Good reasons not to jump start

Depends on whether you blow something up doing it (see the bold bit in my list above).

Also, almost nobody will hand-crank something reasonably substantial – even if they know how which I guess 99% of pilots don’t, and it does need training, as well as muscle.

There is one thing with three phase alternators: You definitely need power from the battery to get the alternator active as there is no other way to produce the magnetic field in the rotor even with a running engine. So with a really duff battery you may hand prop the engine and go fly but you will not regain an active alternator. Also if the battery has only minimum charge and you switch on some device maybe at idle speed at the holding point I guess the voltage will drop to zero and the alternator will stop working – and you will be unable to reset the alternator in flight. A DC dynamo will start working any time, even with a dead battery as it has a non-electronic regulator that would not eat all minimal voltage in its diodes like with three phase alternator regulators. Vic

@Peter: yes indeed, few will hand-prop, even if their engine allows it. Engines with a gearbox, like my Rotax, make it very hard, though reportedly not completely impossible.

@Vic: as I understand it, the reason alternators need a minimum of power from the battery is they need to feed the field coil – otherwise no primary magnetic field, and no power can be produced. Ex nihilo, nihil fit Why dynamo’s do not suffer from this is unclear to me, perhaps they use a permanent magnet rather than a field coil?
But if the battery gets no charge current it ought to be clear from both the volt- and ammeter before take-off. The dangerous scenario, as pointed out, is that the battery is dead so that it does not build up any charge even though it seems to. Again, this danger has inherently nothing to do with jumpstarting; except that jumpstarting will be the most common way of encountering it.

Jan_Olieslagers wrote:

Why dynamo’s do not suffer from this is unclear to me, perhaps they use a permanent magnet rather than a field coil?

That’s right!

Well, the aircraft generators that I’ve seen do have a field coil, but they also have a permanent magnet that’s enough to get the system bootstrapped. (I’ve had the field coil fail in an old style generator on a Cessna 140, it basically became detached from its terminal, and at that point the generator stopped generating).

Batteries should (ideally) stay on a charger when not used. In cold climates, that’s more or less a rule. A flat battery in sub zero degrees is goodbye battery, led/acid at least.

Batteries should (ideally) stay on a charger when not used

This may be very relevant at this point.

Sorry, no, a DC dynamo has no permanent magnet, but the rotor has a steel core with the windings and commutator ring where the generated current is picked up at the carbon brushes as DC. The remaining minimal magnetism in the steel is enough to get the dynamo going even without battery supply to the field coils in the stator. All minimal voltage is wired to the coils while starting up and that builds up to the nominal voltage of 7 V, 14 V etc. till the regulator connects to the rest of the electrics. With a three phase alternator it is the other way round: The rotor has just two plain slip rings that get power via the charging light from the battery to produce the (rotating) field and the power goes from the stator coils to the diodes in the rectifier: There is no commutator so the three phase AC has to be made DC in the rectifier and voltage controller. As diodes are required all minimal voltage would be eaten by them so no way an AC alternator could self excite up to the nominal voltage. I have never seen any permanent magnets in an alternator, at least not in automobile types and I doubt there would be in aircraft types. How would these look in the confined space ? There are new types of lightweight alternators in production with permanent magnets that are available only for non certified aircraft I believe, don´t know. These should work without batteries but then they are not regulated as the field (magnet) cannot be regulated, so the overvoltage gets burned in heat sinks like Zeners. Decades ago I had do without electrics on my sixties BMW bike that had a “modern” three phase alternator from more recent types. I think I had a dead battery in Scotland so no way to get power again from the alternator all the 1500 km to home. But luckily the ignition was and is the standard mageto so I could return home no problem but certainly no lights all the way. Vic

There’s a world of difference between a ‘dead’ and a ‘discharged’ battery. A ‘discharged’ battery (master switch left of over lunch) will still have some power, enough to energise the panel lights or engage but not turn the starter. A lead acid battery in this state will recover from normal charging, although with a high initial current. It will certainly suffer some permanent loss of capacity though, depending on how deep the discharge was.

I’d have no hesitation in jump starting and flying such a battery VFR, subject to observing initial charge current declining and voltage in the normal range. The 2-3 hour recharge figure is irrelevant, because the time needed to recharge sufficient to start the engine again is going to be c. 10 mins, or a typical taxi time. (10mins x 70A = 4.2 AH). Admittedly it’s a bit nervy watching the charge at 70A, but it’s rated to do that and if it can’t, you might prefer to find out now rather than on some black night!

The time to get the capacity to IFR requirements is more problematic because if the requirement is to power the equipment for 30 mins, an aircraft battery in typical condition might be pushed to do that anyway. For instance, I took off in a rented 172, failed to notice the alternator had not started (The movement on the Cessna centre zero ammeter was imperceptible), and got a low volts warning after 30 mins, daytime VFR!

A dead battery (master switch left on for days) is a completely different matter because it will have little or no residual voltage to resist the inrush current when you connect jump leads from another battery that can source 1000’s of amps into a what is a dead short. The result can be a big arc, fire or at least melting of wires. In addition a completely discharged battery is almost certainly wrecked and likely to have little or no capacity to hold charge or regulate voltage spikes. I wouldn’t dream of trying to fly away on a previously dead battery.

vic wrote:

There are new types of lightweight alternators in production with permanent magnets that are available only for non certified aircraft I believe, don´t know. These should work without batteries but then they are not regulated as the field (magnet) cannot be regulated, so the overvoltage gets burned in heat sinks like Zeners

Why would they do that? Any modern design would surely use a switch mode regulator (with efficiency of at least 85%) rather than a linear regulator – otherwise, pretty much all the advantage would be lost.