AoA instrumentation
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AoA instrumentation
I'm kicking off a bit of a research project into the use of AoA gauges.
I'm aware of the use that the USN makes of AoA gauges, pretty much across the fleet. I also know that the Jaguar used to have a cockpit AoA indicator with selectable warning limits depending upon configuration.
I've also seen various aftermarket devices recommended particularly for kit aircraft.
I believe also that the 737 has an AoA indication with a limit which displays on the MFD once flaps are selected. Is that across all models?
However, can anybody tell me what other aeroplanes have AoA indication - in particular I'm trying to get a handle on how they are used? - is anybody but the USN, for example, flying to AoA values (perhaps for approach or climb), rather than airspeed or Mach values? I've also spotted that Boeing hold some patents on a combined AoA/Mach indicator for combined limits - did that ever get fitted onto anything?
And what do Airbus do? Any other fast jets?
All wisdom gratefully appreciated.
G
I'm aware of the use that the USN makes of AoA gauges, pretty much across the fleet. I also know that the Jaguar used to have a cockpit AoA indicator with selectable warning limits depending upon configuration.
I've also seen various aftermarket devices recommended particularly for kit aircraft.
I believe also that the 737 has an AoA indication with a limit which displays on the MFD once flaps are selected. Is that across all models?
However, can anybody tell me what other aeroplanes have AoA indication - in particular I'm trying to get a handle on how they are used? - is anybody but the USN, for example, flying to AoA values (perhaps for approach or climb), rather than airspeed or Mach values? I've also spotted that Boeing hold some patents on a combined AoA/Mach indicator for combined limits - did that ever get fitted onto anything?
And what do Airbus do? Any other fast jets?
All wisdom gratefully appreciated.
G
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Hi,
Some old versions of A320 used to have AoA indicators in the cockpit, left of the captain seat. Small analog gauge/needle pointer. Did not seem to be used for any kind of purpose other than providing extra info to the flight deck.
Haven't seen any on latest A320 models.
Flex
Some old versions of A320 used to have AoA indicators in the cockpit, left of the captain seat. Small analog gauge/needle pointer. Did not seem to be used for any kind of purpose other than providing extra info to the flight deck.
Haven't seen any on latest A320 models.
Flex
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Airbus takes the AoA information along with the rest of the air data, sends it to the Flight Augmentation Computers which then compute and display relevant information on the PFD speed tape:
Green Dot (max l/d),
Vls (lowest selectable speed w/ autopilot),
Alpha Prot (below which sidestick requests an AoA, bank limited to 45, speed brakes retract, a/p disconnects, pitch up trim inhibited),
Alpha Max (cannot be exceeded in normal law)
Additionally the Alpha Floor function utilizes AoA and other parameters to determine when to apply TOGA power to extract the aircraft from a dire low energy situation.
Airbus has recently developed the Back Up Speed Scale, which in the event of a triple air data failure replaces the airspeed tape on the PFD with an AoA scale of a conventional design. Keep it in the green range, respect the chevrons. So simple I could do it.
Green Dot (max l/d),
Vls (lowest selectable speed w/ autopilot),
Alpha Prot (below which sidestick requests an AoA, bank limited to 45, speed brakes retract, a/p disconnects, pitch up trim inhibited),
Alpha Max (cannot be exceeded in normal law)
Additionally the Alpha Floor function utilizes AoA and other parameters to determine when to apply TOGA power to extract the aircraft from a dire low energy situation.
Airbus has recently developed the Back Up Speed Scale, which in the event of a triple air data failure replaces the airspeed tape on the PFD with an AoA scale of a conventional design. Keep it in the green range, respect the chevrons. So simple I could do it.
Had a little time in a North American Saberliner 40 over 30 years ago, it had an AOA indicator on the glareshield, approaches were flown with reference to this.
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Originally Posted by Cardinal
Airbus has recently developed the Back Up Speed Scale
PBL
I believe also that the 737 has an AoA indication with a limit which displays on the MFD once flaps are selected. Is that across all models?
I have flown business jets with a needle & dial display, with angle of attack as a percentage of Vs and you could bug a Vs percentage which would relate to a small "traffic light" head up display on the glare shield.
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I believe that all current Boeing production models offer the AOA as a customer option. I know that DAL and AA were the driving force behind this offering.
The B787 has a form of synthetic airspeed combined with AOA to help handle unreliable airspeed issues should they develop.
The B787 has a form of synthetic airspeed combined with AOA to help handle unreliable airspeed issues should they develop.
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Yup, customer option on the 737 for the AoA display (round dial on the PFD). However i'm not aware of any european airline using that. Quite questionable anyway as all unreliable airspeed problems in our fleet in the last 2 years or so were directly linked to malfunctioning AoA sensors, not to static port problems. Since wrong AoA inputs affect both airspeed and altitude values displayed to the pilot it is not a trivial thing.
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Essential on VSTOL eg Harrier (GR3/T4) where it allows you to assess the amount of engine v wing-borne lift required - the old '"8 Units, lad, 8 Units" chant.
Do a Hover - it avoids G
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Genghis
One of the very great advantages of having AoA available in a Harrier was that by using it (and NOTHING else) you could optimise any slow approach even if you did not know your actual weight, the ambient conditions or the delta of your particular engine versus the standard donk.
It went like this:
a. Fly conventionally with gear and flap down
b. Decide what RPM/JPT you wanted to use for this approach
c. Set that RPM/JPT
d. Control flight path with stick
e. Pull nozzle lever back to hover position
f. Speed reduced smartly and so required an ever increasing AoA to maintain desired flight path.
g. Once AoA reached the optimum of 8 note IAS and use nozzle lever like a throttle to maintain this IAS
You are now flying a fully optimised approach despite all the variables present on any given day. Amazing eh?
A few comments re some of the above:
Re a. Anything between 250 and 180 kt would do.
Re b. The selection of RPM/JPT would be based on how vital it was to achieve max performance on that approach. Clearly the greater the power the less wing lift needed and the lower the stabilised IAS but the greater the engine life counts used. If you used full throttle it also meant that you had no power available to do an overshoot and the use of nozzle lever to tickle speed (and lift) up would involve a fair height loss. Better to leave a margin from full throttle of say 3% which was about 1000lb of thrust.
Re f. Ideally you would start moving the nozzle lever forward to stabilise the increasing AoA as it approached the optimum of 8. The wing stalled at rather more than 12 in these circumstances so you could use more than 8 if you were with it. Avoid more than 12 at touchdown or you could scrape the tail bumper.
The above ’fixed throttle and varying nozzle’ technique was my favourite but rejected by the RAF who taught to leave the nozzle at 60/65 and vary the throttle to control speed/AoA. This certainly gave a crisper glide path response but you never knew the counts you would use or what your power/overshoot margin was at any time without much monitoring of the engine.
JF
One of the very great advantages of having AoA available in a Harrier was that by using it (and NOTHING else) you could optimise any slow approach even if you did not know your actual weight, the ambient conditions or the delta of your particular engine versus the standard donk.
It went like this:
a. Fly conventionally with gear and flap down
b. Decide what RPM/JPT you wanted to use for this approach
c. Set that RPM/JPT
d. Control flight path with stick
e. Pull nozzle lever back to hover position
f. Speed reduced smartly and so required an ever increasing AoA to maintain desired flight path.
g. Once AoA reached the optimum of 8 note IAS and use nozzle lever like a throttle to maintain this IAS
You are now flying a fully optimised approach despite all the variables present on any given day. Amazing eh?
A few comments re some of the above:
Re a. Anything between 250 and 180 kt would do.
Re b. The selection of RPM/JPT would be based on how vital it was to achieve max performance on that approach. Clearly the greater the power the less wing lift needed and the lower the stabilised IAS but the greater the engine life counts used. If you used full throttle it also meant that you had no power available to do an overshoot and the use of nozzle lever to tickle speed (and lift) up would involve a fair height loss. Better to leave a margin from full throttle of say 3% which was about 1000lb of thrust.
Re f. Ideally you would start moving the nozzle lever forward to stabilise the increasing AoA as it approached the optimum of 8. The wing stalled at rather more than 12 in these circumstances so you could use more than 8 if you were with it. Avoid more than 12 at touchdown or you could scrape the tail bumper.
The above ’fixed throttle and varying nozzle’ technique was my favourite but rejected by the RAF who taught to leave the nozzle at 60/65 and vary the throttle to control speed/AoA. This certainly gave a crisper glide path response but you never knew the counts you would use or what your power/overshoot margin was at any time without much monitoring of the engine.
JF
I think what GTE was refering to as the AofA display on the 737 is, in fact the Pitch Limit Indicator (PLI), which displays the pitch attitude co-incidental with onset of stick-shaker.
I think every EFIS equipped Boeing has these.
I think every EFIS equipped Boeing has these.
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The pitch limit display (eyebrows) with flaps other than up is of course standard. But there is a customer option (god knows, boeing has even more than porsche) for a true AoA display within the PFD. I believe there is a picture somewhere on Chris Bradys webpage.
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On the type of Bizjet we fly on, there's no value of AoA per se (Falcon50).
However, the AoA information is used for stall margin indications on the PFD, changing with the A/C configuration (slats/flaps/airbrakes/landing gear).
It is also used for triggering stall warning / protection (automatic extension of slats under a certain speed).
As John Farley said, stall happens at a fixed AoA value, for a given configuration, regardless of variables.
However, the AoA information is used for stall margin indications on the PFD, changing with the A/C configuration (slats/flaps/airbrakes/landing gear).
It is also used for triggering stall warning / protection (automatic extension of slats under a certain speed).
As John Farley said, stall happens at a fixed AoA value, for a given configuration, regardless of variables.
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Denti, I think thats what I said back on past 9, or at least that's what I meant to say.
Your right on regarding the Boeing options list. Only the checkbook has limits to the possibilities.
Your right on regarding the Boeing options list. Only the checkbook has limits to the possibilities.
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Originally Posted by fauconpoilu
As John Farley said, stall happens at a fixed AoA value, for a given configuration, regardless of variables.
One of the "variables" which I suggest you regard is speed. Stall AoA at, say, 0.8M is rather different from stall AoA at, say, 170 kts CAS, in almost any airplane capable of that performance variation.
I did find a source for what appears to be the paper I referenced on BUSS, by Joelle Barthe. It doesn't contain much information about the details of the system. Installing it requires, on some aircraft, replacing the ADIRUs with a model in which the AoA sensorics passes through the IRs.
PBL
One of the "variables" which I suggest you regard is speed. Stall AoA at, say, 0.8M is rather different from stall AoA at, say, 170 kts CAS, in almost any airplane capable of that performance variation.
It was always my understanding that critical AofA was a fixed value for a given airframe.