When Unit Conversion Goes Wrong: Four Expensive Failures

Unit errors are not a theoretical concern. They have destroyed spacecraft, grounded airliners and killed patients. What makes them dangerous is that a wrong-unit answer is usually a plausible number — it passes the eyeball test in a way that a genuinely corrupt value would not.

Mars Climate Orbiter, 1999

NASA lost a $327 million spacecraft to a units mismatch.

The orbiter used small thruster firings to manage angular momentum. Lockheed Martin's ground software produced impulse figures in pound-force seconds. NASA JPL's navigation software expected newton-seconds.

A pound-force second is about 4.45 newton-seconds, so every value fed into the trajectory model was too small by a factor of 4.45. The error accumulated over nine months of cruise.

The spacecraft was expected to reach periapsis at about 226 km altitude. It actually arrived at roughly 57 km — far inside the atmosphere — and was destroyed.

The Gimli Glider, 1983

Air Canada Flight 143 ran out of fuel at 41,000 feet over Ontario.

Canada was converting to metric, and the Boeing 767 was among the first Air Canada aircraft specified in kilograms rather than pounds. The fuel quantity system was inoperative, so crew calculated fuel load manually using a dipstick reading in centimetres.

Converting volume to mass requires fuel density. The figure used was 1.77 — pounds per litre. The correct figure for kilograms was 0.803 per litre.

The aircraft ended up with roughly half the fuel it needed. Both engines flamed out in cruise.

What followed was extraordinary. Captain Robert Pearson was an experienced glider pilot and flew the powerless 767 in a forward slip to lose altitude. First Officer Maurice Quintal identified a closed RCAF base at Gimli — unaware that part of it had become a motor racing circuit, in use that day. The aircraft landed on the strip, the nose gear collapsed, and the resulting friction helped stop it short of spectators. Nobody was seriously hurt.

Institute for Space and Astronautical Science, 1996 — and a broader pattern

Unit confusion in aerospace is persistent rather than rare. Documented incidents include mixed metric and imperial fasteners causing assembly errors, thrust figures misapplied between systems, and altitude clearances misread between feet and metres in airspace where conventions differ.

ICAO standardisation on feet for altitude was driven precisely by this: the risk of inconsistency between aircraft in shared airspace outweighs the metric argument.

Medication dosing errors

The most consequential unit errors happen in hospitals, one patient at a time.

Common patterns:

• Micrograms versus milligrams — a thousandfold error. The abbreviation "µg" handwritten can resemble "mg", which is why many health systems now mandate writing "microgram" in full.
• Pounds versus kilograms — paediatric doses are weight-based, and entering a weight in pounds where kilograms are expected produces a 2.2× overdose.
• Total dose versus dose per kilogram — not strictly a unit error but the same class of failure.
• Trailing zeros — "1.0 mg" misread as "10 mg" if the decimal point is faint. Standards now forbid trailing zeros and require a leading zero: "0.5 mg", never ".5 mg".

These patterns are well documented by medication-safety organisations, and the mitigations — standardised abbreviations, weight recorded only in kilograms, independent double-checks for high-risk drugs, computerised order entry with dose-range checking — exist because the errors were common enough to demand systemic fixes.

What the failures have in common

1. The wrong answer looked reasonable. A fuel figure, a thrust value, a dose — each was in a range that did not obviously alarm anyone.
2. The error crossed an interface. Between contractors, between systems, between a written order and its execution. Unit assumptions are usually implicit at exactly these boundaries.
3. A transition was under way. Canada mid-metrication, aerospace mixing conventions. Transitions are the highest-risk period precisely because both systems are live.
4. Individual competence was not the issue. These were capable professionals. The defence has to be in the system, not in vigilance.

Practical defences

• Label units on every value — in variable names, in spreadsheet column headers, on every interface. mass_kg, not mass.
• Specify units in interface contracts and validate them at boundaries.
• Sanity-check magnitudes. Does this dose look like a normal dose for this drug? Does this fuel load look like a normal load for this route?
• Use dimensional analysis. Carry units through the calculation; if they do not cancel to the expected result, the formula is wrong.
• Independent double-check for high-consequence values, performed separately rather than by reviewing someone else's arithmetic.

This site exists to make conversion fast and correct. It cannot tell you that you selected the wrong unit — that check belongs to you, and for anything consequential, to a second person.