The X-31

Spill hit an estate sale this week, and came away with a nice little gem.

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This coin contains metal from the X-31 aircraft.

How did they get the metal from the aircraft? Glad you asked.

The use of digital fly-by-wire controls in high performance aircraft, covered by Spill here, 2, 3, 4, 5, meant that unconventional flight controls could be used on planes to maneuver in ways not previously possible. In particular, thrust vectoring could be used to control aircraft at very high angles of attack.

A joint US and German test program conceived and built the Rockwell/MBB X-31 research plane to explore this use of DFBW control in conjunction with high angles of attack and thrust vectoring.

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X-31 in flight. Notice the three “paddles” used to vector the thrust.

Two were built, and a highly successful test program showed the X-31 was capable of maneuvers that were then astonishing. Since it was purely a research aircraft, it was quite small, had a very small fuel load (typically, only 4100 pounds) at take off had a thrust-to-weight ratio of 1:1, which meant it could accelerate vertically right off the runway.

After several years and hundreds of test flights, one of the two X-31s had its pitot tube replaced by a Kiel tube. The pitot tube us the pointy stick pointing out of the nose of the jet. It measures the dynamic pressure of the air. Between it and a static air pressure sensor, the pitot system provided air data to the flight control computer to determine speed and altitude.

Remember, in a DFBW system, the pilot doesn’t control the airplane directly. He uses the flight controls to tell the computer what he wants the plane to do. The computer uses those imputs, along with air data from the pitot system, and attitude data, to determine which controls should be deflected, and how much.

Obviously, if the air data was corrupted, the computer would provide corrupted control deflections.

One of the most common failure modes for pitot tubes is icing. Moisture from clouds or humidity freezes on the pitot tube, constricting the flow of air through the tube, which makes the computer think it is going faster than it is. To combat this, most pitot tubes have an electrical heater, just like the rear defroster on your car. The normal pitot tube on the X-31 was replaced by Kiel tube, which gave more accurate air data at high angles of attack. But it didn’t have a heater. Given that the flight test rules for the program prohibited flying the X-31 in known icing conditions, this wasn’t thought to be a significant hazard.

Of course, Murphy gets a vote. The engineers knew there was no pitot heat. The pilot didn’t.  And of course, the X-31 encountered pitot icing. Not immediate, but gradual accumulation of ice led to a steady degradation of airflow, and hence data. And that led to instability, as the flight computer tried to make the plane do things that it didn’t want to do.  The X-31 exectuted an uncontrolled pitch-up, and as soon as the pilot realized he had no control, he safely ejected.

The loss of the X-31 is unusual in that it was very carefully documented. It took place almost directly over the airfield, and was being tracked by powerful cameras on the ground.

Here’s the short version:

[youtube https://www.youtube.com/watch?v=WIUtUrKMw2k]

If you’re interested, a 40 minute video investigates the chain of errors that led to the mishap. It can be found here.

And here’s a brief history of the program.