The Stuff QA Nightmares Are Made Of

There’s a White House petition to make the United States change from imperial units to metric units of measurement.

I got two words for you: Gimli Glider, a plane that almost fell out of the sky during the Canadian switchover to the metric system:

At the time of the incident, Canada was converting to the metric system. As part of this process, the new 767s being acquired by Air Canada were the first to be calibrated for metric units (litres and kilograms) instead of customary units (gallons and pounds). All other aircraft were still operating with Imperial units (gallons and pounds). For the trip to Edmonton, the pilot calculated a fuel requirement of 22,300 kilograms (49,000 lb). A dripstick check indicated that there were 7,682 litres (1,690 imp gal; 2,029 US gal) already in the tanks. To calculate how much more fuel had to be added, the crew needed to convert the quantity in the tanks to a weight, subtract that figure from 22,300 kg and convert the result back into a volume. In previous times, this task would have been completed by a flight engineer, but the 767 was the first of a new generation of airliners that flew without a flight engineer and flew only with a pilot and co-pilot.

A litre of jet fuel weighs 0.803 kg, so the correct calculation was:

7682 L × 0.803 kg/L = 6169 kg
22300 kg − 6169 kg = 16131 kg
16131 kg ÷ (0.803 kg/L) = 20088 L of fuel to be transferred

Between the ground crew and pilots, however, they arrived at an incorrect conversion factor of 1.77, the weight of a litre of fuel in pounds. This was the conversion factor provided on the refueller’s paperwork and which had always been used for the airline’s imperial-calibrated fleet. Their calculation produced:

7682 L × 1.77 kg/L = 13597 kg
22300 kg − 13597 kg = 8703 kg
8703 kg ÷ (1.77 kg/L) = 4916 L of fuel to be transferred

Instead of 22,300 kg of fuel, they had 22,300 pounds on board — a little over 10,000 kg, or less than half the amount required to reach their destination. Knowing the problems with the FQIS, Captain Pearson double-checked their calculations but was given the same incorrect conversion factor and inevitably came up with the same erroneous figures.

You can read the whole story of the Gimli Glider in the excellent book Freefall: 41,000 feet & Out of Fuel, which I read on a plane trip to Florida a couple years ago.

No, I have five words for you. The other three are Mars Climate Orbiter:

On November 10, 1999, the Mars Climate Orbiter Mishap Investigation Board released a Phase I report, detailing the suspected issues encountered with the loss of the spacecraft. Previously, on September 8, 1999, Trajectory Correction Maneuver-4 was computed and then executed on September 15, 1999. It was intended to place the spacecraft at an optimal position for an orbital insertion maneuver that would bring the spacecraft around Mars at an altitude of 226 kilometers on September 23, 1999. However, during the week between TCM-4 and the orbital insertion maneuver, the navigation team indicated the altitude may be much lower than intended at 150 to 170 kilometers. Twenty-four hours prior to orbital insertion, calculations placed the orbiter at an altitude of 110 kilometers; 80 kilometers is the minimum altitude that Mars Climate Orbiter was thought to be capable of surviving during this maneuver. Final calculations placed the spacecraft in a trajectory that would have taken the orbiter within 57 kilometers of the surface where the spacecraft likely disintegrated because of atmospheric stresses. The primary cause of this discrepancy was engineering error. Specifically, the flight system software on the Mars Climate Orbiter was written to take thrust instructions using the metric unit newtons (N), while the software on the ground that generated those instructions used the Imperial measure pound-force (lbf). This error has since been known as the metric mixup and has been carefully avoided in all missions since by NASA.

Now, imagine every computer system you know being rewritten to use or convert from imperial measurements to metric measurements. All the Why1K bugs in critical embedded systems. Forget flying. Forget driving. I’ll be walking.

You might be thinking, Gee, Director, aren’t you hitting the luddite thing a little hard these days? Well, I just got a smart phone and a Roku box. It’s an instinctive pushback against my entrance to the 21st century.

(Petition link via tweet.)

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