Switches - which way is on?

[4Greens]

Had an interesting talk the other day with some old aviators; we had a discussion on why, at least, in the old days mag switches etc had to be switched up for 'ON'. None of us were quite sure why.

Would be interested in views from the experts.

[Herod]

Or even non-experts? A lot of switches, particularly on the overhead panel, are down for "off". I believe it may stem from the idea of sweeping the hand downwards during an emergency drill; perhaps an early example of ergonomics. As an aside, ordinary light switches in many Continental countries are down for "off", perhaps it comes from there?

[Agaricus bisporus]

in the old days mag switches etc had to be switched up for 'ON'. None of us were quite sure why.

When mag switches were mounted outside the cockpit on the side of the fuselage so the ground crew swinging the prop could see them a hand resting on them would turn them off, not on which is safer than a hand brushing over them and switching them, and the prop, live. It takes a more positive action to select a switch up than down so up - live down - safe is the better way of doing it.

How's that?

[Shaggy Sheep Driver]

Mag switches are up for 'live'. In this position, the swith is 'off', or 'open circuit. When they are down, the switch is 'on' completeing the circuit to ground the magneto and thus disable it.

This is a failsafe system in that if the switch fails, or the wiring to the switch, it goes 'open circuit' and the mag is 'live'. If it worked the other way around, any fault in the switch or wiring would result in an engine failure!

But it does mean you should always treat a prop as 'live' if you are turning it by hand. If there is a fault in the switch or wiring, the plugs will spark and the engine could start.

[fantom]

Hmm...

Remember being introduced to the Yakee 320 sims in MSP where the switches were reversed.

'Because we are used to it that way'.

Progress? I don't think so.

[Flanflinger]

Think it is more USA logic. All switches in american houses (and elsewhere) work that way.

[Mike6567]

I seem to remember on the 707 landing lights were down for on - seemed logical.

[India Four Two]

Part of the confusion here is the different switch conventions between the UK and North America.

In the UK, a normal single-pole switch is "down for on". In the US and Canada, it is "up for on". US aircraft follow the local convention. Here's a FS image of a panel I'm very familiar with - the Bellanca Citabria:



In this picture, the left three switches are Off and the other switches (Panel Lights, Master, Avionics and Mags) are all On.

As SSD points out, when the Mag switches are ON (up in this case), they are in fact open-circuit. When the Mags are OFF, the circuit is closed and the magneto primary coil is grounded, thus preventing it from generating any voltage.

Part of the shutdown checks is to run your hand along the row of switches to confirm they are all down.

I suspect that the reason that mag switches on British aircraft are also down for OFF (I'm thinking Tiger Moth - I'm not familiar with other types), goes back to WWI when domestic light switches were used for mag switches and they were installed in the conventional sense - down to close the circuit and thereby ground the mag.

[Lordflasheart]

logical ?

1dot quote -

It's Boeing 'logic' of towards the windows is ON ie down on the overhead panel and up below the windows

I believe, historically, that Boeing reversed their 'logic' between the 747-Classic and the -400. Does that count as logical ?

It was always the cause of much hilarity when anyone dared to suggest that one model of the BAC-111 had the switches going one way, and another model, the other way, so the CAA would not allow both types to be flown concurrently. I was never sure if it was only a flat-earth joke.

LFH

[bcgallacher]

I believe that switch operation was a customer option on B747 as I have maintained them with switches operating in both directions - with the same company!

[Rick777]

Boeing is all about customization. The make it like the customer wants--at least as far as switches go. When I flew 707s we had planes from several different companies so there were no two exactly alike. Most of them had down for on for the landing lights, but some had up.

[dixi188]

IIRC most BAC 1-11s were "ON" towards the windscreen as the biggest market was going to be the USA.
But of course BEA had to be different.

[Fareastdriver]

Eurocopters are the same.

[Jhieminga]

It was a customer option on a lot of aircraft. Also when there were still FEs on the flightdeck, towards the top of the panel for ON could have been customary. Once that position disappeared, toward the nose of the aircraft for ON was more likely.

[4Greens]

Judging by the responses to my post, there seems to be little logic behind the switch position except perhaps that on a Tiger Moth when you were swinging the prop.

Still interested in any input.

[PowerDragTrim]

Shaggy Sheep Driver is correct.
The switches in, say a Tiger Moth, operate in the same way as a domestic switch - down for ON, up for OFF.
But in the case of a magneto, it becomes live when the earth line is cut - i.e. switch up, and so this is the switch position when the engine is running.

[Shaggy Sheep Driver]

Not only Tiger Moth, but Chipmunk, Cub, and Citabria to my knowledge. In fact I'd say that where tumbler switches are used for mags, it's up for 'live' (switch open circuit in that position, just like a UK domestic light switch).

[ICT_SLB]

"IIRC most BAC 1-11s were "ON" towards the windscreen as the biggest market was going to be the USA.
But of course BEA had to be different."

Dixi,
Not just BEA - all CAA-registered 1-11s had the "UP = ON = Towards the rear" Overhead Panels. As you said, all 1-11s for the US met FAA regs with all switches "Forward = ON". Switches on the main panels & pedestal were UP = Forward = ON for both agencies.
I remember we had to bring an ex-Philippine Airways (FAA) onto the British register and swap all the Overhead panel switches.

[Spooky 2]

I think this subject has been covered before but at least in the case of the Boeing line, the customer dictated the switch logic.

Most noteably was TWA who treated the overhead panels the same as the vertical FE panel, up was on, down was off. This logic exteneded beyond just the landing lights. TWA in turn did a lot of early work with Lufthansa when they started up operations in the Connies and early 707/727. I believe that LH used the same logic in their Boeing's as well. I flew a former LH 727-100 briefly and it was configured this way.

[India Four Two]

except perhaps that on a Tiger Moth when you were swinging the prop.

4Greens,

Interestingly, the Canadian Chipmunk, unlike the UK-built one, has the mag switches in a similar position to the Tiger Moth's, where they can be seen by the ground crew:

(switch open circuit in that position, just like a UK domestic light switch).

SSD,
... but a Canadian or US domestic light switch is 'up' for closed circuit, so from a North American point of view, mag switches (in Cubs and Citabrias) are installed 'upside down'.