Angle of attack indicators -- do we need them?

[Coanda F. Eckt]

And if so, when would we use them? I guess in most cases during unreliable airspeed or in manouvers at the edge of the flight envelope. Then again flying at the limits in a FBW protected airplane (i.e. the bus) you would probably relie on the protection and not spend time wathcing an AoA indicator.
Those of you who fly Boeing machines with AoA indicator installed, do you ever actually use it?

[GOLF_BRAVO_ZULU]

In Serial 25 of http://www.pprune.org/forums/showthr...=223467&page=2 , Mr Farley sums it up very well.

[Mad (Flt) Scientist]

Of course, when someone says that they want one for use "at the edge of the flight envelope" the hairs on the back of some peoples' necks stand up; do you want one so that IF by chance you get to the edge of the envelope you can use it to more safely return to a more normal flight condition (good!) or do you want one to have increased (and possibly misplaced!) confidence in going to the edge of the envelope in the first place (not good!).

Its the "safer car design" argument - do we save lives by making a design 'safer', or do people simply adjust their conduct to the presence of the new feature, taking additional risks to end up at the same overall hazard?

[None]

There are a lot of edges to the flight envelope.
I liked having the AOA indicator for turns to final and final approach. What I liked about having the AOA was that I could see errors before any other indicators would give the clue...normally before I could feel them in aircraft handling.
It's another indicator that tells me more info that's more meaningful than an ASI.
However, since we conform to the stabilized approach criteria in these big transport jets, I have learned to get along without having the AOA anymore. We're not normally anywhere near the edge of the envelope.

[misd-agin]

Accident investigators use the AOA data all the time.

Unfortunately the pilot isn't trusted with the information that the investigators use to find out what happened.

The a/c have the data, it's just sent to the FDR and the pilots aren't allowed to see it.

[Check Airman]

Originally Posted by misd-agin

Accident investigators use the AOA data all the time.

Unfortunately the pilot isn't trusted with the information that the investigators use to find out what happened.

The a/c have the data, it's just sent to the FDR and the pilots aren't allowed to see it.

The 777 has an option to include AOA data on the PFD. Never seen an airline use it though.

[misd-agin]

Originally Posted by Check Airman

The 777 has an option to include AOA data on the PFD. Never seen an airline use it though.

Wow. I'd take that option in a heartbeat.

[GearDown&Locked]

Originally Posted by Check Airman

The 777 has an option to include AOA data on the PFD. Never seen an airline use it though.

I believe the NG has that option too.

[kellykelpie]

Hi. I fly airbus and the Flight Augmentation Computers (FACs) use AoA sensors to calculate the aircraft Gross Weight. We had a problem two days ago on a four sector flight where the gross weight the FACs came up with was tonnes different than what we had entered in the MCDU. We knew the FACs were wrong just by pax numbers and cargo (we had no cargo). Yet both FACs agreed. Because of this our characteristic speeds were all way out (up to 20kts). Apparently if the AOA senser is just .3 of a degree out it can create 3000kg difference.

[Coanda F. Eckt]

An AoA indicator on the PFD is an option on 737NGs, the 767-400 and the 777. Don't know how common it is though.

Of course using the AoA to fly at the edge of the envelope would be in upset recovery, avoidance maneuvers and the likes -- but in a 'protected' FBW aircraft wouldn't you relie on the protection in such a situation, rather than using the AoA indicator?

Also an AoA will not be garanteed accurate. On jets the AoA sensor is usually not located on the wing, but on the fuselage ahead of the wing; the measure at this loaction will not be the same as true AoA for a number of reasons.

Still, given the importance of AoA, and how many automatic systems relie upon it, of course it is peculiar that the pilot doe not get the same information. Probably has a lot to do whit habit/custom and being trained to measure AoA as a function of airspeed.

I'd take an AoA indicator any day, but I'm not sure how much I'd actually use it.

[Mad (Flt) Scientist]

Originally Posted by kellykelpie

Apparently if the AOA senser is just .3 of a degree out it can create 3000kg difference.

A decent rule-of-thumb for the lift-curve slope is 0.1 CL per degree, so a 0.3 degree error is worth 0.03 CL. Your clean CLmax is probably about 1.50, so if you're flying at, say, 1.6Vs (a ballpark cruise number) your CL would be about 0.60, at which point the alpha-induced error is about 5% (0.03/0.60) so I'd expect a 5% error in an estimated weight. That would put your weight at about 60 tonnes, which is possibly plausible, so that error seems plausible too.

And to achieve accuracy of more than about 0.2-0.3 degrees takes a lot of effort and probably isn't worth it for various reasons...

[Mad (Flt) Scientist]

Originally Posted by Coanda F. Eckt

Also an AoA will not be garanteed accurate. On jets the AoA sensor is usually not located on the wing, but on the fuselage ahead of the wing; the measure at this loaction will not be the same as true AoA for a number of reasons.

An AoA sensor "on the wing" isn't going to tell you "true AoA" at all... you're going to have spanwise differences, to start with, and wing bending/flexing, and the sensor will be in the upwash field too.

Also the AoA that is actually useful is the one that is measured relative to the fuselage reference - that's the one used for 99.9% of the analyses used in design and certification. While the fuselage side-mounted sensor will also not be a direct reading of this parameter (which is why test aircraft routinely use noseboom sensors) its a LOT easier to calibrate/correct since the flow conditions are more dependable - and we position the sensors in the location best suited to minimise the flow effects.

[kellykelpie]

Interesting. Thanks Mad Scientist.

[TruBlu351]

They're good on a Hornet, so when you see 60*, you can back it off a bit!

Also helpful in flying max range cruise or endurance as you can just set the speed for the best AoA in each case.

AoA also for stick shaker/pushers.

[Old Smokey]

Originally Posted by misd-agin

The a/c have the data, it's just sent to the FDR and the pilots aren't allowed to see it.

Ummm, yes, we do get to see it, after it has been converted to more "pilot useable" presentations such as Pitch Limit Indicators (Eyebrows), Low speed indication on the speed tape, plus a few hundred other functions, some of which you probably don't want to encounter, such as Stall Warning etc.

I have flown an aircraft with an AoA indicator, nice to have, but much better to translate the information to more useful presentations as alluded to above.

Regards,

Old Smokey

[overstress]

Fly approach at x units at any weight. No tedious calculating of Vapp and is independent of any weight discrepancy.

Flare the a/c to x+y units - again removes any interpretation.

AoA too high? 'Push' the indicator down with the thrust levers.

I don't know why we don't have this on every glass equipped a/c.n