AoA and ASI low speed awareness

[Check Airman]

Hello,

I'm having a senior moment, and would appreciate some clarification. On a modern jet, how is AoA information translated to IAS indications on the speed tape?

Let's say you think you weigh 100T, but your actual weight is 110T. You'll notice on final that your computed approach speed is awfully close to the top of the low speed awareness tape on the PFD.

This indicates the system is calculating a stall speed higher than the one you've calculated. How does it realize that you don't weigh 100T, and that it needs to show speeds for 110T?

[FatFlyer]

On a "bus" the speed bug is driven by the FMGC when in managed mode according to the zero fuel weight entered plus fuel remaining but the Vls speed (lowest selectable speed- V ref) which is displayed as an amber line, is computed by the flight augmentation computer according to the AofA sensors. If you were heavier than expected, the speed bug might be in this yellow arc which would indicate an error, the auto thrust ( if engaged) would not allow the speed to go below this Vls speed. Can't remember what a Boeing does.

[Check Airman]

"the Vls speed (lowest selectable speed- V ref) which is displayed as an amber line, is computed by the flight augmentation computer according to the AofA sensors."

That's the crucial part. How exactly does the FAC in your bus convert AoA to speed? In other words, how does the FAC determine your stall speed independent of the FMC data. On my current aircraft, the FMC doesn't send that information to any other systems.

PS. Why can't pprune have a QUOTE button like every other forum?

[Trent 972]

PS. Why can't pprune have a QUOTE button like every other forum?:)
(A380 FCOM)
The Air Data and Inertial Reference System supplies parameters to the cockpit display (PFD and ND) as well as to other user systems such as FMS, FADEC, or PRIM.

Each ADIRU is divided in two parts:
The ADR (Air Data Reference) part and the IR (Inertial Reference) part.
The ADR part of the ADIRU provides air data parameters such as:
Total pressure, Static pressure, Altitude, Mach number, Computed airspeed, True airspeed, Static temperature, Total air temperature, Barometric vertical speed, Angle of attack, Side slip angle.

Vs1g is computed by the PRIMs.
It is a function of:
slats / flaps position, aircraft weight & CG, aircraft altitude, landing gear position
It is also computed by the FMS, for performance prediction, and characteristic speeds displayed on the MFD.

[Clandestino]

You don't need FM(G)S or FACs or PRIMs to calculate your stalling speed. Total pressure, static pressure, AoA and ADC to make some sense of it all are quite enough and that's what drives low speed cue on Q400. It's interesting to watch it bounce in turbulent approach or climb in turns, just as Alpha max red strip does on A320.

Basically, low speed awareness tapes are just alpha information superimposed on speed tape. If you are trying to fly 110 ton jet with 100t approach speed, you'll need higher AoA to fly it down the glide, that's how ADC knows you are flying closer to stall.

[HazelNuts39]

A330 FCOM 3.04.10: The characteristic speeds displayed on the PFD are computed by the FE (Flight Envelope). (...) Computations by the FE and FMGC are based on the gross weight information transmitted by the FCMC (Fuel Control and Monitoring Computer).

In Alternate Law (FCOM 3.04.27 p.7): Unlike VLS, which is stable, VSW (stall warning speed) is g sensitive so as to give additional margin in turns.

[Check Airman]

Basically, low speed awareness tapes are just alpha information superimposed on speed tape. If you are trying to fly 110 ton jet with 100t approach speed, you'll need higher AoA to fly it down the glide, that's how ADC knows you are flying closer to stall.

The AoA measures how close we are to the critical AoA. In level flight, that AoA will be reached at different speeds depending on weight.

Given that the critical AoA remains constant, how does the computer know that we will stall at a higher speed? In my airplane, the FMS doesn't send any sort of W&B info to the ADC's. So how does the computer know where to place the stall speed at different weights?

[HazelNuts39]

In level flight, that AoA will be reached at different speeds depending on weight.Since stall speed, and other speeds that depend on it, depends on weight, 'the computer' needs to 'know' the weight to calculate these speeds. For a specified configuration it is possible to calculate weight from AoA, airspeed, altitude and inertial data, but not from air data alone. Therefore the calculation is unlikely to be done in the ADC of 'your' airplane.

[TyroPicard]

For the A320... the FAC first computes VS1G (stall speed)...
"When the aircraft is below 14,500 ft and 250 kts, it computes this from current AoA, speed/Mach, altitude, thrust, and CG."
Presumably the FAC will then apply a factor to VS1G in order to display VLS on the speed tape.
Any clearer?

[Check Airman]

Since stall speed, and other speeds that depend on it, depends on weight, 'the computer' needs to 'know' the weight to calculate these speeds. For a specified configuration it is possible to calculate weight from AoA, airspeed, altitude and inertial data, but not from air data alone. Therefore the calculation is unlikely to be done in the ADC of 'your' airplane.

I figured that if all other parameters are known, weight can be roughly deduced. I remember reading that FBW Airbii would actually warn the pilot of a gross error in the FMGC weight.

However, my current plane (E145) has only a generic FMS with no performance database. The flight is not affected if we do not enter the weights.

Is it possible that the plane calculates the weight using the other known parameters, then uses the computed weight to put marks on the ASI?

[FE Hoppy]

Stall is AoA based.

AoA doesn't care about your weight.

You will stall the wing above AoA max regardless of your airspeed when you get there.

The low speed awareness tape on an EDS is AoA based hence it's relationship to the Airspeed tape it's superimposed on changes with g load.

[HazelNuts39]

AoA doesn't care about your weight.At a given speed, it does. Weight times loadfactor (that's where the inertial data come in). We're talking about the speed scale, right?

[FCeng84]

The speed tape indications for stall warning speeds are AOA based. Neither weight nor CG data is required. By comparing current AOA with known stall AOA (which is a function of current flap/slat setting) the AOA margin to stall is computed. This is translated into a speed margin to stall that is then applied to current speed.

On Boeings the speed tape includes a lower amber band. The top of the amber band corresponds to a 1.3g margin to stall such that turns up to 40 deg bank angle can be performed provided speed is maintained at/above the top of this amber band. Below the amber band is the red and white barber pole. Stick shaker fires when current speed drops below the top of the barber pole.

To repeat, all of these calculations are based on determining the margin from current speed to critical speeds based on the margin between current AOA and stall AOA. No knowledge of weight or CG is required.

[chubbychopper]

If the wing is not contaminated, it matters not one jot what the IAS is, or indeed the weight. The stall will happen whether you like it or not at VS.

The AOA indicator (if you are lucky enough to have one fitted) will indicate your margin relevant to VS. VS is 1, and you will approach it as the AOA increases through 0.6 (1.3 Vs) to 0.8 (1.2 VS which just happens to correspond to V2), and then to stick shake which will occur at a pre-determined AOA (not IAS) determined by the manufacturer. At 1.0 the airplane will stall. Irrespective of WAT.

It makes no difference whatsoever if the airplane is fast or slow, with flaps or not, fat or thin, high or low, cold or warm - AOA will tell you the truth on an UNCONTAMINTED wing. If you have an AOA indicator you need not even bother to calculate Vref, V2, best glide, or even whether you can climb another 2000 at your current weight.

Magic springs to mind when considering the usefulness of the gauge, and stupidity for it not being fitted to all turbine aircraft. Almost everything in a FBW airplane is determined by AOA, so why not let the drivers read it?

[Lyman]

Does the word "Negligence" come to mind? Stupidity does not apply. Stupid people do not design/build.

There are no fingerprints of stupids anywhere in the delivered cockpit.

Greedy? Shortsighted? Ignorant? Complicit? Conspiratorial? Oh yeah.....

[chubbychopper]

Perhaps stupid, perhaps negligent.

With hindsight perhaps the crew of many an airplane that departed into uncontrolled flight as a result of exceeding the envelope by way of excessive AoA might wish to have had the benefit of the simplest and most reliable gauges that are fitted to 90% of corporate jets.

Perhaps its indication would have highlighted the actual predicament faced by those who valiantly fought to save the AF airbus prior to a fully stalled decent into the ocean. Perhaps it would have made more sense (or less negligence) to have a gauge indicating the ACTUAL state of play, rather then an aural warning that, despite deriving its information from an AoA probe, relayed the information to the crew by way of an aural warning that was CANCELLED when it went beyond arbitrary limits.

Perhaps one day those responsible for regulating the safety of our industry will realize that some principles of flight will never change, and AoA is one of them. As much as they may be displeased by it, AoA rules outweigh all of those that regulators may dream up.

There is only one essential instrument required during flight in any aircraft in VMC - irrespective of WAT - and its an AoA gauge! It tells you all you need to know - holding speeds, approach speeds, ref speeds and much more if you are taught to read it from day 1.

[Check Airman]

To repeat, all of these calculations are based on determining the margin from current speed to critical speeds based on the margin between current AOA and stall AOA. No knowledge of weight or CG is required.

Stall AoA is constant. Now if the aircraft weight was also constant, I could easily understand how AoA information could be converted to speed information. My question is how the devil does the computer know at what speed the critical AoA will be reached?

Let's say we reach the stall AoA at 100kt when the weight is 100T. Let's also say we reach the stall AoA at 110kt when the weight is 110T. How do the aircraft systems sense that we are at a higher weight, and thus determine to change the markings on the ASI?

[Lyman]

By sussing dynamic inertial loads? Any mass in motion can give up her weight if enough strategically located accelerometers are on line. Don't tell your girlfriend.

[PEI_3721]

The airspeed equivalent AOA – min speed, stick shake, stall etc, are computed without the need for aircraft mass by relating weight to Cl (Cl alpha curve) with use of look up tables.
This involves some compromise (linear assumptions) and different tables for each configuration.
Furthermore, some aircraft require adjustment for cg, which can be approximated by trim setting.
An example is in CONVERSION PRINCIPLE-ANGLE OF ATTACK TO AIRSPEED (http://www.freepatentsonline.com/3614036.pdf)

[chubbychopper]

Check Airman,

Now I THINK I see what you are asking. You understand that the AoA gauge does not need to know the aircraft weight to tell you your margin from the stall, but you are asking how the computer transposes that information into a forecast speed which it paints onto the speed tape?

Is that your question?

If so, I dunno for sure, but my guess would be that weight of the airplane is still not required. If it knows that the margin is at (for example) 1.3VS at a given moment, and if at that moment your IAS is 130 kts, it is easy to calculate that VS will happen at 100kts.

[john_tullamarine]

How does it realize that you don't weigh 100T, and that it needs to show speeds for 110T?

Much of the basics are captured in other posts but the following may make things a tad simpler. As a sideline point, some of the ideas in a few posts are a bit wide of the mark but that's aviation - ask ten pilots a question and get twenty different answers ...

The basics are comparatively straightforward even if there is a lot of wind tunnel work and computer code cutting to make it work in the cockpit.

The underlying computer models built into the magic black boxes will know the relevant parameters which determine lift from onboard measuring systems - incidence (AoA), density, speed, and a reference area to balance up the units. In addition (and most folks either forget or are not aware of these) Reynolds Number and Mach Number affect the shape of the lift curve slope (incidence against lift coefficient) as well as incidence.

If we take the simple case of cruise flight, for instance, we can put

lift = weight = some constant x lift coefficent x speed squared

so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplane should be flying for that lift coefficient.

If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.

Stall is proportional to the square root of weight so the magic box can figure the correct stall speed and adjust the presentation made to the pilot accordingly.

Why can't PPRuNe have a QUOTE button like every other forum?

If you look along the top line (starting with the bold symbol) you will find the quote (http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/editor/quote.gif) symbol.

Simpletons like me just highlight quoted material by italicising or similar.

Given that the critical AoA remains constant

A simplistic (if useful for basic pilot training) view. But be aware that Re and M come into the lift curve shape as well as incidence (AoA). To save my having to try and write a complicated equation, you might like to have a look at any of the web sources which cover this sort of stuff, such as here (http://www.allstar.fiu.edu/aerojava/lift_drag.htm).

The flight is not affected if we do not enter the weights.

.. because the boxes can figure out what the weight is to a reasonable accuracy.

You will stall the wing above AoA max regardless of your airspeed when you get there.

.. but be careful of some sideline issues. Published stall data usually is based on certification testing at low pitch rates. Things are a bit different at high rates - accelerated or g stalls. Indeed, at very high pitch rates, the stall angle can be pushed much higher due to the formation of spanwise vortex flows above the wing.

[italia458]

I think this Boeing document on AoA might answer a bunch of questions in this thread.

http://www.boeing.com/commercial/aeromagazine/aero_12/aoa.pdf

[Check Airman]

Now I THINK I see what you are asking. You understand that the AoA gauge does not need to know the aircraft weight to tell you your margin from the stall, but you are asking how the computer transposes that information into a forecast speed which it paints onto the speed tape?

Is that your question?

That's exactly my question.

[Check Airman]

The underlying computer models built into the magic black boxes will know the relevant parameters which determine lift from onboard measuring systems - incidence (AoA), density, speed, and a reference area to balance up the units. In addition (and most folks either forget or are not aware of these) Reynolds Number and Mach Number affect the shape of the lift curve slope (incidence against lift coefficient) as well as incidence.

If we take the simple case of cruise flight, for instance, we can put

lift = weight = some constant x lift coefficent x speed squared

so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplance should be flying for that lift coefficient.

If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.



So it seems that the the weight is calculated to display speed tape information. I think I'll go re-examine the report on the Emirates A340 incident in Melbourne. I wonder what the speed tape looked like in that instance.

[Check Airman]

I think this Boeing document on AoA might answer a bunch of questions in this thread.

http://www.boeing.com/commercial/aer...ero_12/aoa.pdf (http://www.boeing.com/commercial/aeromagazine/aero_12/aoa.pdf)



A very useful document. Thanks. I'll have to have a read.

[italia458]

That's exactly my question.

The computers calculate that.

Lift = 0.5 rho V^2 S Cl

You can disregard the 0.5 for this. rho (density) can be calculated using static pressure and total air temp probes, you'd need a humidity measurement to get precise figures of density. V^2 is your TAS squared. S is the surface area of the lifting surface which can be stored in the computer. Cl would be related to your angle of attack in a certain configuration. That graph would change quite a bit and there are calculations/transformations for all changes. If you read the article by Boeing I just posted here it'll explain quite a bit. The Cl vs AoA graph would change depending on a number of variables that is explained more in the Boeing article.

The total lift at 1G will = the total weight. By calculating what the total lift is (including the lift on the horizontal stabilizer), they can know what the total weight of the aircraft is. They could even calculate the Center of Gravity! Knowing all this for 1G, they could apply transformations that would calculate the weight of the plane when it's not at 1G.

All of this stuff is easy to calculate. But does your airplane have the capability? - I don't know. I'm not intimately familiar with Boeing or Airbus systems on each aircraft so I don't know what their capability is.

[HazelNuts39]

Perhaps it is even simpler. The system interprets the difference between the current AoA and the stall AoA as a margin between the current speed and the stall speed. So in the example shown below, if the current speed is 120 kts at an AoA of 9 degrees, it would place the stall speed bug at 100 kts.

P.S. The curve shown as an example to illustrate the principle is defined in the text box on the graph, with V/Vs=SQRT(CLmax/CL). It is typical of a large jet transport in clean configuration at low Mach numbers, and will be different for each airplane type. For any given airplane type, it is subject to the variations described in the Boeing article linked in post #22.
http://i.imgur.com/JgWoG.gif?1

[john_tullamarine]

Cl would be essentially a graph of your AoA vs TAS

Probably not.

Generally presented as a curve relating incidence to lift coefficient. If a bit more generalised, it will be a parametric presentation involving M and Re.

[italia458]

John... thanks for pointing that out, not sure why I said that. I made a correction to my post.

[Check Airman]

Perhaps it is even simpler. The system interprets the difference between the current AoA and the stall AoA as a margin between the current speed and the stall speed. So in the example shown below, if the current speed is 120 kts at an AoA of 9 degrees, it would place the stall speed bug at 100 kts.

That suggestion seems the easiest for me to comprehend. Can anyone verify if it's correct? It seems elegantly simple.

[john_tullamarine]

It seems elegantly simple.

Just depends on which way you apply parallax to your contemplation.

If the machine knows the incidence (plus the other stuff), it knows the (approximate) weight which gives the Vs and V/Vs. So there is no problem representing things by a simple relationship between incidence and V/Vs. If that explanation floats your boat comfortably then run with it.

One needs to be a little careful extolling the virtues of incidence measurements - useful at high alpha but still subject to errors like everything else we deal with and a fairly coarse measurement - which is one of the reasons why the certification fraternity tends to stick with the weights and speeds side of things.

Just how the OEMs implement this stuff in the black boxes I can't say, but the underlying basics philosophically are fairly straightforward.

[italia458]

That suggestion seems the easiest for me to comprehend. Can anyone verify if it's correct? It seems elegantly simple.

It does look to be correct. But you would have to analyze different configurations and compile relationships for each config. Usually when something is depicted to be very simple, there has been a lot of simplification (that would seem obvious) - meaning that it no longer applies to the full extent of applications. I would be careful to assume that this relationship applies - as indicated - at all altitudes, at all speeds, at all pitch rates, at all Reynolds numbers, at all Mach numbers, etc. There are a huge number of effects that will change this graph. However, knowing that this is a big generalization, I don't think there is anything wrong with it.

[Check Airman]

Thanks guys. I don't expect it to be a nice smooth curve like the one shown, but I just wanted to know that the computer is relating AoA to V/Vstall. That's easy enough for my little brain to comprehend.

[Mansfield]

I think pretty much everything has been covered on this thread, but I thought another anecdote might provide a different view of the same concept.

I just re-qualified on the MD-80, thus gaining an opportunity to refresh my original instrument scan from years gone by, and relieving me of the need to remember what all those colored little bugs and notches and hash marks are supposed to mean.

However, the check airman demonstrated a simple technique for checking your actual weight before beginning the approach. The 80, of course, has an alpha protection mode incorporated in the autothrottle speed mode, so that it will theoretically prevent you from decelerating into the protected stall margin by adding whatever power it might be able to scrounge up. We manually bug up the min maneuvering speeds prior to approach based on what we think we weigh, which is of course paperwork plus known fuel. If, upon dialing the speed back for the first speed reduction, you get an ALPHA annunciation on the FMA at a speed somewhat higher than the min maneuvering bug, then you know you weigh more than you think you do. The computer is telling you that you are already at a higher angle of attack than you think you are, and thus the protected stall margin is displaced to a higher speed range.

What you hope to see is the ALPHA annunciation illuminate just as the speed bug is dialed down past the manually set min maneuver bug; that says your weight is correct.

John:

Just depends on which way you apply parallax to your contemplation.

What a marvelous metaphor...must keep that one squirreled away for future use, with appropriate acknowledgement, of course.