Tuesday, 8 September 2009


This week Annette has been writing about gremlins. Gremlins found their origins in the wartime RAF. These are discussed in The Gremlin Question, an evocative bit of writing on the subject from the RAF Journal, 1942
Included, in full, is the following song,  no doubt ‘sanitised’ for its appearance in print.
When you're seven miles up in the heavens,
(That's a hell of a lonely spot)
And it's fifty degrees below zero
Which isn't exactly hot.
When you're frozen blue like your Spitfire
And you're scared a Mosquito pink
When you're thousands of miles from nowhere
And there's nothing below but the drink
It's then you will see the Gremlins...

Gremlins were blamed for weird failures that occurred in the air, but were not reproducible on the ground. As aircraft became more complicated these problems became more common, but the squadron that produced this song seems to have been especially bothered by them. There are several clues why. They were a photo reconisance unit, and the pink Mosquitos and blue Spitfires flew as high as the technology of the day permitted. The song says seven miles but the best Mosquitos could make it up to 42000 ft. As mentioned in the song, it gets pretty cold up there.

There we have it, cold soak an aeroplane and parts start to shrink and jam. Back on the ground, at normal temperatures, a sticking part will often free up again.

Weird things can happened on the ground too, when the Supermarine company was ramping up Spitfire production, prior to the Battle of Britain. They installed new tooling. A jig for manufacturing wings was made with the framework bolted firmly to the factory floor. Yet wings made in this rock solid new fixture were found to vary in size. Some would fit snugly into the new fuselages being built alongside them, others would be too big or too small and had to be scrapped. Gremlins? No, the factory was built on the banks of a tidal river, on reclaimed land, and as the tide came and went the wing jig was distorted. High tide wings came out different to low tide wings.

No wonder that the RAF thought the gremlins were working for the opposition.

The writer Roald Dahl served in the wartime RAF. After seeing combat in Greece he was sent, in 1943, to serve with the British Air Attaché in Washington. While there he wrote about gremlins and presented Walt Disney with the idea for a movie about them. After this, gremlins became part of the fantasy vocabulary.

They still retained an attachment for aircraft though. There’s a Twilight Zone episode from 1960, starring William Shatner, who sees a gremlin on the wing of a plane creating mischief around the engine.

So, now that gremlins have been outed, are we safe from them? I doubt it. The folds and wrinkles of software systems offer massive scope for gremlins. In fact, there is probably a whole sub-species of gremlins just there to invoke problems around data arrays. These are probably the count-from-zero, count-from-one gremlins who hang around multi-platform systems just waiting to strike.

The Thames Barrier is a safety critical system which just has to work when called for. The control system, on the insistence of the designers, has been implemented completely without software.

On safety critical systems, when software MUST be used, it is tested extensively then changed as little as possible. What is arguably the greatest engineering achievement of our civilisation, the moon landings of forty years ago, were carried out with the minimum of flight rated software. What software was used, not much, was stored in such a way as to be almost impossible to change.

Even so, the gremlins still managed to strike. Defeated on the software front they went back to tampering with hardware. The famous Apollo 13 problem, caused in part by up rating the heater coil in a liquid oxygen tank from 28 volts to 110 volts, and not changing out a switching relay, could have been fatal. As we all know, some very smart guys saved the day. On that occasion the gremlins were trounced, but you can be sure they are still out there. Lurking, inside any machine more complicated than a bottle opener.


  1. Hi Terry,

    Once upon a time I drove a Ford Tempo that had gremlins. Every time I went around a corner the car would stall out. I got very good at restarting the engine in neutral in the middle of a turn. I took the car several times to be fixed and every time I took the car in to the mechanics it acted perfect. No stalling.

    Finally in a fit of exasperation I made one of the mechanics go for a ride with me. The car stalled and he could see that I wasn't a ditzy woman.

    Guess what? It turned out to be a problem with the computer system in the car's engine.

    I grew up with older model cars that were rebuilt from salvage yard finds and had carburetors- no fuel injection systems. Occasionally they need a poke with a stick to get the carburetor unstuck but they had much fewer gremlins infesting them.

  2. THAT'S A GREMLIN? When did gremlins get boobs and goggles and good hair?