The Anatomy of a Pitchup!

[wsherif1]

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.

[AirQuake]

Didn't the Co-Pilot on that Northwest flight survive an almost identical 'upset' some months earlier?

AQ

[Notso Fantastic]

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?

[alf5071h]

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?

[Notso Fantastic]

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?

[Astra driver]

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.

[Notso Fantastic]

Alf
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?

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.

[wsherif1]

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.

[Notso Fantastic]

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?

[Full of Foehn]

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

[wsherif1]

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.

[The African Dude]

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!! :ooh: )

[411A]

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....:ooh: :ooh:

PS: Later fan-powered varients were much better but still had the stab trim 'problems'.

[HotDog]

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.:uhoh:

[wsherif1]

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."

[wsherif1]

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.

[The African Dude]

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.

:confused:

[wsherif1]

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.

[Mad (Flt) Scientist]

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.

[PickyPerkins]

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, http://home.infionline.net/~pickyperkins/pi.gif

[wsherif1]

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. Due to this smooth transition in attitude there was little loss of kinetic energy and therefore no imminent stall threat. There was no requirement for any fast reactive action to immediately shove the nose over to return to a normal flight attitude.

[The African Dude]

wsherif1,

Without meaning to sound rude - why are you ignoring mine, and others input?

Andy

[wsherif1]

The African Dude,

Your comment,

"Aircraft continues at same speed with a higher wing angle of attack - how could they not be generating more lift in this configuration?

Strong Vertical relative wind in updraft.

[Notso Fantastic]

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.

I'm sorry, but this just does not make sense. Somehow, we are to believe that an aeroplane can be pitched up 25 degrees caught in a violent updraft, and still maintain altitude? If an aerodynamicist can confirm this I will believe it, but what you say above does not make sense.

A sudden updraft has a similar AoA effect of instantaneously pitching an aeroplane nose up. Stability is designed into aeroplanes whereby they will restore, and the tailplane will provide a nose down effect.

I repeat, the only way this makes sense (nose up and maintaining altitude) is if you are in a sustained downdraft.

[Mad (Flt) Scientist]

Did someone say "aerodynamicist"?

OK, let's start at the beginning. The story provided is that an aircraft, in essentially level flight encounters an "updraft", pitches UP some 20-25 degrees with no changes of significance to 'g' or to altitude.

The mechanism of encoutering a shear in the atmosphere are as follows:

As the aircraft enters the region of the gust, the angle of attack (and sideslip, but we're concerned here with a gust in the symmetric plane) are affected, because the aircraft's airspeed and angles are a result of the inertial velocity (which is very slow to change) and the velocity of the air mass, which undergoes an abrupt change.

If an aircraft were to encounter an upwards or downwards moving air mass, the effect would be negligible on airspeed (for most practical combinations of airspeed and gust speed, at least for an airliner type) and significant on angle of attack. The increased movement of the airmass would be manifested as an instantaneous change to the aircraft angle of attack (and would be evident on any AoA metering instrument, such as stall vanes). For an UPDRAFT, there would be an instantaneous increase in the angle of attack; for a DOWNDRAFT there would be an instantaneous decrease in the angle of attack. If one were to assume 50kt instantaneous gust and a 400kt (TAS) one would see an angle of attack change of approximately 7 degrees in the appropriate direction. One would also experience a corresponding 'g' bump as the gust was entered and the 'edge' of the gust passed over the wing. (Since it's a sharp-edged event, the AoA sensors on the nose will 'see' the gust before the wing)

Now, consider the natural aerodynamic response of an aircraft to an instantaneous change in angle of attack. All aircraft are STABLE in flight. Therefore the effect of that stability is for the aircraft to pitch down, or up, so as to reduce the alpha 'spike'. Therefore, an aircraft will aerodynamically pitch DOWN following a sharp-edged updraft, and pitch UP for a downdraft. Any other behaviour would result in an aircraft which had divergent pitch stability and was, essentially, unflyable.

Therefore I simply fail to understand the mechanism you are proposing which would pitch an aircraft nose-up following an updraft.

The centre of lift argument is fallacious - what matters is the neutral point, which will not be moved about by updrafts or downdrafts.


...but, anyway, let's imagine that an aircraft somehow ends up in a 25 degree nose high attitude in level flight relativbe to the earth.

In the absence of any wind component, this implies a 25 degree angle of attack. For any commercial clean wing, this is a stalling angle of attack; there should be stall warnings going off, possibly pushers firing and all kinds of similar activity. None of this is mentioned, therefore one must conclude that the aircraft is at a significantly lower angle of attack. The only way to achieve this is by changing the relative wind to add a significant DOWNDRAFT component, which allows the velocity vector relative to the airmass to more closely approximate the direction of the nose, while still allowing level flight relative to earth. Since this is also the same component consistent with the pitch up behaviour, I see no mystery.

Incidentally, it is theoretically possible for the stalled aircraft to be maintaining level flight in an updraft, if the rate at which the aircraft is falling out of the sky like a brick is matched by the updraft. This is not only inherently unlikely, and would require some rather impressive post-stall manoeuvrability more commonly associated with e.g. Sukhoi-27s at airshows, but still provides no explanation for how the aircraft got to a post-stall attitude in the first place.

Frankly, the reason why "the industry" believes this to be an impossible scenario as described is because it conflicts with the fundamental longitudinal design of every aircraft built.

[Ignition Override]

Excellent topic, y'all.

There is a small section in our manual (swept-wing, twin-turbofan) which claims that in some severe turbulence situations, it might be best to not just engage the autopilot, but switch one of the knobs to pitch hold, in order to avoid chasing airspeed by moving the yoke forwards and back.

I've never read anything else about this 'pitch hold', nor seen other (captains years ago) pilots use it, nor seen it discussed in any systems review or sim briefing etc. Of course (I) we've never been in any turbulence worse than 'strong' moderate :suspect: , fortunately. I would rather be in a bit of moderate for a short while than have these damned computerized systems tests in the future :rolleyes: :{ .

[Notso Fantastic]

Mad Scientist- that is my reading of the situation and seems to be broadly in line with what I was trying to say.

My worst experience with this sort of situation was flying into Genoa in a 737-200. Joining the hold in some violent CBs, as we were going around the holding pattern, we were going from very strong downdrafts to updrafts at each end. Speed was falling back to 200 kts clean with full power on at one end whilst we struggled through the downdraft ( I actually saw about 195 with full power), and as we moved to the other end, speed was increasing and the thrust levers were coming back to idle at 290 kts. It was an extraordinary situation. We managed 2 holds in serious turbulence before chickening out and diverting to Milan Linate.

[alf5071h]

M(flt)S Using your excellent post as a basis, could you or any one else provide an explanation or hypothesis for the incident that I described earlier. I could not establish whether the aircraft entered an up or down draft, but it pitched up and climbed rapidly. Whatever the phenomenon, there was a period where full forward stick only just balanced the pitching moment – this could be a manifestation of instability.

[Mad (Flt) Scientist]

alf5071h - this is all pure speculation without the data but....

If the aircraft were at, say, 15 deg nose up pitch attitude and approx zero AoA in the climb before the incident, and were to pitch up to 30 deg AoA instantaneously, that's 15 deg AoA. To achieve that with the controls would take a great deal of nose-up pitch command - in the absence of such a command the aircraft would naturally be very quick to return to the trim near-zero AoA value.

If you were applying nose down-pitch command, but the pitch attitude were not changing, then I would conclude that you were in a stabilised condition. This would actually imply that your AoA was more negative during the event than prior to it, even though your pitch attitude was more positive. The simplest explanation for that is a downdraft.

Now, to address the altitude change - the aircraft went from FL130 to FL180 in one minute. One of the first things I would check with the data would be your geometric altitude, not your pressure altitude; in a significant atmospheric disturbance, I'm not sure I trust pressure altitude to be consistent through the event. If the shear/gust/downdraft were also associated with a significant pressure gradient, some or all of the apparent altitude change might have been due to pressure, not altitude. You mention it was at night, so a geometric altitude variation would not be so apparent.

Those would be my initial comments; as I mentioned, there would be a lot going on you might not be aware of, and only looking at the FDR traces would really resolve some of it.

[bookworm]

Couldn't alf5701h's excursion have been a strong horizontal gust? A sudden increase in headwind would initiate a phugoid, though you may have brought it under control before it became apparent as such. If the gust decayed gradually, it might not have been noticeable.

[Mad (Flt) Scientist]

Phugoid is too long period a motion, typically it's measured in minutes, this was over in a minute or so it seems.

A strong headwind would indeed have caused increased lift at the fixed AoA. But there should have been no difficulty whatsoever in pitching the nose down to maintain altitude, since if anything the increased airspeed would have provided more absolute tailplane/elevator pitch authority. So the comment about running out of pitch control seems to work against that.

[PickyPerkins]

Mad (Flt) Scientist
Great post- agree with it all. The CAB report on the crash of NW 705 (described in the opening post of this thread from wsherif1) seems to agree as well:
http://home.infionline.net/~pickyperkins/NW705_02.gif

But what does this mean? Does it mean that the initial response is nose down followed by nose up?
-----------------------------------------------------

Could you walk us through the traces below from the same report?

http://home.infionline.net/~pickyperkins/NW705_01.gif

The initial part of the top (altitude) trace is the normal climb, which then increases (according to the text of the report) to three and a half times the normal rate of climb.
I assume that this increased rate of climb is due to an up draught.
The next trace shows a falling air speed - not surprising.
The next traces are deduced from FDR data but are not direct traces.
The first shows a PITCH UP increasing from about 4 to 20 degrees during this climb.
This is not a "weather cocking down into the up draught", but might be what the CAB called “the ultimate effect of the updraft is an altitude and nose up attitude increase“.
The last trace shows an initially unchanging angle of attack.
---------------------------
So does an updraft initially induce nose down quickly followed by nose up, or what?
The pitch trace shows no sign of nose down, with a climb rate three and a half times normal.

Signed, Puzzled.
------------------------------
Ignition Override
The same report contained strong words of advice to use the attitude indicator as the primary instrument in turbulence, and even then to go easy on the controls. They didn‘t seem to like auto pilots much.:

"From all the evidence available to the Board, it is abundantly clear that flight on instruments in heavy turbulence can present a difficult problem to any pilot who departs too far from the recommended practice of using the attitude indicator as the main reference instrument for maintaining control. If the pilot places undue emphasis on any other flight instrument during his normal scan routine, a serious miscue with drastic consequences can occur. Similarly, attempts to maintain "perfect" attitude control can be equally hazardous, because of the high loadings induced, the danger of over controlling by the use of large control displacements, and the possibility of inducing an undesirable oscillatory motion of the aircraft. "Loose" attitude control, or moderate counteracting control inputs, appears to be the best method of counteracting the effects of heavy turbulence. ………… Little is gained by trying to maintain rigid. attitude control since this can produce excessive aircraft loadings without appreciably affecting the altitude and airspeed excursions that occur during severe encounters. Large pitch attitudes of 40 degrees nose up can occur in severe turbulence but moderate counteracting elevator inputs will prevent excessive speed reductions that could result in a stall. The use of the autopilot on Manual Mode offers some advantages but considerable stabilizer trim activity can occur in some types of' turbulence and could present a serious danger if the autopilot was disengaged either deliberately or inadvertently at a time when the trim varied appreciably from the in-trim setting .”

Cheers, http://home.infionline.net/~pickyperkins/pi.gif

[Captain Stable]

Can I throw a question in here for the technically-minded?

Initially, Notso's thesis made perfect sense to me. However, it occurs to me that, in most modern aircraft, the centre of gravity is aft the centre of lift (thus necessitating a net downforce by the tailplane).

A strong updraft would act primarily on the CofG, n'est ce pas? thus making the initial response by the aircraft a nose-down pitch.

Or am I talking a load of whatsits?

[411A]

...again.

Yes, Ignition Override, 'pitch hold' works as advertised, altho on the TriStar, it was called 'turbulence mode'.

I have personally used same on a number of occasions, and the pitch excursions are indeed kept to a minimum.
Worked like a charm.

[Notso Fantastic]

The way I see it, a pitch changing effect on the CofG would require significant (and sustained) vertical acceleration to cause any effect of acting as a lever around the centre of lift. The CofG and centre of lift are very close and I can't see an updraft having the sustained effect to cause a pitch change on such a mass- there is just not enough lever arm moment. We're told there is not much change in altitude, so the acceleration force acting on the CofG is nothing more than momentary. I think the weathercocking of the tailplane is overriding.

I think the effect we see in the flight recorder traces is an encounter with a downdraft causing pitch up. This is countered with large control inputs nose down, just as the plane comes out of the downdraft into the surrounding updraft. Combine the effects of weathercocking again, but nose down this time, with a strong control input which is already held nose down, and that explains the violently large and rapid nose down pitch. Did it exceed structural limits and break up? It obviously goes into strong negative 'g' territory from 12.20 onwards. I can't see anything more complicated than that scenario.

I've found the accident report now. I thought I didn't remember this one! The inflight structural break up explains the damage to the artificial horizons- it's not possible it was structurally sound and pitching 90 degrees nose down! There is no way the integrity of the fuselage could be partially maintained if it pitched enough to damage the AH stops- sadly I think the spinning and pitching that occured to the cockpit as it disintegrated and after caused the violent movement that damaged the stops. We're looking for mysterious effects that just aren't there because of the faulty premise given to us earlier.

[bookworm]

Phugoid is too long period a motion, typically it's measured in minutes, this was over in a minute or so it seems.

Not sure I completely agree with that. In the simple approximation the phugoid period is sqrt(2)*pi*v/g. So at 135 m/s (about 270 KTAS) the period is about 60 seconds. I'm not suggesting the aircraft went through an entire cycle, just a quarter cycle.

The traces in PickyPerkins's post look mostly like phugoid motion, don't they? If you picture the airspeed at 11:50 as an equilibrium value, a sudden increase with a maximum at about 12:00 leaves the aircraft with excess lift. What follows is broadly, constant AOA, with altitude increasing, airspeed decreasing 180 degrees out of phase, pitch attitude leading the altitude by 90 degrees. Thus it looks like the phugoid will have gone through a half cycle by about 12:25.

However, it looks like the crew intervenes at just after 12:20 with a great big downward punt on the controls, pushing the nose, and therefore AOA, down by 15 degrees. And they did that at just the wrong time, because the nose is about to go back down of its own accord, so instead of halting the oscillation, they exacerbate it.

Of course, since the modes are coupled, it's more complicated than that, but that looks like the broad idea.

How do the numbers work? An airspeed excursion from 325 KTAS to 365 KTAS (250 KIAS to 280 KIAS) is "worth" a bit less than 1200 ft of energy. It looks like the amplitude of the excursion was rather more than that, but that may be disguised by the aircraft being in a climb to start with. Or it may be that the airspeed started below 250 KIAS -- we don't know what went on before 11:50.

[Notso Fantastic]

We;re not talking phugoids here. The weather conditions here were typical Florida storm activity- no doubt severe turbulence with powerful up and down drafts. I believe any phugoid effect would have been obliterated by the immediate vertical wind components.

[bookworm]

That's not what the data says, NF. An updraft or downdraft would have resulted in spike in AOA, and a sharp change in pitch attitude as it returned to its trimmed value, over the timescale of that mode which might be a few seconds at most.

No doubt that was happening on a small scale, contributing to the confusion that must have it very hard for the crew. But I don't see how you can dismiss significant horizontal gusts in those conditions.

[Notso Fantastic]

Where is the data support for gusts? Speed changes don't seem very great until the aircraft departs. This is an early 60s accident with very primitive flight recording. The conditions were serious Cbs. The main effect is violent vertical drafts. I think there is absolutely no evidence to support a hypothesis that horizontal gusts played any part whatsoever. This was an aeroplane torn apart by serious vertical acceleration probably coupled with large control inputs. Since those days, we've learnt to 'ride' it rather than try and maintain attitude/altitude.

[PickyPerkins]

For those wanting to see the full report on the crash of NW 705, a pdf version can be seen at
http://www.avsaf.org/reports/US_reports/1960/1963.02.12_NorthwestAirlines_Boeing-720B.pdf

If this link does not work for you, go to
http://www.avsaf.org/reports/US_reports
then click on
1963
then click on
Feb. 12 1963
and you will get to the same place.
Cheers, http://home.infionline.net/~pickyperkins/pi.gif

[bookworm]

The full graphs at the end of the report are interesting, thanks for the link. It's difficult without more detail on the "Flight Path Analysis" to draw firm conclusions: it may be that the AOA trace was inferred using some fairly weak assumptions -- it's not from the flight recorder.

I don't think we'll ever know with certainty what happened. The CAB cites the interaction of vertical gusts with control inputs. I find it difficult to see how the vertical gusts could in themselves cause the pitch-up in a way that is consistent with the data. I can certainly see that they could cause confusion and control inputs that made matters worse.

If the aircraft were to encounter a strong downdraft, a pitch-up would be accompanied by a spike of downwards acceleration, until the AOA recovered to its trimmed value. The excursions between 7:00 and 11:00 in the trace show that sort of effect.

You can argue that there's some evidence of that in the traces around 12:05 (though the acceleration never falls below 1g), but you can easily also argue that it was in response to a control input, as has been inferred in the elevator and stick force traces. Between 12:05 and 12:10 there seems to be a nose up control input, and if the aircraft was already above its trimmed airspeed, that's only going to make matters worse.


The pitch-up seems to be a combination of being above trimmed airspeed (whether or not I can persuade you that that was due to a horizontal gust), and a nose-up control input. There's little evidence that it was caused by a downdraft. Regardless of the cause of the pitch-up, what broke the aircraft was the irrecoverable dive induced by the subsequent sustained full nose-down control input.

To address wsherif1's original point, I think the CAB does at least speculate (page 42) on why the nose-down input was held for so long -- a combination of the negative g (perhaps they couldn't even reach the controls?) and the lightening or reversal of stick force at high down-elevator loads on the 720.

[Notso Fantastic]

Bookworm- I think that is as far in agreement with the events as I can see. Flight recording then was extremely primitive and caution should be attached to these readings.
PickyPerkins, I've been thinking about that excerpt from the CAB report and I think it is simplistic. An updraft will induce an instantaneous weathercocking nose down. Due to the inertia of the aeroplane, it will tend to continue on the same horizontal path with increased AofA. When it comes out of the updraft, its AofA may even be negative, giving a strong pitch up momentum- if it then rapidly moves into downdraft territory, there will be a cumulative effect and it will provide a very powerful increased weathercocking nose up effect. To counter this, anything up to full nose down control may have been applied. I think the catastrophic event occured just after 12.20.

[PickyPerkins]

I see that when the altitude returns to the same as it was at the beginning of the event (red arrows), the airspeed has also returned to the same or a very slightly lower value.

http://home.infionline.net/~pickyperkins/NW705_03.gif

This means that very little enery was gained/lost overall from the up/down draughts during this period of 32 seconds, although at the end of this time (at 12m 32s) the elevator was full down, the AoA was -11 degrees, the aircraft was pitched 90 degrees down, and the crew was experiencing a negative 2.5 g acceleration. All this with little or no change in total energy.

Isn't almost no change in total energy (potential + kenetic) during altitude excursions a characteristic of a phugoid?

Of course once the crew intervened it ceased to be a simple phugoid, but there was still negligible overall energy input from the up/down draughts.

Cheers, http://home.infionline.net/~pickyperkins/pi.gif

[PickyPerkins]

I thought it might be interesting to try to put together some diagrams of what people on this thread have said ought to happen to an aircraft meeting a short duration down draught.

The diagrams below depict an aircraft flying from left to right, first through still air, then through a down draught, then into still air again.

Case 1
http://home.infionline.net/~pickyperkins/Plane_2.gif

This diagram assumes that the aircraft weathercocks instantaneously to the new relative wind direction as soon as it enters the down draught by rotating about its center of gravity. If there is no change in engine thrust or trim, the lift vector, L, will be greater than in still air because the relative wind speed is greater, and it will act at the angle “a” to the vertical adding to the induced drag.

However I think that the increased lift and drag may be negligible and the aircraft will effectively emerge again into still air at the same altitude and speed that it started with.

For example, the lift, L, in the down draught will be increased over the still air lift by the square of the increase in the relative wind, or by a factor of (v*2+ d*2)/v*2. The vertical component of this lift is [(v*2+ d*2)/v*2][v/((v*2+d*2)*0.5)] = (1+(d/v)*2)*0.5.

We can get an idea of how big this factor (1+(d/v)*2)*0.5 is from some measurements made by NASA in 2000, when they flew an instrumented Boeing 757 through convective turbulence and in two encounters rated as “severe turbulence” measured maximum down draughts of 15.00 m/s and maximum up draughts of 18.41 m/s. However, these flights deliberately avoided direct entry into regions with the strongest radar returns.

If we assume a down draught twice as strong as the strongest measured by NASA, then d=30 m/s and the increased factor derived above (1+(d/v)*2)*0.5 = 1.02 if the aircraft is moving at 280 kts (as was NW 705), i.e. the vertical component of lift increases by only 2% with a down draught twice as strong as the strongest measured by NASA. So as I suggested above, these effects may be negligible and the aircraft may effectively emerge again into still air at the same altitude and speed that it started with.

Case 2
This case assumes that the aircraft takes a finite time to weathercock into the new relative wind.

http://home.infionline.net/~pickyperkins/Plane_3.gif

In the first transition region the plane will experience a jolt downwards which will cause it to start to lose altitude, and during the second transition a very similar jolt upwards which will leave the aircraft flying about horizontally and at about the same speed but at a lower altitude. The wind and lift vectors are essentially the same as in Case 1. The difference between Case 1 and Case 2 is only that the jolt down initiates a loss in altitude which continues until the second, upward, jolt.

Case 3
http://home.infionline.net/~pickyperkins/Plane_4.gif

This case assumes that in addition to everything that happens in Cases 1 and 2, each jolt will start a phugoid, the first initiating a pitch up rotation, while the second initiates a faster, potentially larger amplitude, pitch down rotation (assuming that there is no input from the pilot).

This sequence in Case 3 is reminiscent of the words in the CAB report on the NW 705 incident quoted in an earlier post (but talking about an up draught rather than a down draught):

Up draught causes
an initial weather cocking pitch down into the relative wind
followed “ultimately” by a pitch up and a gain in altitude.

Down draught causes
an initial weather cocking pitch up into the relative wind
followed “ultimately” by a pitch down and a loss in altitude.

I am sure that these diagrams are all too simplistic and that other effects come into play, but these sketches incorporate my impression of what people on this thread have been saying ought to happen when an aircraft flying straight and level meets a short-duration down draught with no input from the pilot.

An extended abstract of the NASA turbulence measurements is available in pdf format at:
http://ams.confex.com/ams/13ac10av/10ARAM/abstracts/40038.htm

Cheers, http://home.infionline.net/~pickyperkins/pi.gif