Radio off during engine start

[shumway76]

This is a question with regards to light piston engined aircraft.
It is always stated that "radio equipment should be switched off prior to starting the engine to prevent damage to radio equipment due to transient voltage" or something like that.

would greatly appreciate if an "electrician" could explain this in technical terms.

I can only think of 1 (possible) answer:

When the starter is operated, the battery will supply more CURRENT to the starter. This higher CURRENT will also be sent to any other equipment that is connected to it (switches on / contact make). Therefore the higher current could cause damage to sensitive electronics within the radio if the radio was not switched off (contact broken).
So basically it's the current that is damaging. "Transient voltage" in this term is not quite accurate?

[RJ Kanary]

The inrush current of a heavy electrical load oftentimes isn't what kills sensitive solid state devices.It's the back EMF of when the load is disconnected that does the dirty work.Hundreds, even thousands of volts may be present briefly during this event.
And if that didn't kill anything, the surge of the charging system trying to fill the battery up has a second chance to kill what didn't get murdered during the start. :)

[easaman]

The problem is not the current of the battery, it is the voltage!
You are not only supposed to switch off the radio, you should switch off all avionics for the engine start of a small airplane!
While your battery is delivering the current to the starter, battery voltage drops and depending on the airplane,
the system voltage regulator is increasing the voltage which may lead to voltage spikes, which the aircraft electronic does not like!
You may be 100 times lucky, but one day it has killed your avionics.
Some airplanes have voltage surge limiters installed to protect the aircraft electronics during engine start.
Cheers
easaman

[shumway76]

easaman,

Understood your explanation, but leads me to more questions:

The voltage regulator is supposed to maintain a certain voltage.

Now, when the starter is engaged, obviously the voltage will drop. The voltage regulator will try to maintain the desired voltage (eg. 28V) but of course will not be able to due to the high demand from the starter.
So let's say the voltage drops to about 10V, and fluctuates to maybe 14V, how is this harmful to the radio?
It is mentioned that transient voltage (as you mentioned voltage spikes) are the harmful one, but I find that there is no more than 28V, so how can the "spikes" be harmful?

Or am I getting this all wrong? Or are "spikes" you refer to not voltage of more than what the radio can handle (in this case 28V) but "spikes" as in rise & drop (non-consistent) of voltage, not OVERvoltage.

[easaman]

There is a variety of different voltage regulators on the market and some are more prone to produce spikes than others!
Just imagine how accurate a carbon pile voltage would be when the battery voltage drops and it tries to increase the voltage to maintain it at a certain level.
Not to forget EMF and counter EMF etc. due to coils which come also in the game.
Cheers
easaman

[shumway76]

Apologies for asking too many (silly) questions), but I am not an electrician, only having very basic knowledge of electrics.

First - the mention of voltage "spike" - is this a spike above the normal 28V or a spike as in non-cosistent or sudden jump in voltage, but not above 28V that is damaging to the radio?

Secondly - back EMF - correct me if I'm wrong - Back EMF occurs when the flow of current suddenly stops and that's when back EMF comes in? And this back EMF causes a sudden "spike" in voltage?

Third - As the starter is engaged, the carbon pile voltage regulator 'tries' to increase the voltage by reducing the regulators "resistance" (but to no avail since battery voltage is already low due to high demand by the starter). However, when the starter no longer needs the high current flow, the voltage from the battery will return to normal, but the voltage regulator did not regulate quickly enough, causing the voltage to actually be above the normal 28V momentarily. Is this possible?

[happybiker]

Shumway.

On a light aircraft, the 28V power to the starter motor is supplied through a solenoid which is a heavy duty relay with contacts that can handle the start current. The coil of the solenoid to close the contacts is an inductive load which is energised by the start switch. When the start switch is opened, the inductor will resist the drop in current and the stored magnetic field of the coil will create a back e.m.f. This emf, as pointed out in a previous post, can be extremely large and many times the battery voltage. If this high voltage creates an arc across the start switch it will be connected to the battery bus bar momentarily and any electrical equipment on that bus bar. Radio equipment switched on is particularly susceptible to any voltage spike generated in this way.

A more technical description of this can be found via this link

Flyback diode - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Flyback_diode)

[hedgehopper]

Gentlemen,

It is also worth noting that it's not the aircraft's voltage regulator in question here, more likely the radio equipment / avionics it's self!

As the starter is engaged, bus volts drop, the "equipment's voltage regulator" compensates and raises the voltage within the individual unit(s) and then with the sudden raise in voltage as the starter disengages is unable to react quickly enough to stop an over voltage situation and damage occurs!

HH

[shumway76]

I'm getting a better picture now, thanks (even though there are several versions).

But one more question - If the radio is not off during start, is the damage to the radio instantaneus (due to bad luck on the day when the regulator did not regualte "on time")
OR is it 'cumulative', that is constant engine starting with radio not off slowly having a detrimental effect on the radio electronics?

[Phalconphixer]

Hi Guys...
It's not so much that physical damage can be caused to the radios and or other sensitive systems during an engine start it's more a case of the microprocessors in these equipments not being able to handle the low voltage encountered by the current drain on the battery.

The radios require 14 or 28 volts (depending on aircraft type and equipment age) for correct operation and in the event that the aircraft battery is getting a little old or worn it will not be able to sustain a voltage high enough to power the starter motor plus the other equipment.

Inside the radios, there is a power supply unit which converts the incoming 14 or 28v to those voltage levels required for the various circuits, in the case of the microprocessor this unit normally requires 5vdc for correct operation. If the supply voltage drops too far, this 5v might well drop below the threshold level required by the microprocessor and it will freeze or stall, whilst other circuits in the radio that need perhaps 12v may not be affected to the same extent. Thus the radios built in fault detection circuits may come into operation and display a fault indication.

Generally speaking, simply recycling the radio should be enough to restore normal operation. The radio should be switched off and then back on again; the radio itself cannot do this automatically because the microprocessor has 'stuck' and is out of synch with the rest of the unit.

Basically if the radio control unit has digital, LED, or similar 7 segment frequency displays switch it to OFF before engine start.

Hope this helps...