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Flight 77 Information needed,

Crazy Chainsaw

Philosopher
Crazy Chainsaw
Joined
Aug 12, 2006
Messages
8,339
Can some one give me more information on the effect of flying at high speeds close to the ground on instrument readings.
A pilot friend of mine pointed me to this.


"Airdata are vital to successfully complete an aircraft's mission and are derived from the air
surrounding the aircraft. References 14 supply pertinent information regarding airdata
measurement and calibration. These airdata encompass indicated and true airspeed, pressure
altitude, ambient air temperature, angles of attack and sideslip, Mach number, and rate of climb.
Typically, pitot and static pressures are sensed and converted (by mechanical means in the
instruments themselves) into indications on the altimeter, vertical speed indicator, airspeed
indicator, and Machmeter. Similarly, measured local flow angles establish angles of attack and
sideslip, and the outside air temperature is measured and indicated in the cockpit. (Instruments
that can perform the conversion, such as airspeed indicators, altimeters, and Machmeters, do not
correct for errors in the input values.) These measured parameters are commonly input to the
airdata computer, which, using appropriate algorithms and correction factors (or calibrations, as
discussed later), can provide other parameters, such as true airspeed, required by the aircraft's
avionics or flight control system.

The presence of the aircraft in the airstream causes input errors to the measuring
instruments the aircraft disturbs the air that it flies through, thereby also disturbing the airdata
measurements. Figure 1 shows the airflow around an airplane wing. The air above the wing has
lower pressure than the ambient air, while the pressure below the wing is higher than the ambient
air. Compressibility and shock waves also disturb the air and affect the measurements.
Compressibility effects become important above approximately Mach number 0.3. As a result, the
static pressure around an airplane varies considerably with location. Local flow angles also differ
from the free-stream flow direction. In straight-and-level flight, the airflow rises to the wing
leading edge and falls below the trailing edge, causing errors in flow direction measurements. To
some extent these errors can be studied in wind tunnels, but wind-tunnel measurements cannot
replace in-flight measurements."

http://http://dtrs.dfrc.nasa.gov/archive/00000221/01/104316.pdf
 
Anyone have a tape of the sound of crickets?
Atleast then there would be something on this tread.
 
Crazy Chainsaw said:
Anyone have a tape of the sound of crickets?
Atleast then there would be something on this tread.
Click to expand...

you expect a highly-detailed explanation of a technical issue in less than 2 hours? At least I get paid at my job for that. You probably won't hear anything 'til tonight.
 
I scanned that article, and understood what I saw. It just says that there can be errors in the altitude and speed measurements, notes what some of the errors are due to, and discusses ways to figure out where to place sensors and methods to calibrate.

The only way I can figure this relates to flight 77 is in regard to JohnDoeX's discovery that the plane was 480 feet off the ground when it was supposed to have struck the light poles. After reading this article, it's easy to imagine that the static pressure sensors for altitude may not have been optimized on the 757 for high-speed flight at lower altitudes.
 
The only thing the article doesnt really mention is pneumatic lag. I dont know much about it except that its severity is a function of Rate of Descent(in the case of AA77, 4000 ft/min). The faster the airplane climbs or descends the worse the lag is. Modern aircraft use Instantaneous Vertical Speed Indicators that adjust for it. I've done many Air Data checks on Boeings and when I'm working the cockpit portion of the test, I always notice that the IVSI will "level out" while the altimeter is still in a climb/descent. Not sure about the difference being 500 ft, I usually only notice a lag of 100-200 ft, but then again you dont use a RoC/RoD of higher than 2000 fpm at the test set...
 
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All flight instruments have to operate accurately (within a certain error band) over the range of the normal flight envelope.

Things like the airspeed indicator have to be able to operate accurately beyond normal operating speeds (something like 1.4 times Vmo, IIRC).

The altimeter reading DOH! is using were taken from a system that was operating outside its normal range. There is a thread about it somewhere on LC. An engineer told him that he'd need to find out if the pressure altimeter system had been certified up to that speed.

If it had, then the altitudes are valid (with the normal errors).

If it had not, then the altitudes can't be confirmed as being accurate; they could be out by more than the normal tolerances. If that were the case, then someone would have to run the appropriate tests on that system to determine how large the error could be expected to be.
 
kevin said:
you expect a highly-detailed explanation of a technical issue in less than 2 hours? At least I get paid at my job for that. You probably won't hear anything 'til tonight.
Click to expand...


No I just wanted to know if anyone else found simular articles on this!
Sorry but I have not got the money to pay you would you take a wooden sculpture of a naked Female model in trade I seem to have one around here somewhere she modeled and paid for it but never picked it up.
 
Another way to look at it, and by using it you can ignore any pressure errors, is to use the pitch angle data.
The airpseed data would still be pretty accurate, so you can work out from there the rate of descent and flight path angle.
 
Obviousman said:
All flight instruments have to operate accurately (within a certain error band) over the range of the normal flight envelope.

Things like the airspeed indicator have to be able to operate accurately beyond normal operating speeds (something like 1.4 times Vmo, IIRC).

The altimeter reading DOH! is using were taken from a system that was operating outside its normal range. There is a thread about it somewhere on LC. An engineer told him that he'd need to find out if the pressure altimeter system had been certified up to that speed.

If it had, then the altitudes are valid (with the normal errors).

If it had not, then the altitudes can't be confirmed as being accurate; they could be out by more than the normal tolerances. If that were the case, then someone would have to run the appropriate tests on that system to determine how large the error could be expected to be.
Click to expand...

The person who pointed me to this article was a fighter pilot, now an air line pilot he laughed at me when I told him about the flight 77 data problems.

He said because of the way the air flow near the ground effects the instruments on the 757 at that speed, the instruments could only be off a couple of thousand feet.
I was just trying to find out if that was true he said other problems can be at play to, such as rapid decent trapping moisture from Ice inside the instruments.
He said it was simple data error, Just want to make sure he is right.

He said the reason that NASA was so interested in better instrument at the time of the test was so that stealth air craft could fly low to the ground with out using radar altimeters giving away their positions.
 
Crazy, is this a concern about black box recordings, or a concern about a pilot hitting the Pentagon using cockpit readouts?
 
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