AoA and ASI low speed awareness
 

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?

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.

"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?

(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.

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.

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.

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?

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?

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?

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.

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?

Speed tape calculations do not require weight
 

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.

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?

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.....

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.

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?

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.

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

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.