Starting Technique.

[gaxan]

I have recently converted to a Rallye 180, starting is performed with the alternator OFF , which is then brought on line once the engine is running - Why? In 20 years of flying various types I have never come across this technique, but I have heard of it before in various flight test reports.
If various electrical services such as radios have not been switched off on the previous shut down I can seee that this technique could prevent spikes but beyond that I am puzzled. Asking the people who operate the aircraft has resulted in the answer " we have always done it like that".
WHY ??

[DubTrub]

I have never come across this technique Nor have I, but it makes some sense...after all, the alternator does sfa during the start process.
Perhaps it's something electrical that the French designers introduced?

What does the POH say?

And does the "others" technique work?

Nice aeroplane the 180. Never, ever, EVER, go fly a 100hp Rallye after flying a 180.:sad:

[SkyHawk-N]

I have heard that this is done to protect the voltage regulator (?). I was once told to do it when starting a PA-28 on an extremly cold day, I just did it without asking why! :hmm:

Skyhawk.

[Tarq57]

Not 100% sure about this, but alternators require power to produce power, albeit not very much. Maybe this is recommended so that all the battery juice goes to the starter motor instead of energising the alternator. I would've thought the amount required to spark up the alt. was quite small, though. Anyone know?

[fly_sd]

Yes, I also understand that it is to protect the components in the solid state devices such as the regulator. My guess is that while starting the engine the voltage will drop due to the large current draw and once the engine fires and the load on the batt drops you can get large voltage spikes due to the sudden change of current over time - di/dt (back emf I guess).

[englishal]

I've come across this on a number of aircraft - mostly twins and French aeroplanes, but never really thought about it. I assumed it is to ensure less load on the starter motor so it can turn at full tilt - when the alternator is generating power the load on the engine increases (which is also why many sprint racing cars run with no alternator, to maximise power output).....Could be to protect the circuitry as well I suppose, though I don't know why it would used be in some aircraft and not others....

I fly a Rallye with the 160HP engine, they are beautiful aeroplane to fly and it is aerobatic :ok:

[Them thar hills]

I agree with Flying SD's reply.
Electronics are rather frail when it comes to voltage spikes, unless properly protected.
Good old electromagnetic voltage regulators are a bit crude but the technology is robust. OK for generators.
TTH

[fly_sd]

I'm guessing here but like in cars, older aircraft probably used generators which do not use any sold state components. With an alternator you have a voltage regulator which governs the current fed into the alternator which in turn regulates output. If the reg malfucntions you can get serious overvoltage at the o/p of the alternator which can fry the elecrnics. I think most modern aircraft have overvoltage protection but I believe some of the older ones did not. I guess the voltage spikes on startup could fry the reg and in turn could cause a serious problem with overvoltage.

I believe only about 1-4 amps of excitation current is fed to the alternator coils so I don't think that a big load for the batt.

The only other thing I guess that could be destroyed is the bridge rectifier which is made up of diodes and a high voltage could cause them to break down.

[TheOddOne]

It's interesting that ALL the Rallye marques that I have flown (100, 160 & 180) have this in the POH but other types with identical engines, don't, not even the Cessnas I've flown which were made in Reims (The 'Reims Rocket' being the most interesting!) The alternator switch in the Rallye is even separate from the Master, next to the fuel pump (from memory).

The PA28 has a 'split' red switch so that you can isolate the alternator, but I routinely flick them both on together. My checklist even has the 2 functions brought together with a forward stroke 'master/alternator switches ON'. I have once only taken the alternator 'off-line' in the air. I experienced what I thought was a serious electrical problem, so turned everything off and relied on COM1 running off the battery to get me back on the ground. The alternator had a fault and was somehow draining the electrics, so it was a good learning point, fortunately in good weather and close to base.

Cheers,
TheOddOne

[Sleeve Wing]

Interestingly this also applies to some aerobatic aircraft (Pitts, Extra.) where only a small battery is fitted, sometimes just enough power to start the engine on a winter's day.
Even the the strobes/anticoll. left off to make sure all the power goes to the starter.
Only a loud voice then required !!!
:ok:

[Whopity]

I think the answer is really quite simple. The Alternator is of no value during start up yet the field if not switched off independantly, will draw current that is an unnecessary additional load. There is no merit in having the alternator ON so its is switched OFF until it can be of use.

[RatherBeFlying]

The C-172 etc. master switch has a split between Battery and Alternator for all the good reasons mentioned above.

IF your check list includes switching on the alternator after start, then yes, do the start with the alternator offline.

The Citabria after start drill has these switches turned on: Right Mag
Alternator
Radio MasterNote that all these switches are individual separate toggles; so, the checklist has to cover each item.

[fly_sd]

It's interesting that ALL the Rallye marques that I have flown (100, 160 & 180) have this in the POH but other types with identical engines, don't, not even the Cessnas I've flown which were made in Reims (The 'Reims Rocket' being the most interesting!) The alternator switch in the Rallye is even separate from the Master, next to the fuel pump (from memory).
The PA28 has a 'split' red switch so that you can isolate the alternator, but I routinely flick them both on together. My checklist even has the 2 functions brought together with a forward stroke 'master/alternator switches ON'. I have once only taken the alternator 'off-line' in the air. I experienced what I thought was a serious electrical problem, so turned everything off and relied on COM1 running off the battery to get me back on the ground. The alternator had a fault and was somehow draining the electrics, so it was a good learning point, fortunately in good weather and close to base.
Cheers,
TheOddOne


I believe one of the diodes in the rectifier going bad has the effect of draining current from the system if you work your way through the current flow. For all of the reasons mentioned it does make sense to be able to isolate the alt from the system if it develops a fault.