HOT PROP ALERT By JOHN H. KEVERN 33 Root St.

New Hartford, NY 13413 Most aircraft provide no warning when the engine is stopped and the mag switches are inadvertently left ON. In this condition, any movement of the prop could cause an accidental start or kick that could result in an injury.

Right Mag

*0

—

P Leads Twisted

O •——————t

Shie ded

°

' f) V f

i

Left Mag

U

Both Right Left Off

1

3n '/

•Uv-,

Most aircraft engine ignition systems use a two pole rotary switch to enable or disable the magnetos and it is standard to have at least four switch positions, Off, Left, Right and

Radio Shack

position but OFF.

VDC 273-066

The most simple form of a Hot Prop Alert circuit uses the aircraft battery (always hot) to supply the

or equivalent

third pole of the ignition switch with power to activate a pulsing buzzer

mounted in the engine compartment. This configuration would provide an audio alert whenever the switch is in

any position but OFF. When the engine is running, the engine noise is much louder than the buzzer, however, if the engine is not running, the pulsing buzzer provides a warning that the ignition switch is ON. A variety of Pulsing Buzzers are

available from Radio Shack. An example is the Pulsating Piezo Buzzer

«

^c

Both. If a three pole rotary switch is installed, the third pole can be used to activate a pulsing buzzer whenever the ignition switch is in any

Three Pole Four Position

Buzzer silent when engine is running

\/

.

Oil Pressure Sender vnn v?n nnq

J_

or equivalent

Low Oil Pressure Switch VDC 273-066 that draws an insignifi-

Model 360 009 that includes a switch

cant amount of battery power. If you dislike the continuous operation of the pulsing buzzer when the engine is running, you can use an engine oil presure sender like the VDO

that is closed when the oil pressure is below 7 PSI. The addition of this switch will turn the buzzer off when the engine is running and developing normal oil pressure.

Readers are invited to submit entries to EAA, Hints For Homebuilders, Att: Golda Cox, EAA Aviation Center, P.O. Box 3086, Oshkosh, Wl 54903-3086. Entries will be reviewed by a panel of EAA judges. Readers whose hints are published in any EAA magazine will be awarded one of three monthly prizes by Snap-on Tools - a 3/8" Drive Socket Wrench Set, a 1/4" Drive Socket Wrench Set or a

Nine-piece Long-Handle Com-bination Wrench Set. Members are also invited to submit hints of an

Always Hot

Rotary Switch

Pulsing Buzzer

Most simple J[ / x circuit

r~

+12V

J,

302 3

ll»Jlllll.»