Magnetoes - Start up on left only or both?

[distaff_beancounter]

Something has puzzled me for a long time, & I have not been able to get an answer else where, so here goes :)

Many of the well known types of SEPs & MEPs are fitted with similar Lycoming engines. Yet some manufacturers' POHs (eg Piper) mostly state that engines should be started up with both mags on, while other manufacturers (eg Grumman) usually say start up with only the left mags on.

(A) WHY?
(B) For those A/c where the POH says left only, can they suffer any damage if started on both mags?

I ask (B) because on very cold winter mornings (like recently), some of the types that are supposed to start on left only, just won't, but will start fairly willingly if both mags are selected. So it seems to be a question of POH, versus wearing out the starter and/or flattening the battery.

Can any of the techies enlighten me please :confused:

[Keef]

I'm not an expert, but my understanding goes something like this:

Some aircraft are fitted with magnetoes of the "shower of sparks" variety. These have some springs and bits inside that do clever things to make starting easier. The showery version is always (?) fitted as the left mag, in which case you want to be on that one (only) to start.

Other aircraft don't have SoS mags, so you start on "both".

The POH should tell you whether to start on "Left" or "Both".

If it won't start on "Left" but will start on "Both", and the POH says left, then my next question:

When did you last have your magnetoes serviced? 500 hours is generally the longest they'll go without an overhaul.

The lack of start on "Left" indicates the SoS bit isn't working right, I suspect.

[distaff_beancounter]

Keef
Thanks for the nice non techinical answer, that even I could understand! :D

I appreciate that the magnetoes are totally seperate from the engine, but I sort of assumed that Lycoming would recommend the same magnetoes for all aircraft types, using the same engine. This is presumably not so.

Nearly all the aircraft that I fly are on Public Transport CofA, and are regularly maintained on the required CAA schedules, so presumably the magnetoes are checked. (BUT, I will ask the maintenance company)

[Final 3 Greens]

Distaff

If you have the patience, try this article......

http://www.vnwa.com/Oncourse/Articles/Ignition.htm

In some a/c. the left mag only is an impulse magneto (e.g. early PA28) so you start on the left mag only.

Hope this helps.

[TheKentishFledgling]

In some a/c. the left mag only is an impulse magneto (e.g. early PA28) so you start on the left mag only.

Which is why, IIRC, you cannot hand swing a lot of engines with mags selected.

tKF

[Final 3 Greens]

Kentishfledgling

I am no expert on handswinging, but doesn't 'Contact' mean mags on for starting, whereas 'switches off' means mags off whilst turning a few blades before starting?

[TheKentishFledgling]

I am no expert on handswinging, but doesn't 'Contact' mean mags on for starting, whereas 'switches off' means mags off whilst turning a few blades before starting?

I'm definately no expert either, but AFAIK, "contact" means that the required mags are on. If this is only one, then second I think is turned on when the engine's running.

tKF

[nonradio]

DB: A good spark from a mag needs a high rotation speed; bit difficult especially if you are using an Armstrong starter;) - so amongst other gizmos we have an impulse coupling: this just a device that allows the mag shaft to spin more quickly for a very short time when starting by means of a spring loaded 'clutch'. Bingo! Trouble is, you may recall the normal firing of the plug at normal RPM is before top dead centre which means, of course, that the piston hasn't reached the top of the cylinder, hence any firing of the charge would be trying to force the piston back down! Not good esp. if hand swinging ("kickback"). Thus the impulse coupling automatically retards the spark. If you have only one mag fitted with an impulse coupling it's best to have that selected only.
Hope that makes sense:D

[Shaggy Sheep Driver]

Most piston aero engines have an impulse mag incorporated into the left mag. At > about 300 rpm the mag works normally. At less than that RPM, it reverts to 'impulse' mode, controlled by bob weights.

Impulse mode is for starting the engine at low RPM (especially during hand swinging) when there are 2 problems:

1) The rate of rotation of the engine, and therefore the mag, is insufficient to generate a healthy enough spark to fire up the engine.

2) Mags are timed to fire before the piston reached Top Dead Centre in the cylinder, so that by the time the fuel burn is producing power, the piston is just beginning its downward stroke. If the mage fires before TDC at very low rpm, the power will develop while the piston is still on its way up the bore, and will drive the piston back down the way it came - kickback, when the props runs backwards a few turns, and nasty if you are hand-swinging.

The impulse 'mode' overcomes both these. The timing is retarded so it fires after TDC, and the mag, instead of being directly driven by the engine, is held stationary while the engine turns and winds up a 'clock spring', then released to 'flick' over rapidly to give a good fat spark. You can hear this 'impulse click' if the engine is pulled through by hand. You can also hear it on Lycomings when the are shut down; as the prop turns the last few revolutions before stopping, you can hear 'click, click, click' of the impulse.

Certainly for hand swinging, the left (impulse-equipped) mag only should be used. The call "contact" by the pilot confirms impulse-equipped mag on (in response to "Contact" by the 'swinger - never "switches on"; sounds too much like "switches off" and a mistake can easily be fatal). Once started, thenother mag is switched on. For electric starting, it's not that critical and using both would not be a problem except you might get the odd 'kick back'.

A shower of sparks (as fitted to Yak 52) uses a separate electrical 'make and break' device to enable the mag's secondary coil (or a separate coil) to produce a continuous stream of HT sparks which are 'distributed' by the mag in the normal way, but at retarded timing as per the impulse mag. It is only used during starting (on the Yak, the pneumatic starter button operates it) and once the engine is running the normal mags are switched on. If you take the cover off a Yak mag, you will se that the rotor has 2 'fingers'. One is for normal operation; the other (behind the main one, and therefore more ‘retarded’) is fed from the 'shower of sparks'.

Hope this helps

SSD

[A and C]

As has been stated above most engines have the impulse unit on the left mag and for all the reasons stated above the left mag should be used for hand swinging.

The normal type of mag switch fitted to most light aircraft selects ONLY the left mag when you turn it to the start position so you cant get it wrong when using the electric starter.

The shower of sparks system is something that I have not found on a light aircraft in the best part of 20 years and so for flat engines it is more or less obsolete , it is however very popular with the radial engines I suspect because of the lower crancking speeds on start.

[Tinstaafl]

Hey, A & C,

Guess which system our Islanders use... :p

[Skylark4]

Shaggy has got almost all of it in his post.
I think the impulse is on the right mag on a Chippie. Someone please correct me if I am wrong.
The Grob 115E is started with the mags selected `Both` but operating the starter button kills the right one. If the left mag is not co-operating you sometimes get a serious kickback on releasing the starter button.
The left mag has a special circuit added which feeds a voltage across the winding to produce a fat spark without a high crank speed. Can`t for the life of me remember what it`s called. Still has the impulse mech though. Probably to produce the retarded spark.

Mike W

[Volume]

(B) For those A/c where the POH says left only, can they suffer any damage if started on both mags?
I just know what frequently happens to older Robin planes, having seperate switches for both mags and the starter. The engine kickback when starting with both switches on can easyly make you lose one toth of the starter wheel or even break the starter shaft.

The Grob 115E is started with the mags selected `Both` but operating the starter button kills the right one.
I have read in an newspaper article, that any ´normal´ ignition switch kills the right magneto when start is selected. Can anyone confirm this ?

[distaff_beancounter]

Final 3 Greens
Thanks for the website reference, that was very helpful.

Volume
Thanks for your reply. So it seems that there is some possible risk in ignoring the POH & starting on both mags, instead of left only.

[AC-DC]

To the best of my memory even SoS systems use the left magneto while the right magneto is grounded.
All large injected flat engines use SoS.

[TheKentishFledgling]

Hope this helps

Muchos, thanks SSD :)

tKF

[stiknruda]

I would normally have a look at the mags before hand swinging an unfamiliar aircraft IF the pilot was not sure - what I guess I am saying is that normally aircraft that do not have electric starts have crew that understand what is required: but not always!!

IF you had a flat battery and wanted a swing but were not sure what was what re mag selection for contact then I'd have a look under the bonnet.

My Aeronca is hand swing BUT has impulse couplings on both new magnetos (exchanged 12 hrs ago). So mags to both for swinging.

My electric start equipped Pitts only has an impulse coupling on the LEFT mag - so when started on the key, the START position grounds the RIGHT mag so that only the LEFT is hot. Once it starts to run, you alleviate the pressure on the key which causes it to return to BOTH and the RIGHT mag kicks in.

IF I had to hand swing the Pitts then obviously I'd only select LEFT until she was started, then would pop around to the controls, select BOTH and adjust throttle to idle.

On my Bendix mags and my replacement Slicks the impulse coupling adds about half an inch to the depth of the mag. If you don't know what you are looking for - have someone show you before you really need to know.

Stik

[TheKentishFledgling]

If you don't know what you are looking for - have someone show you before you really need to know.

Remind me to ask you to show me when we next meet plz Stik :)

tKF

[javelin]

Further to recent posts.

The shower of sh#te magnetos are fitted to quite a few engines, our is one, a Lycoming IO360 A1A (also fitted to the IO360 A1B6 ex Bulldog engines that are now appearing everywhere- great engine, superb balanced crank, cheap one available in Gloucester). Both mags are of this type and are truly sh#te ! No volts, no start, they need power to provide the sparks to get the thing going, once going they are OK. The Robin engine soemtimes features another of Mr Bendixes, whacky ideas, the dual mag - 2 mags in one, thus satisfying the need for dual ignition. Still not good. IMHO the best system apart from Klaus Xavier's Lightspeed system is a simple Bendix/slick mag with impulse on one or both. I have never had trouble hand or electric starting these and always use both on as the start method. I still have two hands and arms, although the right one is slightly longer. It's either hand swinging or self abuse - the jury is still out :D

[A and C]

I should have known that as soon as I made such a sweeping statment about the S of S system some one would be waiting in the wings to shoot me down quoting some obscure type that uses the system.

It would seem to me that the S of S system is only used when the starter cranking speed is to low to use an impulse unit , so if you have a big engine mounted a long way from the battery then the S of S is the only way to get the sparks to start the engine.
As for the Bulldog engine I would guess that it was fitted to meet some military cold start requirment and in normal operation is just an expencive waist of weight.

[LowNSlow]

javelin I had an A-65 powered Cub that had no impulse mags and a wooden (thus low inertia) prop. Starting that was a hoot when warm. My Auster has a steel prop soo is less prone to kick-backs but I still start with only the left hand mag as the start is generally cleaner than when using both mags.

[Tinstaafl]

Was having a slight dig at pommy design choices, not you, A & C

BTW, an Islander isn't that obscure...

[Skylark4]

The volts throught the windings on the Grob 115E is `Slickstart`. (Slick Mags of course).

I don`t remember Shower of Sparks on the Bulldog. Standard impulse on left Mag as far as I recall, but then what would I know, not me Chief, I`m Radio.

Mike W

[A and C]

Despite being a pom I to find some of the british design choices rather odd , I do have to say that compaired with the numbers of american aircraft built most british aircraft come under the heading of obscure and all that i have worked on from the DH82a to Concorde have been a pain in the a*** to work on.

Perhaps it is my liking for the Bunderburg overproof that helps me see these aircraft without the rose tinted specs.

[ShyTorque]

I never heard the term "shower of sparks" applied to the mags on a Bulldog's Lycoming.

As far as I remember from my time QFI'ing on them about 12 or 13 yrs ago, it was an impulse mag only. That's certainly what RAF CFS and Bulldog Standards Flight taught me (and they gave me a 100% pass on the CFS engines exam paper :cool: ).

I recall there was a single rotary mag switch, with positions "OFF / L / R / Both / Start". I never heard of anyone trying to hand swing one though.

[Rob_L]

The Hughes 300c had shower of sparks on both mags. Not an obscure type. Many moons ago went to Thirsk to do some airtest work on a 300 cropsprayer. On start the motor disassembled itself and and lay smoking under the aircraft. The operator said that this happened at least once a week and thay had repair kits in stock!!!!!!!!!!!

Assured that the engine was timed correctly I asked if the ignition vibrators had been checked. A puzzled look greeted this question.
On removing the covers the points on one vibrator were corroded together and the other gap was about half an inch apart. A half hours maintenance of the vibrators and the end result was no more self destucting starter motors.

[Tinstaafl]

Hi A & C,

I agree with you about British design.

I think the Islander is a pig of an a/c to fly. Its only saving grace is its short field performance.

Doesn't excuse the stupid & avoidable design choices that were made.

Much prefer American a/c.