, generally nose up (aircraft flying faster requires forward stick). What’s the alternative point of view?
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Notso Fantastic,
Your comment,
"A strong updraft will pitch an aeroplane nose down."
Unless the resultant angle of the aircraft's forward momentum and the vertical component of the updraft increases the lift and moves the center of lift forward on the swept wing. pulling the nose up.
The vertical updraft component of the relative wind was so strong that NW 705 was unable to return to a level flight attitude even with a full forward pitch control input. Full nose down trim on the horizontal stabilizer was also ineffective, until the aircraft returned to a normal relative wind direction, and then it was drastically effective!
AirQuake,
Your comment,
"Didn't the Co-Pilot on that Northwest flight survive an almost identical 'upset' some months earlier?"
I believe you are referring to the Eastern Co-pilot that shoved the nose over into a steep dive attitude in an Eastern DC-8, when his airdpeed indicator went to zero!
The Captain put the DC-8 engines into reverse and recovered. (The DC-8 was the only aircraft that you could reverse the engines in flight.)
As an aside, the same co-pilot was aboard another Eastern DC-8 that disapeared over Lake Ponchetain in Lousiana, (no spelling checker!), a year later.
Your comment,
"A strong updraft will pitch an aeroplane nose down."
Unless the resultant angle of the aircraft's forward momentum and the vertical component of the updraft increases the lift and moves the center of lift forward on the swept wing. pulling the nose up.
The vertical updraft component of the relative wind was so strong that NW 705 was unable to return to a level flight attitude even with a full forward pitch control input. Full nose down trim on the horizontal stabilizer was also ineffective, until the aircraft returned to a normal relative wind direction, and then it was drastically effective!
AirQuake,
Your comment,
"Didn't the Co-Pilot on that Northwest flight survive an almost identical 'upset' some months earlier?"
I believe you are referring to the Eastern Co-pilot that shoved the nose over into a steep dive attitude in an Eastern DC-8, when his airdpeed indicator went to zero!
The Captain put the DC-8 engines into reverse and recovered. (The DC-8 was the only aircraft that you could reverse the engines in flight.)
As an aside, the same co-pilot was aboard another Eastern DC-8 that disapeared over Lake Ponchetain in Lousiana, (no spelling checker!), a year later.
Last edited by wsherif1; 7th Jan 2005 at 21:31.
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sherif, the way I see it, if an updraft pitches an aeroplane nose up, then it will diverge and the pitch up moment will continue and it will immediately break up unless controlled- ie negative stability? How do you explain the second paragraph of the first post- if you have a nose up attitude and you are maintaining altitude, then the only physical way this is possible is in a strong downdraft?
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In the case of the NW flight, wouldn't a very strong and sudden horizontal headwind result in a pitch up and subsequent increase in altitude? Then, a sudden shearing tailwind (or even relative wind returning to the original vector) would result in a rapid nose down pitching moment due to trim.
Kind of like a high altitude microburst effect, but maybe from the result of horizontal vortices that sometime produce tornados later in their development.
FoF
Kind of like a high altitude microburst effect, but maybe from the result of horizontal vortices that sometime produce tornados later in their development.
FoF
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Notso Fantastic,
Your comment,
"What is the date of this accident, and what prompts this to be raised at the moment?"
The NW705 accident over Miami, FL occurred on February 12, 1963. This accident proved that a swept wing, aircraft design can pitch-up in a strong updraft. Pilots should be aware of this possibility.
The "Industry" still insists that a swept wing design will always pitch-down in an updraft.
Your comment,
"What is the date of this accident, and what prompts this to be raised at the moment?"
The NW705 accident over Miami, FL occurred on February 12, 1963. This accident proved that a swept wing, aircraft design can pitch-up in a strong updraft. Pilots should be aware of this possibility.
The "Industry" still insists that a swept wing design will always pitch-down in an updraft.
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I agree with Notso, but also with wsherifs "Unless the resultant angle of the aircraft's forward momentum and the vertical component of the updraft increases the lift and moves the center of lift forward on the swept wing. pulling the nose up."
Is it possible that the personal experience was a downdraft? How can you confirm it was an updraft if you maintained altitude?
And, from the other side, against "if an updraft pitches an aeroplane nose up, then it will diverge and the pitch up moment will continue" - updraft against an aircraft which is nose-up will also add a tailwind component, which would bleed airspeed and initiate a pitch down motion.. would it Notso?
I'm happy to be corrected, of course (and I don't mean that as in "come on then!" - honestly!!
)
Is it possible that the personal experience was a downdraft? How can you confirm it was an updraft if you maintained altitude?
And, from the other side, against "if an updraft pitches an aeroplane nose up, then it will diverge and the pitch up moment will continue" - updraft against an aircraft which is nose-up will also add a tailwind component, which would bleed airspeed and initiate a pitch down motion.. would it Notso?
I'm happy to be corrected, of course (and I don't mean that as in "come on then!" - honestly!!
)
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The aeroplane done it...
Well, maybe.
To those that have actually flown the B707 (and the very similar B720) will vividly recall that if, full elevator (to the stops) is used to any great extent, the stab pitch trim (jackscrew) will become stalled (as in, binding...not moving) due to aerodynamic loads caused by the elevator, and pitch control can become lost...or at best, very difficult.
This was (or could become) a rather major problem under some circumstances.
The 707 was a wonderful aircraft, but you definitely needed to keep in mind the odd quirks associated therewith....
PS: Later fan-powered varients were much better but still had the stab trim 'problems'.
To those that have actually flown the B707 (and the very similar B720) will vividly recall that if, full elevator (to the stops) is used to any great extent, the stab pitch trim (jackscrew) will become stalled (as in, binding...not moving) due to aerodynamic loads caused by the elevator, and pitch control can become lost...or at best, very difficult.
This was (or could become) a rather major problem under some circumstances.
The 707 was a wonderful aircraft, but you definitely needed to keep in mind the odd quirks associated therewith....
PS: Later fan-powered varients were much better but still had the stab trim 'problems'.
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During my L1011 conversion at Palmdale in 1975, I hired a C150 and flew my lady friend to Tehachapi from Fox Field. We had a leisurely lunch and took off to retrun to Fox field. Climbing at 500 fpm coming through the gap at 4,500ft, ROC suddenly increased to 1500fpm with a rapid rise in altitude. I pitched down, increased power but the aircraft was still rising in a pitch down attitude. We fell out of it at 10,000ft. I was innocently asked by my girfriend during the inadvertent climb as to where we were going and my answer was I really don't know.
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Notso Fantastic,
Your comment,
"I would have thought any damage to pitch down stops in the VGs would not be caused inflight, but either in wear and tear during rocking on start up or during impact damage or damge post break-up."
Quote from the accident report. "The nosedown rotational pitch stops of both vertical gyros received severe impact damage, as a result of a rapid rotation of the aircraft about its pitch axis."
Your comment,
"I would have thought any damage to pitch down stops in the VGs would not be caused inflight, but either in wear and tear during rocking on start up or during impact damage or damge post break-up."
Quote from the accident report. "The nosedown rotational pitch stops of both vertical gyros received severe impact damage, as a result of a rapid rotation of the aircraft about its pitch axis."
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Notso Fantastic.
Your comment,
"Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?"
The aircraft was pitched up by the resultant AOA from the forward momentum of the aircraft and the vertical component of the updraft, increasing the lift, and moving the center of lift forward on the swept wing, MECHANICALLY PULLING THE NOSE UP, (overpowering the tail plane force). THERE WAS LITTLE INCREASE IN G, OR ZOOM IN ALTITUDE. THE AIRCRAFT CONTINUED ON ITS PROJECTED FLIGHT PATH, IN THIS ATTITUDE. There was a mechanical lifting of the aircraft a few hundred feet with the updraft. Due to the small increase in G there was little loss of kinetic energy and the aircraft continued on course with no imminent stall threat.
Your comment,
"Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?"
The aircraft was pitched up by the resultant AOA from the forward momentum of the aircraft and the vertical component of the updraft, increasing the lift, and moving the center of lift forward on the swept wing, MECHANICALLY PULLING THE NOSE UP, (overpowering the tail plane force). THERE WAS LITTLE INCREASE IN G, OR ZOOM IN ALTITUDE. THE AIRCRAFT CONTINUED ON ITS PROJECTED FLIGHT PATH, IN THIS ATTITUDE. There was a mechanical lifting of the aircraft a few hundred feet with the updraft. Due to the small increase in G there was little loss of kinetic energy and the aircraft continued on course with no imminent stall threat.
Last edited by wsherif1; 11th Jan 2005 at 06:31.
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wsherif1
Aircraft continues at same speed with a higher wing angle of attack - how could they not be generating more lift in this configuration?
The aircraft could not continue on it's previous trajectory with an increase in lift due to increased AoA.
Aircraft continues at same speed with a higher wing angle of attack - how could they not be generating more lift in this configuration?
The aircraft could not continue on it's previous trajectory with an increase in lift due to increased AoA.
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Notso Fantastic,
Your comment,
"Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?"
The resultant AOA from the forward momentum of the aircraft and the vertical component of the strong updraft, increased the lift and moved the center of lift forward on the swept wing, mechanically pulling the nose up. (Overpowering the tail plane force.) The resultant pitch-up attitude depends on the strength of the vertical updraft and the length of the moment arm, from the c.g. to the tail plane.
The aircraft continued on its projected flight path, in this attitude (25 degrees nose up.) Due to this smooth transition in attitude, and no zoom in altitude, there was little loss of kinetic energy and therefore no imminent stall threat. There was no requirement for any quick reactive action to immediately shove the nose over to return to a normal flight attitude.
Your comment,
"Can you explain how an aeroplane would have a nose up attitude in an updraft and not rocket upwards?"
The resultant AOA from the forward momentum of the aircraft and the vertical component of the strong updraft, increased the lift and moved the center of lift forward on the swept wing, mechanically pulling the nose up. (Overpowering the tail plane force.) The resultant pitch-up attitude depends on the strength of the vertical updraft and the length of the moment arm, from the c.g. to the tail plane.
The aircraft continued on its projected flight path, in this attitude (25 degrees nose up.) Due to this smooth transition in attitude, and no zoom in altitude, there was little loss of kinetic energy and therefore no imminent stall threat. There was no requirement for any quick reactive action to immediately shove the nose over to return to a normal flight attitude.
Last edited by wsherif1; 11th Jan 2005 at 20:59.
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Stall is created by angle of attack; regardless of the kinetic energy your aircraft has, or the airspeed for that matter, if you take the angle of attack past the critical value THE WING WILL STALL.
If an aircraft was on it's projected flightpath (by which I assume you mean level flight, gamma=0) with a 25 degree nose-up attitude, that means a 25 degree angle of attack.
That is perilously close to stall conditions, if not actually well beyond them (depending on type and configuration) for any aircraft in the transport category.
If an aircraft was on it's projected flightpath (by which I assume you mean level flight, gamma=0) with a 25 degree nose-up attitude, that means a 25 degree angle of attack.
That is perilously close to stall conditions, if not actually well beyond them (depending on type and configuration) for any aircraft in the transport category.
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It occurs to me that there is one way in which an aircraft could enter a stable and markedly pitch-up attitude without any change of altitude or ground speed, and therefore without any sensation by the crew of acceleration in either the vertical or horizontal direction.
And that is by encountering a strong tailwind. The only sign would be a drop in indicated air speed and the need to pitch up to maintain altitude, and possibly without much need to change the throttle setting.
This might happen near the top of the “hump” over a lenticular cloud, without any reciprocal phenomena later.
Or rather, the reciprocal action by the pilot would be to just to ease the nose down again as the airspeed picked up due to the decreasing tailwind as the aircraft leaves the “hump” behind (with no change in altitude).
There might be up/down draughts as well - I am just trying to discuss the pitch up.
What do you think?
Cheers,
And that is by encountering a strong tailwind. The only sign would be a drop in indicated air speed and the need to pitch up to maintain altitude, and possibly without much need to change the throttle setting.
This might happen near the top of the “hump” over a lenticular cloud, without any reciprocal phenomena later.
Or rather, the reciprocal action by the pilot would be to just to ease the nose down again as the airspeed picked up due to the decreasing tailwind as the aircraft leaves the “hump” behind (with no change in altitude).
There might be up/down draughts as well - I am just trying to discuss the pitch up.
What do you think?
Cheers,