Like railroaders with their "highball" signals and dockside longshoremen with their loading signals, airplane mechanics have long been developing their own hand signals by which pilots and crew chiefs can exchange information when the full-throated roar of engines makes hearing impossible. Now, with Army Air Forces Training Command schools turning out thousands of new mechanics a year, these signals are becoming familiar on flight strips and landing fields from Newfoundland to Guadalcanal, Los Angeles to Cairo.
Thus, when a mechanic standing in front of a plane makes a circular clockwise motion with his right hand, then makes a straight "thumbs-up" movement, the man in the cockpit knows that all is clear to start his engine. When the mechanic puts his hands out, palms down, and moves them up and down, the pilot knows that something off-key has been detected and that engines are to be idled immediately. To help the standardization of these and other signals, the most common ones have been gathered into a booklet published by the Industrial Training Department of Douglas Aircraft Company.
Like most working signals, these used by aviation mechanics arose from necessity for conveying information. They are not, like the "handy" craze which swept the nation a few years ago, meant to keep you guessing, but have been adopted to get the idea across quickly. There is little chance of mistaking that when the man on the ground pulls his forefinger across his neck in a throat slitting fashion, he means for the pilot to cut his engines; stop everything, because danger is present. Quickly, clearly the order is executed, and the first blossom of flame that the mechanic may have spotted is choked off.
In making adjustments during ground run-ups and engine checks, simple universal signals are used. The man in the cockpit watches the gauges of his instrument board while the mechanic works on the valves and pumps under the engine cowling. A thumb-up means more of whatever he is adjusting; a thumb-down, less. By using his fingers, the pilot can signal how many notches, or pounds of pressure, or degrees he wants added or subtracted, and the mechanic makes his settings accordingly.
If a man is working behind an engine, under a wing, or in a nacelle where he cannot be seen by the man in the cockpit, signals are relayed by a third man who stands on the ground where he can see both. Practically deaf and dumb because of the noise made by the engines, experienced mechanics can still "talk" to each other with their hands.
When adjustments are being made on various accessories, the men have a signal for each one. Moving the clenched fist of either hand back and forth as if working the fuel wobble pump, the man in the cockpit signifies that his fuel pressure gauge shows that the fuel pump is not adjusted correctly. Then he can thumb up or down and furnish the right number of fingers to show exactly how much correction is needed. If the oil pressure is wrong, he holds up the thumb and forefinger forming an "O".
Extending the forefingers of right or left hands forms a "V" not for Victory, but for Vacuum. When adjusting engines for idle rpm., the hand is rotated in either direction. Again the thumb is held upward for more idle rpm., down for less rpm. Holding the three longest fingers of the hand downward, and spread apart, forms a suggestion of an "M" for Mixture. This refers to the carburetor adjustment which controls the carburetor fuel-air mixture. The thumb up or down indicates whether the mixture is to be made rich or lean.
A downward "V" made with the first two fingers, crossed with the forefinger of the other hand, forms an "A" to indicate the automatic pilot hydraulic pressure needs adjustment. In testing the rhythmically expanding and collapsing rubber de-icer boots, sucking a finger in the mouth indicates that de-icers are to be tested. One finger on the other hand shows that the first sequence (on the C-53 this would be outboard wings, inner boots) is to be tried; two fingers means the second sequence (on the C-53 this would be the outboard wings, upper and lower boots). Third and fourth sequences are similarly noted, but the fifth sequence is indicated by putting the left thumb in the mouth, motioning over the shoulder with the right thumb toward the tail assembly. When the compass is being swung, a thumb-up means the ship has swung beyond the cardinal point; a thumb-down means it is short. The appropriate number of fingers are flashed to show how many compass points the ship is off.
When the pilot wants the battery cart connected into the airplane, he shoves the forefinger of one hand into the vertically-held fist of the other, symbolizing the plugging-in operation. Disconnect is indicated by "unplugging" the forefinger. The same signal done with hands held horizontally indicates that the pilot wants the landing gear safety pins pulled. These are used on the ground to keep the landing gear from accidentally collapsing, but must be removed before take-off so that wheels can be retracted.
Before take-off, the engine cowl flaps which control cooling of the power plant must be tested. Holding the heels of his hands together, the mechanic moves his palms and fingers apart in a kind of half-hearted clap. This means that the pilot should test his cowl flaps for open and closed positions.
"Can I taxi out?" asks the pilot by holding both thumbs up. "All clear, come on," replies the ground crew chief by pointing over his shoulders with his thumbs. "Pull the wheel chocks," signals the pilot, pointing outward with his thumbs. He taxis to the head of the runway, guns one engine, then the other; runs up both at once. Then, just before releasing his brakes and rocketing down the field, he glances at his mechanic, makes a vibrating "O" with thumb and forefinger, accompanies it by a wide grin. In any language that has come to mean "everything right, you're on the beam."
This article was originally published in the March, 1944, issue of Air Tech magazine, vol 4, no 2, pp 47-49, 80.
Photos credited to Douglas Aircraft Co.
The PDF of this article includes two pages of photos illustrating the signals described in the article.
Photos are not credited but are certainly from Douglas.