Jump to content

Mags (In an aircraft). What are they?

Guest Cralis

Recommended Posts

On my intro flight on the weekend, the instructor asked me to flick 2 switches down... they were labelled "Mag...'. It was on the downwind leg of our approach.


Any idea what this was for? I've heard it mentioned before. Something like 'Magnetos' or something. Is it like a diesel engines startup? When you start a diesel car, you gotta wait for that little coil thing to heat before you start. I don't think it's the same, as the Jab I went on runs on unleaded, I believe.


Another question: Why don't they make diesel engines for planes like the Jab? More fuel efficient, and they can delivery plenty power.



Link to comment
Share on other sites

In simple terms it is a generator that provides the energy for the ignition spark. Generally an aircraft will have two just in case one fails. A normal check before flying is to run the engine and check that both are working. In the subsequent flight if one fails the working one will keep the engine ticking over. What concerns me is turning one off in flight while preparing for landing.


There is a lot going on during the landing phase and this to me seems to be tempting fate. If one of the magnetos has failed when you turn the working one off the engine may cease to run. The downwind leg is normally flown at 1000ft AGL in our aircraft so there isn't much room for error. You are of course in the circuit and would normally be able to glide to the strip but this is no reason to potentially kill the engine! Besides, what are you going to do about it if it has failed? You are about to land anyway!


Some people do check the magnetos on shut down but I have never heard of doing it as part of the downwind checks. Nor would I recommend it.


The downwind checks I use are BUMPFISCH


Brakes - still got pressure


Undercarriage - still there if it is fixed, three greens if retractable


Mixture - rich


Propeller - fine


Fuel - on and sufficient for go around if need be


Instruments - set and correct


Switches - magnetos on, landing lights on, etc.


Carby heat - set


Hatches and harnesses - secure and locked


No magneto testing though...



Link to comment
Share on other sites

Thanks Airsick..


I may be wrong then. I'm sure it was Magneto to off. And... again, I might be wrong, but I think it was two switchs... Ah, a lot was going through my head at the time, so I may be wrong - but I'm 90 sure it was 'Mag' to off, and I'm 50% sure it was two switches...



Link to comment
Share on other sites

You might have condensed memories or mixed up what / which switches you flipped down on down wind


I did type conversion with the same school and instructor (going by your photo elsewhere) as you and can guarantee that you were not asked flick down mags on downwind


If you did well then the engine would have cut off.


Think of the mags as the ignition switch on your car – only two of them incase one of their circuits fails.


The ONLY switches you would have used on down wind would have been “Fuel Pump” on and First stage of flaps late down wind – just before base perhaps assuming you had slowed down enough



Link to comment
Share on other sites

Ah! Wait... 025_blush.gif.9304aaf8465a2b6ab5171f41c5565775.gif


Fuel pump is rining a bell...


Are the switches close to the Mags switches?


Sorry guys - I think it was fuel pump.. Damn, 087_sorry.gif.8f9ce404ad3aa941b2729edb25b7c714.gif


Then the question changes to... why would you switch off the fuel pump? Where is this fuel pump pumping the fuel?


Sorry about that, guys.


Does anyone maybe have a photo of a J170 panel? I can tell you what it was from that. I was in the left seat... and the switches were just above/to the right of my left knee.



Link to comment
Share on other sites

there are a few different switch arrangements in a Jab but normally above and to the right of your left knee will be mag 1&2, master switch, starter, fuel pump, strobe, avionics and finally flap somewhere in the middle.


I hope he was just getting you to check that both mags were on as part of the down wind checks and not getting you to shut an engine down......:raise_eyebrow:



Link to comment
Share on other sites

Guest Pioneer200

The Jabiru will have 2 fuel pumps,one engine driven and the other commenly known as an auxillery or boost pump. This second one is usually only used in the critical stages of flight (take off, low flying ,landing ) etc as a back up for if the engine driven one was to fail.


Why not leave it on all the time?


I suppose it will not last as long if on all the time!



Link to comment
Share on other sites

It should be noted that while "Mag" is indeed short for magneto, there are no magnetos in a Jabiru. The term is a hang-over from older aircraft which did use magnetos because of their simplicity and reliability. In Jab motors, the spark is generated by coils adjacent to the flywheel when rare earth magnets attached to the flywheel pass by them. The ignition is a transistorised electronic system. The "Mag" switches simply earth the coil on each individual ignition.


I always check them individually on shut down as well as runup. If you find a problem on shutdown you can fix it with less hassle than finding it as you are ready to enter the runway on departure.


On some aircraft, the auxilliary electric fuel pump provides too much fuel flow to be left on all the time.



Link to comment
Share on other sites

Assuming it were a normal circuit, fuel pump may have been turned off, and flaps raised well before the turn to crosswind @700ft .


Downwind checks and activity involve tuning the fuel pump on and flaps to first stage.


My personal advice is not to over analyse what went on as the myriad of suggestions may confuse you or even cause you to think it’s all to complicated.


Wait for the next lesson and I’m sure it will all make sense.


That little Jabiru fairly trundles along the downwind leg. It would have accelerated to maybe 110 knots after levelling out..


At that rate the ground gets eaten up quickly and a lot of activity occurs in a little time.


Had you gone up in their Gazelle things would have been a lot more sedate at 70 odd knots – but not half as much fun heh!


Go to the Jabiru web site and select instrument panels from the left hand menu.


Have a look at the second photo there from 24-5157 it is very similar to 5281 which you went up in and might prompt your memory or help in preparation for your next flight



Link to comment
Share on other sites

Thanks Macnoz. Option 2 does look like the one. And yip... it was definitly guel pump. :) So, we're OK. For some reason, I don't recall the Com1 and 2 bits, but they must have been there. The rest, I recall.


That full glass panel below is pretty smart eh? Nice and neat!



Link to comment
Share on other sites

Glad to help.


5281 has only one radio so comms 2 is not not an option. I forget whether or not it has a transponder -- if it hasnt there is an extra Radio looking thing on that photo I refered you to


Yeah the glass panel is rather nice. the only one I have flown in RAA mode is at Gostner Aviation in Camden, a friendly group that I can recommend -- one of their instructors Mozartmerv is always around these forums





Link to comment
Share on other sites

G'day Cralis, Just thought I'd clear up your mind about the coil thing you talked about, that you wait for on start up on diesel engines... They are called glow plugs used to pre-heat the combustion chambers on diesel engines only, they make it a lot easyer to start a diesel engine, and when the glow plugs die it's nearly imposible to start:sad:


And by the way they do actually make diesel piston engines for aircraft, mostly for the bigger versions like the Cessna 185 & 206 etc... look up www.smaengines.com and it'll tell you all about it......






Ps. I hope your TIF tomorrow goes well:thumb_up::thumb_up:



Link to comment
Share on other sites

There are three fuel systems in the Jabiru. :thumb_up:


Mechanical, ;)


Electrical, i_dunno and


Gravity. 018_hug.gif.8f44196246785568c4ba31412287795a.gif


If we leave the electrical one on, then how do we know the mechanical one is working? i_dunno:crying:question.gif.c2f6860684cbd9834a97934921df4bcb.gif049_sad.gif.af5e5c0993af131d9c5bfe880fbbc2a0.gif


I've never known gravity to fail. :big_grin::big_grin:







Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...