Angle of attack and pitch attitude
 

I've just read Bill Palmer's book 'Understanding Air France 447', and I'm a bit puzzled when he mentions the aircraft at times having a very high angle of attack (30 to 40 degrees) when at the same time the pitch attitude is around 8 to 12 degrees - I can't visualise how this is possible, surely if the AOA is that high the nose must be pitched up at a similar angle? I'd have thought that the only significant difference between the two angles would, for example, be due to the wings being at an angle to the fuselage horizontal centreline.

Disclaimer - I'm not a pilot, just a very interested and reasonably(?) knowledgeable aviation enthusiast.

Consider the case of dropping a level dinner plate. The pitch angle is 0 degrees and the AoA is 90 degrees. Don't use the good china for this experiment.

Ah, now I get it! What a brilliantly simple analogy. And I didn't need to break any china either...

Good to hear that you've cracked it.

Smashing!

Yes. Just translate the feet per minute into knots and you will also see.
Descending 12000 feet per minute is approximately 120 knots
You can have with no wind a ground speed of 120 knots in a stalling situation.
In this case you would have a path angle of -45°. If the nose was pitched 0° you would have an angle of attack of 45° too.

This poses the obvious question : to get out of a stall, one has to reduce the angle of attack. To get a more upward flight path angle, one has to regain lift, so it has to be done after being out of the stall and after reducing the angle of attack.
So, you have to pitch down to the flight path angle to regain speed and lift.
The question becomes, can you recover a positive flight path angle without exceeding some structural speed limit or structural limit, if you pitched down to this kind of angle ?

I know an airbus rated test pilot, he answered that the airplane probably would have exceeded VMO/MMO but not so significantly because of exponential mach drag, preventing the airspeed from increasing too much and the airframe from breaking apart.

Worst I ever saw (my screw-up in ACM) was air speed below 30kts with in excess of 10,000 ft/min rate of descent and 5 degrees nose up pitch. All well below base height. Thank god for nozzles!

Mog

And thank god for the altitude you had available below you I guess Mog. Base height set too high :p

In my world, known as a Fox4 :)

You have to be really trying to get to the structural limits. The test flight manoeuvres to which your friend refers involve full power dives, slightly different profiles in each side of the Atlantic but ostensibly the same.

That there is so much buffer is the topic du jour in Airbus training. Many in flight events have been reported where Aircraft have exceeded Vmo/Mmo briefly due to wind shift and subsequently ended up in alpha floor, due to pilot exuberance at dumping speed to stay out of the red.

Can't really add much to MechEngr's perfect explanation, except a couple of polishing details.

1) Most aircraft wings are installed with a slight positive "angle of incidence." That is, the line between the wing trailing edge and leading edge points slightly "up" relative to the fuselage centerline (or cabin floor), maybe 1-2 degrees. So that the floor can be level for pax in cruise, even when the wing has a positive angle of attack (and probably other reasons.) Exception might be for aerobatic aircraft, where the aircraft may often be flying upside down, and thus symmetry up or down is desirable.

2) In a "slow-motion" version of the level-plate analogy, on final approach down a 3° ILS glide slope - at approach speed - the aircraft attitude is usually slightly nose-up (2-4°). For additional lift at the slower speed, and to make sure the stronger main landing gear touch down first. While the descent path through the air is ideally 3° down. For a net AoA of 5-7° (plus any built-in angle of incidence).

Concorde, of course, had, and needed, an even higher approach attitude/pitch (10° plus), With a landing AoA even higher, thus the need for the drooping nose, so that the crew could still see the runway ahead.

https://upload.wikimedia.org/wikiped...Fitzgerald.jpg

Another polish for you pattern ;), did you know the "angle of incidence" is the "angle of attack", I didn't until put right by a highly credentialed test pilot just a week or so ago on Pprune, apparently "aircraft wings are installed with a slight positive "angle of incidence" should be referred to as simply "incidence" or "rigging incidence". Angle of incidence is synonymous with angle of attack, the same thing.

I do hope that's being said tongue-in-cheek. :O

Great analogy, in other words the angle of attack is the angle between where the aircraft nose is pointing (In the case of the dish let's suppose its falling flat therefore the "nose" would be more or less pointing towards the horizon, and where the aircraft is going: vertically down.

The angle between where our dish or hypothetical aircraft its pointing and where its going is roughly 90 degrees, which could happen fully stalled stick held back pancaking towards the ground.

The confusion comes due to the horizon (Which should be removed from the equation when considering AofA), in this particular case with reference to the horizon the pitch angle is virtually zero, though with respects to the trajectory (Pointing vertically down) its close to 90 degrees

Not so, read the last few posts on the link, or you might also refer to the glossary of "Handling The Big Jets" by D P Davies page 4 where he says, "Incidence - The angle between the wing chord line and the free air stream, (Also referred to as "angle of attack"). The word "incidence" has been adopted and used for two different things, both angle of attack and the rigging angle of the chord line to the longitudinal axis, hence the confusion in the likes of you and me. As I just found, a reading of scientific papers will see "angle of attack" and "incidence" being used interchangably.

https://www.pprune.org/aviation-hist...ircraft-3.html

Yes, fair point.

Kermode also acknowledged the ambiguity in his classic "Mechanics of Flight":

https://cimg2.ibsrv.net/gimg/pprune....597d79c9af.jpg

For the angle between the wing chord and horizon just use Pitch. It seems to me that in the US angle of incidence and angle of attack are both used to indicate angle between chord line and airflow.

Only in still air. If there's a headwind component the dishes AoA will be some number less than 90 degrees, if a tailwind the AoA will be some number more than 90 degrees.

It's the relative airflow that matters which is affected by wind.

What is stunning about AF is that probably while still descending through 10,000’ it was already unrecoverable.
The Captain realized this and says something to that effect on the CVR.
AF, Atlas 76 in Houston, Chinese 73, all unrecoverable at significantly high altitudes.

I asked my instructors about this after coming across statements like Flight Path Angle + angle of attack = pitch attitude. They said there are two sorts of angle of attack considered nowadays, the wing alpha when discussing aerodynamic theory and the aircraft angle of attack derived from that formula. I'm still thinking about that.