engine start: magnetos first or starter first?

[Zacke]

Hey guys,

by the (A2A) book with the Comanche one should switch on magnetos first and then hit the starter. With other engines (e.g. the Merlin) the sequence is the other way round. I think it has to do with bringing some oil or fuel to the cylinders first.

Could anybody explain this in detail?

Thanks in advance,
Zacke

[AKar]

GA engines are normally started with the magnetos on from the beginning. Note, that many ignition switches have the mags and the starter behind the same key, so it would not even be possible to start them without having the magnetos on as well.

One exception to this rule are airplanes that only have single magneto with impulse coupling installed. In those, you'd select only one magneto (not both) for starting, specifically the one with the impulse coupling, and engage the starter using separate button or switch. I think Comanche is an example of such airplane, as in the original configuration, but A2A's version is simulated as one with two impulse-coupled magnetos.

-Esa

[Zacke]

Thx!

[frog2019]

I was confused about this because the Pilot's notes (on Shift+2) tells me to switch mags to Both then engage the Starter , while the manual (page 58 under the 'Starting' heading) says "Engage the starter, allow the engine to turn one full revolution then turn the ignition to Both"

[AKar]

I was confused about this because the Pilot's notes (on Shift+2) tells me to switch mags to Both then engage the Starter , while the manual (page 58 under the 'Starting' heading) says "Engage the starter, allow the engine to turn one full revolution then turn the ignition to Both"

That's probably only for added realism, to subject you to conflicting and not immediately obvious information. Just like in real aviation!

-Esa

[guillaume78150]

I have an old british car from the 60s. I always crank the engine without pulling the choke. After two or three rounds, I pull the choke and the engine starts. It helps to "pre-lube". If it does not do any good, it does not harm either…
I start the Comanche engine the same way, starter before engaging magnetos, counting 6 blades.

[AKar]

If it does not do any good, it does not harm either…

Aside complicating the starting process. These engines are not enriched for started by choking, but by fuel injection. With cold-cranking, you do vent the prime charge. Without specific reason, I would not count the blades before ignition with these engines (recalling that in many installations that would be impossible due to ignition/starter key configuration), but would just select the appropriate magneto(s), and crank straight after priming.

-Esa

[Scott - A2A]

As Esa said, it's a technique that has these two steps reversed in different applications with the exact same engine. I take this as a soft recommendation or at least one that two opposing camps exist. I fall into the "it probably doesn't matter which way you do it, but just follow the manual regardless" camp unless someone convinces me otherwise.

[AKar]

If I add one more thing, it is also worth remembering that the Comanche 250 is one of the older post-war GA types. How things are done have subtly changed since: importantly, radials in GAs have become a rarity, oils have improved and ignition arrangements have been changed in newer models. Engine manufacturers make their generic operating instructions reflect the mainstream fleet and continuing usage experience, so these may change over the years as the "typical installation" and common practices change over time (sometimes the books also may not change, which is another source of confusion). Also, airplanes' handbooks often reflect the era of the original manufacture, and the original configuration in their wording. As some sources change over time and others remain unaltered, conflicting methods and procedures exist.

As Scott says, it is best to follow the applicable manuals, however, one should not overthink it. Sometimes the instructions are just different and in conflict with each other with no important technical reasons.

If I absolutely know that this engine installation is effectively similar to that engine installation, and I think that installation has better operating instructions written for it, there are no reasons to not carry them over to this one as operating practice. However, it must be reminded that this relies on operator's judgement, of which two sorts exist: good and bad.

-Esa