Inverted, unloading and reducing angle of attack on Modern Jet Aircraft
 

We are having a real active discussion at work on upset recovery training on a jet and in particular High angle of attack when inverted. My understanding has always been that even when you are inverted with an high angle of attach on the wing you have to unload the wing by "pushing".

The argument comes back that if you "push" when you are inverted then you are going to make the situation worse.

Would welcome some advice.

Unload is reducing the G load, so inverted or anything else, it means pushing less or pulling less, if you look up vg diagrams, it explains nicely. But basically if you reduce the load factor you reduce your stall speed.

As long as you have AOA to provide sufficient lift, rules are the same for all attitudes.

I assume you mean inverted with a positive AoA, ie similar to the top half of a loop. If the nose is still above the horizon you could unload then roll wings level, but you'll invariably be decelerating throughout - so you'd need to have plenty of speed first to avoid running out (stalling). Better to wait until the nose is just below the horizon prior to gently unloading and then rolling wings level. In the latter case the speed will either gently increase or even remain stable. Of course if the nose is already well below the horizon (speed rapidly increasing and g load increasing) you'll need to be more aggressive with the unload prior to rolling wings level. Selecting idle power and perhaps a tad of spoiler might help control the speed, but it's now a careful balance between pulling too hard and overstressing the aircraft or not pulling enough and overspeeding the aircraft. Of course the latter assumes you're well clear of the ground so not a factor. Do you really need to pull 3.5g when say 2.5g may suffice?

Depending upon the trim settings, the unloading may be a relaxation of the pull or indeed a positive - but gentle - push.

It depends.

To unload the wing whilst inverted in level flight, you would have to "pull" (i.e. descend to reduce the g.)
If you were flying a vertical loop and pulling g over the top whilst inverted, then you would "push" to reduce the g and reduce the stalling speed.

We have this slide on our briefing

http://dfa5nxhqcvh4w.cloudfront.net/...image01311.gif

If the wing only knows AOA, even upside down would not pulling in level inverted flight increase the AOA and thereby decrease the stall margin

If the question is " level inverted flight " then that implies minus 1 g. I.E a " push "

unloading would mean " stop pushing " .....

Why does level inverted flight mean -1G?

Hi bohpilot,

I can see why you are confused - the diagram has an error.

The inverted aeroplane's velocity vector would be more towards earth relative to the aircraft axis. (diagram shows the velocity vector impossibly giving lift in that attitude)

For level inverted flight, the aircraft axis would be have to be above the velocity vector axis.

In level inverted flight, the wing is still producing lift so why would the aircraft axis be above the velocity, would they not be the same ?

As long as we are not assuming that the aircraft is in level inverted flight, the diagram would be correct in my opinion. To me, the picture is correctly depicting the descending part of a looping with some substantial "pull" (e.g., following video at 1:06-1:08).
https://www.youtube.com/watch?v=gcfFTZmq7wM#t=1m05

When inverted, the lift direction would have to be towards the belly (not the roof).

Level " right way up flight is 1 g "......
in order to sustain level inverted flight you need the same force in the opposite direction .....-1 g.........

Hi PDR1,

We agree. See post #5

Upset recovery: Unload (reduce alpha to near zero) first, followed quickly by a roll to get normal non-inverted flight. Then pull to get level flight.

If you don't reduce alpha to near zero first, before rolling, then your alpha becomes sideslip, which hits you with a lot of yaw.

If you're flying level upside down, you do pull, not push, to unload.

Maybe the quick (training) rule of thumb is to always pitch your nose towards your velocity vector, period, before rolling to the next orientation you want.

In addition to lowering stall speed, unloading while upright or inverted increases roll rate and effectiveness of ailerons. As remarked by the NTSB in the US Airways 427 accident in Pittsburgh PA, the hard-over rudder accident in the B737-300, the recovery was partly stymied by the crews attempt to pull out. The resultant high angle of attack situation this caused greatly reduced the effectiveness of the ailerons to stop and correct the roll.

As in any airplane in any attitude, if you want to unload the wing (and move away from a stall) you have to move the stick toward the middle, in order to reduce the AOA. If the stick is back and the AOA is high, that means moving it forward. In the high attitude inverted situation, this will allow the (presumably) low-speed critical airplane to pitch below the horizon (via the natural, longitudinal stability, or "lawn darting" if you will) with an AOA close to zero, and therefore a stall speed close to zero. Just the way you want it.

In any technical discussion by non-engineers, a lot of (even basic) detail can get lost in assumptions and mistellings. You didn't say what your friend says will be made worse, but I'll take a stab. He probably took "push" to mean more than moving the stick forward toward neutral, but rather past neutral and even more forward into negative AOA and therefore negative G. That could then prevent the nose from coming below the horizon, and force the speed to decay even more (not to mention what will happen inside a transport plane at negative G).

There is nothing wrong with the diagram. All of these configurations are possible, momentarily. The inverted airplane shows what is happening about 2/3 of the way through a loop.

You're assuming it's trying to show sustained inverted flight, but it's not. If it were, then yes it would need a negative AOA (what's shown is positive) and therefore aircraft axis above the velocity vector, as you say.

In level inverted flight, the wing has to produce negative lift, i.e., the lift vector is pointing out the belly instead of the roof. This requires negative AOA, which means the aircraft axis is above the velocity (seen by an upright observer)

Your last phrase "would they not be the same" belies a deeper confusion, which is enabled by the efficient cambered wings on our airplanes, that make lift at zero AOA at high speed. Imagine for a few minutes that all wings are symmetrical, and really inefficient at that [edit: earlier I typoed and wrote the opposite]. That means that to make any lift, it has to fly at a definite, high AOA. Zero AOA = zero lift. High AOA = high lift.

Even in normal cruise (level right side up flight), then, the aircraft axis has to be above the velocity. If they were aligned, then the AOA would be zero and no lift could be created. In level inverted flight, the exact same thing is happening. The aircraft axis has to be above the velocity, (or below, if this is being considered by someone sitting in the plane. Put it another way, in both cases the velocity is aligned exactly with the horizon, and the leading edge of the wing is pointed a bit at the sky.

When considering large scale effects of all-attitude flight, this simplification is more than close enough to use, and should hopefully make things clearer.

I'm no expert but I have to say that reading through some of these posts makes me wonder if this is really a professional pilots' forum.

Come on guys, get a grip!

Umm, eckhard, so pre AF447 would everyone be agreeing that 'stop aeroplane descending = pull stick back'? My point being better to discuss things (down) here rather than up there! It's possible that not everyone on this forum has had the benefit of flying aerobatics and/or relatively big aeroplanes at extreme attitudes!

I suspect that some of the confusion you are reading is pertaining to the initial scenario, ie is it a positive or negative AoA. I think everyone is agreeing on what recovery actions apply to which scenario.

If you've allowed the aircraft to get that f..led up, then you should let the other pilot have the controls. Someone was not watching or paying attention to what was happening.

I should have said "Just let go of the stick to get alpha down." similar to what you said. Now I remember what we used to say to F-18 pilots. My engineering answer doesn't work well for training aids, as in "pitch toward your velocity vector", fine to say on the ground as an engineer, but in flight, just "let go of the stick a second" is easier.

Yes H Peacock, I agree that it's much better to discuss any subject on the ground and I firmly believe in the concept of 'the stupid question is the one that isn't asked'; however, I am still surprised (and a little disappointed) that professional pilots are confused about the relationship between attitude, angle of attack and flightpath.

Still, let the discussion continue and I'll get back under my rock.

The concept is difficult to grasp, unless you ve been inverted a few times to develop orientation and situational awareness during the upset. And grasped the right reflex for recovery.

And just like controlling drift in a skid with a car, the proper actions are counterintuitive and must be learned.

When inverted the only way out is to push and roll towards the nearest horizon.
100% of non trained pilots will, out of fear and confusion pull whatever the upset.

We are trimmed for +1g for straight and level flight.
If for some reason we end up inverted, this trim situation alone will try and keep the trimmed attitude i.e. maintaining +1 G ending up in an inverted dive, instead of pushing forward to reach -1g and prevent the nose from dropping.

Therefore the lift vector inverted and trimmed for +1g will result in a dropping nose. Add to this the wrong instintive reflex to pull and its a killer non survivable half inverted loop that if started at cruise speed will end up at Vne and well beyond. If the aircraft miraculously does not break apart, to recover from the dive...one would need at least 4g's at normal speed.. beyond Vne, the load factor for the same amount of pull or push on the stick or yoke is proportional to the square of the speed. Double the speed, the load factor increases fourfold.

Invest in additional training, trying aerobatics, in stages, to increase your confidence, sense of orientation, smoothness at the controls, coordination...Across the whole range of attitudes and speeds.

The best investment in safety and to stay out of trouble...still 99,9 % of pilots will not do it.

Wise words indeed. Sadly I agree that most pilots will not have the opportunity to do it.

"Seat-of-the-pants"
 

Isn't this one of the rare cases when the seat of your pants tells you the right thing: If your butt is being pressed into the seat, unload the wing by pushing; If you're being yanked up against the seatbelt, unload by pulling? Irrespective of attitude? Or does the inverted case cause following the seat of your pants cause you to pull into the downward half of a loop?

In all likelihood, the seat of the pants will tell you to do the wrong thing here. After years of teaching acro and unusual attitudes, my general impression echoes what Markkal said: the vast majority of pilots will not build the mental orientation required in the split second available to them and will immediately do the action that comes naturally in all panic-type situations: pull.

Part of the problem is that, as he said, the plane is already trimmed for +1G and even if the pilot does nothing, will quickly begin to nosedive.

The other part of the problem is that through many years and thousands of hours spent within 25 degrees of bank and 15 degrees of pitch, brains build up an incorrect model of how the controls work (on top of simplistic theory taught on the ground): that the ailerons are for turning the plane left and right, and that the elevator is for going up and down. That set of notions works most of the time in standard transport profiles, but breaks down when you get outside those confines. But if that's the model in the brain, when pilot finds himself upside down with the nose plummeting, you can't really blame him when he naturally calls up the only reaction that has always yielded him the "go up" result within that model: to actuate the "go up" control and pull back on the yoke.

The correct model is centered around the lift vector, and how the controls manipulate it. Fighter pilots talk in these terms a lot, as do good aerobatic instructors. (Mediocre ones just list off the steps for performing maneuvers.) The ailerons are for aiming the lift vector (which always points out the roof of the airplane), and the elevator controls its magnitude, or strength. These 2 functions are paramount for airplane control, for the airplane just goes where the lift vector points. A brain inculcated with this model (via careful consideration of the physics on the ground, and hopefully many hours of aerobatic practice in the air) will do the right thing: first, aim the lift vector at the sky, and only then increase its strength with the elevator to pull out.

Jesus wept - and these people only started asking these questions after UPRT became mandatory?? I know that some companies tend to hire for 'low levels of imagination' to prevent having people in their flightdecks getting 'good' or 'creative' ideas... but this takes it to a new level.

To those crew: in your hearts of hearts, you knew this was a 'known unknown' - you just couldn't be bothered to ask the question or dig for the answer... only when the training department 'legitimises' the debate do we start sharing our ignorance?

Am getting too old for this :mad: at 45...

Nope.

If you are nose low and inverted, you will lose a LOT of altitude trying to pull through the vertical.

The best correction is to unload to 0 to +1/2 G (yes, it will take some forward yoke), then ROLL upright, then pull up to the horizon.

Just --- let ---- go ---- of ---- the ---- stick (to unload).
Or is it better to say "Move stick to middle."? I'm a flight control engineer, so training experts jump in here please.
Simple rule. Training brains for dynamic situations requires simplicity. Don't make'em integrate differential equations while in a panic situation.

Yes stop asking questions everyone, haven't we all agreed that we know everything now ?

Your post encapsulates the reason that I rarely visit Pprune, condescension and ignorance ....... which companies hire for " low levels of imagination "?

You didn't really add to the discussion, just abuse .......

tell me do you know what a V/G diagram is ?

UPRT ..... it's a new direction and worth discussing

certainly not worth being abusive about if people are asking ......

perhaps we could discuss CRM ?

Jesus wept ? lol

It's kind of a paradox in upset training, that those best equipped to recover from upsets and spins are also those best equipped to avoid them. It's about understanding and confidence.

Yes you definitely should tend toward the simple in matters like this, but not into the too-simple. And "let go of the stick" is too simple. As has been mentioned earlier, the trim may be doing a lot to you that cannot be left ignored, and the thrust-pitch couple on top of it, even more so.

Without doing quantum chromodynaimcs in the cockpit, "Move the stick toward the middle" is simple enough, and accomplishes something. "Move the stick toward the middle such that you're not seeing any more pitching motion," may be better yet. Hopefully people already have some sort of map of elevator action vs. the scenery moving up or down in the front window, etched in their mind.

Terms like "thrust-pitch couple" and "velocity vector" are too much to have running through one's mind during their first real-life upset, but need to be considered and understood on the ground. A good ability to "do some of that pilot !!!!" is built on top of a good theoretical understanding of what it is, and why, you're doing. You're not gonna quickly and instinctively put the lift vector in the right place, unless you solidly understand what a lift vector is (as well as what effects it, and what it effects) in the first place.

There's no need to have a name like von Karman or Whitcomb to be a successful pilot, Yaeger and Hoover (the country bumpkins) did just fine. But if aerodynamics to the level discussed in basic pilot training material is too much to handle for someone, then there are plenty of hobbies and professions more suitable than flying for that person.

If you push inverted,you will most likely stress the airplane with negative g.

Just roll level...eventually the ADI will display the proper attitude.

Roll upright and recover from the dive...that's the best method

A disoriented pilot, who had never been inverted before, who is inverted with local "positive" G, has very little time to react before he will be nose down, accelerating toward the ground at or above Vne. The risk of unloading - even a bit of negative G - is MUCH less than the risk of a terminal dive. Also, rolling with positive G will stress one wing more than the other, increasing the probability of damage.

UNLOAD, roll, pull. Throttles to a mid position (above idle, below cruise) simultaneously.

Note that the SAME technique works for nose high. The roll is to knife-edge (or slightly beyond 90 deg) in this case, allowing the nose to fall toward the horizon. Also, you do less damage to your situation if you roll the wrong way initially.

There is no point suggesting what one must or must not do, when a pilot is confused and disoriented the chances of him suddenly acquiring the right reflex and skills to get out of trouble are nil.

There is an old adage I've heard somewhere, when !!!! hit's the fan, we will not raise to the level of our expectations but sink to the level of our training.

How this sudden interest in UPRT Loc prevention training will materialise, is puzzling me, we are expected to adress emergency situations, when automation dependency and limited training leaves a lot to be desired in terms of airmanship to deal with normal operations without FD, AP and AT.

Don't get me wrong, I am not faulting crews, but rather the industry regulators, a/c manufacturers and training organisations which have led to this situation and now don't know how to extricate from it.

However, given that most pilots who do not do military training will never see inverted flight in a real airplane, a practiced mantra is better than nothing. Chair-flying the "unload, roll, pull" routine will be as good a fallback position as any, and better than most.

I've taught both air-to-ground and air-to-air tactics, and recovery from all types of stalls and spins. I fully realize that no amount of chair flying or sim training will prepare someone for real, inadvertent, inverted flight (whether a stall or wake turbulence or something else). Ideally, those pilots will go out and pay for some acrobatic and spin training just for the experience...

Following on from Markkal: Slight thread creep, but I wonder at the merits of mandatory UPRT in a civil airliner (simulator) when there are airlines out there who do not encourage manually flown visual circuits. The 3 yearly 5 mins rock&roll regulation satisfying tick the box item might be considered to be a sop to those who feel a need to be seen to be doing something to improve pilot handling. Does it really? For an airliner to get into such a predicament would likely to be accompanied by some major failure first that has resulted in LOC. In the sim UPRT is conducted in a perfectly serviceable a/c. My imagination is struggling to see how an airliner can get into that situation. OK, at the top of the altitude envelope, sneaking over a TS might cause something hairy, but you shouldn't be there in the first place; micro-bursts seem to cause vertical displacements; vortex wash causes roll. What are the numbers of annual worldwide occurrences of these that requires mandatory training? I can think of much more likely scenarios that will cause a combination of handling difficulties with additional instrument display failures. Have an engine malfunction with associated loss of pitot data and see what the guys can do. Bird strikes don't always cause just a loss of thrust. You can add in volcanic ash issues. For me these would be more rewarding & meaningful learning sessions.

Given the discussion above about pushing and pulling on the stick, I'm minded to point out that the stick is connected to the less (perhaps, least) powerful pitch control on the airplane.

An awareness of the stab position and movement and, for aircraft with a pitch/power couple, the thrust, is essential. Either or both of these may have played a role, or be playing a role, in creating the situation you're in.

Stick-neutral is stick-neutral, not pitch-input-neutral.

Stab position and/or thrust may very easily overwhelm the stick input.

You may need not only to move the stick, but make aggressive changes to trim and/or thrust too.