h a l f b a k e r yFewer ducks than estimates indicate.
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In the last few days black box information from Air France 447 has suggested that the crash was mostly caused by failure of the pitot and static systems that provide the airspeed and altitude information. The problem wasn't that they failed completely, but icing caused them to read incorrectly. If
a pilot knows that an instrument is incorrect he or she can work around that. When you don't know which instruments are working and which aren't it becomes much more complicated.
Now the actual idea:
Infrared cameras pointed at the pitot tube and static port. A computer analyzes the information to determine if there is ice forming. If ice is detected, A. de-icing equipment is turned on automatically and B. the PFD reads "inop". This would also work for problems besides icing, like a "remove before flight" flag not being removed. Alternatively the pitot tube could be located next to the window to allow for a visual check, like it is on a cessna. The second part of this idea is to look for discrepencies between GPS data and air data. If the ASI reads 350kts, but the GPS is showing a ground speed of 250, an alarm is triggered.
[link]
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I understand Airbus is offering the BUSS system (BackUp Speed Sensor, I think) for around 300,000 euros per aircraft. |
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It's an attempt to get data as reliable as a glider pilot gets from a tuft of yarn taped to his cockpit. |
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why not use the more acurate gps as the main instrument and keep the on board pressure system as backup |
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everyone would have to switch at the same time. so that everyone is at the right Fl. |
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Would it not be simpler and cheaper to just have a
heater coil around the pitot, with a thermostat to
turn it on at low temperatures? The
camera/computer business sounds unnecessarily
complex. |
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Real aircraft have a toggle switch on the panel clearly labelled "PITOT HEAT". |
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<placeholder for involved, detailed and closely-reasoned technical exposition on both the instrumentation and human factors associated with the accident> |
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<placeholder usurped for a joke about pilots keeping
their pitot tubes warm> |
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Planes operating under Instrument Flight Rules (IFR) require heated pitot tubes. Pitot heat is required to be on when flying into known ice conditions. There's no information coming from the pitot tube that wouldn't be available from GPS. |
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I suspect there was something else going on here - something more than just a frozen-over pitot tube. |
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If you're at sea level, on a windless day, and the air temperature is 15 degrees C, and the air density is 1.225 kg/m^3, and the barometric pressure is 1013.2 hPa, *then* your GPS derived SOG (speed over ground) will be equal to your TAS (true air speed). It - even after all that - may not be equal to IAS (indicated air speed), which can also be affected by the pitot's placement on the aircraft, imperfections in the pitot itself, or in the installation thereof. |
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At the altitude AF447 was flying, their IAS would have been somewhere near 270 knots. Absent any winds, that should have been about 470 knots ground speed - but just saying "absent any winds" makes the whole thing ridiculous, because we're talking about a plane flying around thunderstorms. |
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A pitot tube measures what's happening between the aircraft and the air it's immersed in. The GPS can't handle that task because there's too many variables that it can't see. |
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But GPS does have altitude information which can be used to derive true airspeed, with a better margin of error than a frozen over pitot. |
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//GPS does have altitude information which can be used to derive true airspeed// |
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For one thing, that's not quite correct (there's other factors - I mentioned some - that the GPS can't check); and another thing is that TAS is not what you want. |
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The pitot tube reading is based (partly) off "impact" or "stagnation" pressure - which is how hard the air hits the front of the tube. Since this is directly proportional to how hard the air hits the wings, it's a good measure for both lift and airframe stress. You don't have to calculate around all that air density/pressure stuff - that's what determines the impact pressure, so it comes naturally. IAS is, therefore, both easy (usually!) to get from an in-the-airstream sensor, and useful for what the pilot needs to know - in spite of the fact it's not the "true" speed. True airspeed is, usually, fairly useless info. |
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//a tuft of yarn taped to his cockpit.// I knew I had seen this idea somewhere before... |
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" you're at sea level, on a windless day, and the air temperature is 15 degrees C, and the air density is 1.225 kg/m^3, and the barometric pressure is 1013.2 hPa, *then* your GPS derived SOG (speed over ground) will be equal to your TAS (true air speed). "
[lurch], I hesitate to disagree, but air temperature, density, humidity, etc have no bearing on what your GPS will indicate. Those things will have a bearing on what your steam gauge instruments will indicate though. |
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The GPS can't tell you what your air speed is - indicated or otherwise - but it will tell you exactly what your ground speed is. *I* care what my air speed is because that equals lift, but generally as long as I'm moving forward at a reasonable clip, lift isn't an issue. |
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You might be moving forward at a resonable clip but a strong tail wind will compromise lift. One wants to measure how the plane is interacting with the surrounding environment, as this is what determines whether one can continue watching the in-flight movie. |
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//etc have no bearing on what your GPS will indicate// [phoenix], you are quite right - and that's half the point I was trying to make before I screwed it up by comparing to TAS instead of IAS. |
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