Would not it be useful to have the angle of attack information displayed on the flight display? I know that we have shakers that alert pilot when the critical AOA is exceeded, but still we have cases of pilots stalling the aircraft (sometimes stalling one wing more than other). I am sure designers have considered this but not chose to display AOA. I am curious what the reason could be.
$\begingroup$
$\endgroup$
3
asked Mar 10, 2016 at 13:44
-
2$\begingroup$ of which aircraft/flight display are we talking about? $\endgroup$– FedericoCommented Mar 10, 2016 at 13:53
-
3$\begingroup$ AoA detectors are relatively expensive in GA aircrafts. Many jet airliners have them. $\endgroup$– kevinCommented Mar 10, 2016 at 15:06
-
2$\begingroup$ It has been a recommendation from the BEA after AF447 disaster (p 205) to display the AoA: "It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required." $\endgroup$– minsCommented Mar 10, 2016 at 20:42
Add a comment
|
4 Answers
$\begingroup$
$\endgroup$
10
Many aircraft do include them, because it's very useful.
For example a modern 737:
A Garmin G1000 (one of the more common General Aviation glass cockpits):
Specifically the part that looks a bit like this
And a more traditional analog/mechanical gauge:
The problem is that usually when someone stalls, it's because they've made a mistake - not because they can't see the AoA. Either they're disorientated, or they've made a mistake with airspeed or an obstacle and are exceeding the aircraft's performance envelope.
Typically, the lack of an AoA dial isn't the reason you stall.
-
1$\begingroup$ I would take a further guess that when an accident happens due to a stall, it's because either the sensors have malfunctioned or the pilot doesn't believe the indicators. $\endgroup$– FreeManCommented Mar 10, 2016 at 16:05
-
3$\begingroup$ Would you please make the AoA indicators more clear on the glass indicators? For the 737, I believe it's the 270° circle and
5.0reading, but I'm not sure. For the Garmin, I believe I (not a pilot) can identify most of the indicators, but none jump out and scream AoA to me. Hand drawn red circles would be most useful... ;) Thanks! $\endgroup$– FreeManCommented Mar 10, 2016 at 16:11 -
1$\begingroup$ For the 737 it is indeed the 270 degree circle and 5.0 reading. For the Garmin it's that traffic-light-coloured set of lines to the right of the speed tape: the equivalent of this garmin.blogs.com/.a/6a00d83451bb7069e201b8d0737e3e970c-800wi - the segments light up relating to your AoA and equate to the green/yellow/read areas on the mechanical indicator $\endgroup$ Commented Mar 10, 2016 at 16:15
-
2$\begingroup$ @FreeMan This may be a matter of the point of view, but I think it's better to think of an accident that happens due to a stall being because the pilot has improperly flown the airplane, not because of sensor failure. The sensors are there to warn the pilot that he's messing up, but they are not the cause. $\endgroup$– TerryCommented Mar 10, 2016 at 19:27
-
2$\begingroup$ @AFischbein The AOA sensors on AF447 worked properly throughout. They are vane-style sensors not dependent on the pitot tubes. They did fail, however, on XL flight 888t $\endgroup$– TomMcWCommented Mar 11, 2016 at 1:30
$\begingroup$
$\endgroup$
1
Slightly off-topic, but I recently came across the "side string" which gives a direct reading of AoA. Every gilder pilot is familiar with the yaw string, but in two years of learning to fly gliders, I'd never heard anyone mention the side string.
Now, in the ultralight / open-cockpit world they use a single piece of yarn mounted on a stick, which acts as a combination yaw and side string.
-
4$\begingroup$ In every single type of glider I have flown, the change in noise and attitude were so clear near the stall, that I would have never paid attention to a side string like that. $\endgroup$ Commented Mar 10, 2016 at 19:45
$\begingroup$
$\endgroup$
6
It strongly depends on the aircraft we're talking about. As an example, this is the Head Up Display of an F/A-18 "Hornet":
The number near the α is the angle of attack (AoA or α) measured in degrees.
I presume that only in certain conditions knowing the exact value of the AoA may turn out to be useful (when nearing the limits maybe...). Usually, exceeding the maximum AoA is a the result of a mistake, not the mistake itself.
Fly-By-Wire systems usually do not allow the pilots to exceed the max AoA (Airbuses' Alpha Floor for instance) during normal operations.
answered Mar 10, 2016 at 13:59
-
1$\begingroup$ 20 degrees! Ouch, this guy must be pulling some serious G's. $\endgroup$ Commented Mar 10, 2016 at 22:36
-
-
1$\begingroup$ Do I remember correctly that the "W" symbol indicates where the nose is pointed and the circle with the three lines coming out of it, left right and upwards (at the bottom of the image just to the right of center) indicated the computed actual path of the airplane? If so that's yet another way to get a rough idea of the AoA from this kind of HUD. $\endgroup$ Commented Mar 11, 2016 at 0:33
-
1$\begingroup$ @romkyns: Note the speed: At Mach 0.36 the F-18 will pull 1.7g but not 7.6; this needs far more speed. $\endgroup$ Commented Mar 11, 2016 at 6:02
-
2$\begingroup$ @PeterKämpf, romkyns, IIRC the number 7.6 should be the maximum pulled 'g' during the flight. $\endgroup$ Commented Mar 11, 2016 at 9:29
$\begingroup$
$\endgroup$
3
As others have noted many aircraft do have AOA indicators. Military A/C tend to have them next to the HUD on the left side.
The advantage using AOA over stall speed is that the stall AOA is constant while the stall speed depends upon weight. Greater weights => lower stall speeds.
This difference will be important when there are large differences in A/C weights. If your F-18 has 4x2000lb bombs the stall speed is much different depending upon whether those bombs are dropped or not.
With your average cessna, the weight change of the A/C/ does not change the stall speed significantly. An AOA indicator does not provide as much value relative to its cost and weight.
-
1$\begingroup$ And with your average Cessna (or Piper &c) you tend to fly a lot more by "seat of the pants" than by instruments (or at least I do). $\endgroup$– jamesqfCommented Mar 10, 2016 at 19:40
-
$\begingroup$ An F-18's stall angle is the same whether or not it's carrying those bombs. What changes is how fast it has to fly in order to stay in the air without exceeding that angle. $\endgroup$– VikkiCommented Apr 21, 2019 at 3:23
-
$\begingroup$ You are correct. A typo on my part. As the other paragraphs indicated, it is th stall speed that changes and not the AOA. Thanks for pointing it out. $\endgroup$ Commented Apr 21, 2019 at 13:26