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Drop, no Stop

Have you ever wondered why we check the magnetos at the end of the flight, just before we shut the engine down? I watch people do it all the time as part of their shut-down. But when I ask them why they are doing it, very few have a good answer beyond telling me that that is what they were taught to do. Well, here’s why.

The thing to remember is that the magneto switches in an aeroplane are unlike almost all of the other switches we come across. They are shorting switches. They operate in the opposite sense to 99% of the electrical switches we see in normal life. In a normal switch, say a light switch, when you turn it on you complete an electrical circuit between the power source and the bulb. Hey presto, you have light. When you turn it off you break that circuit and the light goes out.

But magneto switches work the other way around. In other words, when the magneto switch is turned On it doesn’t connect the magnetos to a circuit in order to generate a spark, it actually disconnects a wire (called the P Lead) that otherwise earths the mag and prevents the generation of a spark. It’s designed that way a safety measure, of course. Arranging it this way means that if the switch or its connecting wire fails in flight then the magneto becomes permanently live rather than permanently dead. So it’s a fail-live system not a fail-safe system and it’s obviously better that way if we’re relying on the engine at the time.

The downside of this arrangement, of course, is that if the magneto switch or its connecting P Lead fails then the shorting circuit fails. If that happens we cannot then turn that magneto off, and so it is permanently live. That’s a good thing if you are flying at the time. But it’s not a good thing if you are trying to turn the engine off. And it’s not a good thing if someone later moves the propeller by hand and inadvertently starts the engine.

So, when we come to the shut-down the engine we first check each mag to ensure that the shorting circuit for each mag is working. So what we’re looking for (or listening for) when we operate the switch is a slight drop in the rpm to indicate that the engine is now running on one mag and not two – in other words that we’ve successfully shorted-out one mag. When we go back to both mags we should see or hear a slight rise in rpm and we check each mag in turn. Of course, we’re also checking that the engine doesn’t stop when it is running on one mag. But that is quite another thing and would indicate quite another fault. Don’t confuse the two. The main reason why we do this ‘drop, no stop’ check on each mag prior to shutdown is to ensure that the mags are really dead when we finally turn the switch to Off.

When should we do it? Well, we all tend to do it just prior to shutting down. But there is an argument for doing it earlier than that, before you put yourself in a position where a running-on engine would be an embarrassment. For example, if you had dodgy brakes (or indeed no brakes at all) and you were planning to taxi close to other parked aeroplanes or, say, to the fuel point then you’d probably want to check for a ‘drop, no stop’ before doing that. Or if you were taxiing close to people, kids or pets you might want to check the mags before doing so. After all, if a child were to suddenly run out in front of the propeller then you’d want to be sure that you can kill the engine quickly to prevent an accident, rather than discovering at that point that you can’t stop it at all.

And what if we get no ‘drop’ when we check the mags? Or, in the case of something like an AT3, what if the engine runs on after we have turned off both mags? Well clearly the first priority is to safely stop the engine. The obvious alternatives are to put the mixture control (if there is one) to Idle Cut Off (ICO), and/or to turn off the fuel cock. If you are using the fuel cock, don’t be surprised if it takes a while for the engine to actually run out of juice and stop. At tick-over revs it can take a minute or more to exhaust all the fuel downstream of the fuel cock. Having stopped the engine it is then essential to ensure that the fault is appropriately recorded because that engine is now permanently live until the fault is fixed. And that means that if someone were to turn the propeller by hand then, even if they had first checked that the switches are off and the key is out, there is a real risk that the engine will fire and chop his head off. So also consider a placard on the prop with ‘Do not touch. Live engine’ or something similar written on it.

So next time you do the mag check before shutting down, remind yourself what it is that you are checking and why. And make sure you do it at an appropriate point after landing, which is not necessarily just before shutting down.

Also, we’re approaching the time of year when the wings will sometimes sparkle with frost in the morning sun. Remember that frost or ice on the airframe will have a dramatic and potentially lethal effect on its aerodynamics. The rough frosty surface will hugely increase the aerodynamic drag on any part of the airframe where it remains present. And frost or ice on the wings or control surfaces will greatly reduce their capacity to generate lift. So, don’t skimp on the de-icing. Clean all the frost or ice off the airframe before taking to the air – or you may not get into the air at all.

Be safe, but never stop learning.

David Wood

Deputy CFI