Engine shut down with avionics on

DesertNomad

DesertNomad

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DesertNomad
I always power down my avionics before shutting down and make sure they are off before starting up too.

Yesterday it was quite cold here (below freezing) and while leaning for taxi, I went a bit too far... the engine stumbled... I tried to get the mixture in and catch it, but it died. I shut off my avionics and restarted the engine, then powered my avionics back up. Everything seems ok.

How big a risk is it to shut down with the avionics on? I would think it is less risky than starting with them on.

In the future, I think I will lean for taxi before powering up my avionics.
 
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Modern avionics have voltage regulators and can handle 14v or 28v, unless you have a major spike you would be ok, or unless you were in a middle of a firmware upgrade, then you're toast.
 
Almost none. All avionics are tested for abrupt power down, as well as voltage sag, and spikes. They are almost impervious to any kind of voltage fluctuation within reason. Now, if you get hit with lightning, all bets are off, but just power sags or spikes from the alternator/batt combo on a plane you are fine.

As radios age the capacitors in the power filtering section begin to decay, and may cause trouble, but that is a function of age and not so much power, although there is some relationship.
 
DesertNomad said:
I always power down my avionics before shutting down and make sure they are off before starting up too.

Yesterday it was quite cold here (below freezing) and while leaning for taxi, I went a bit too far... the engine stumbled... I tried to get the mixture in and catch it, but it died. I shut off my avionics and restarted the engine, then powered my avionics back up. Everything seems ok.

How big a risk is it to shut down with the avionics on? I would think it is less risky than starting with them on.

In the future, I think I will lean for taxi before powering up my avionics.
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Very very little risk on a shut down.
 
That, and there are diodes in the Cherokee electrical system to prevent voltage spiking.
 
Not to worry on shut down,especially if you go through a separate avionics switch on the panel.
 
Meh, I wouldn't make a habit of it but I think you're fine.

I'm alway OCD about stuff, start with my alternator off, etc, I've killed the engine with the avionics on a couple times by accident, never seen any damage, still felt stupid. Thus is life
 
James331 said:
I've killed the engine with the avionics on a couple times by accident, never seen any damage, still felt stupid. Thus is life
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Yup - definitely felt stupid. I was alone in the plane which made it a little better... just taking it over to my local shop for some minor work.
 
i did this once or twice on my new mooney while leaning for taxi.... it isn't going to break anything... Just not a good practice to get into.
 
There is no effect on the electrical system if the engine quits other than a small voltage decrease when the alternator stops. It's when you switch the master off, or the alternator field off, that you can generate a voltage spike. Starting with the avionics turned on is not a good idea, either. Both the starter and its contactor can make robust voltage spikes when they're switched off.
 
The airplanes I've worked on all have bus protection, if undervoltage is sensed, a relay will trip, removing power and preventing damage to electrical components.
 
Dan Thomas said:
There is no effect on the electrical system if the engine quits other than a small voltage decrease when the alternator stops. It's when you switch the master off, or the alternator field off, that you can generate a voltage spike. Starting with the avionics turned on is not a good idea, either. Both the starter and its contactor can make robust voltage spikes when they're switched off.
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Because I often suspected that most concerns WRT "voltage spikes" during engine start, alternator turn on, etc were pretty much OWTs I actually made some measurements last year looking for excessive voltage on the main bus with a digital oscilloscope under various conditions. What I found was that none of the usual suspects including engine starts caused more than a few volts disturbance. Of course this is just one airplane and the batteries were relatively new (3 year old Concord RG-XCs) but I was surprised at how little cranking the engines affected things.

That said, I did manage to damage a radio once when I accidentally attempted to start an engine with the master off while connected to a 60A external supply and I still start with avionics off most of the time out of habit.
 
I always leave my avionics on.
Do you turn your radio off everytime you start your car and turn it off also?
 
pigpenracing said:
I always leave my avionics on.
Do you turn your radio off everytime you start your car and turn it off also?
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On the typical car system when the key is in the start position all external circuits are defeated while the engine cranks. Only a few essential systems(ign, FI of course) are powered. The radio is nearly always defeated for engine crank.

It would be beneficial to turn the avionics off during engine crank as the lower voltage during cranking and early charge leads to capacitor damage slowly over time. It's certainly nothing critical, but will likely extend the useful life of the power regulation circuits of most avionics.
 
I never heard of lower voltage hurting anything, capacitors or anything else.
Low voltages can cause the electronics to do funny things, including changing memory states, this is why many battery powered electronic devices have reset button you push with a pen or paper clip. Of all the things to worry about, I would rank this just above being hit by a meteor. A spike is another matter, this is why you ground yourself if working on electronics, especially ICs, to protect from static electricity.
 
Shut down of the engine itself due to a stall likely ain't gonna do ****. No big spikes, no nothing.

Start-up; you can get noise from the starter, and when you open a solenoid there is a potential for an inductive flyback voltage. And, in theory, this could be a bad thing for avionics. Now, I should point out that I ain't no A&P so I don't know diddly squat about the details of your wire harness, but in automobiles, one typically puts devices (diode) in the system to allow the flyback voltage to dissipate harmlessly. (And, in the auto industry, one doesn't spend $0.35 on a diode needlessly.) One is also aware that the redneck tow truck guys like to use 24 volts to jump 12 volt cars because it will crank and start better. So, one designs electronics to withstand 24+ volts.

Mr. Gismo noted above that he looked at the voltage profile on his aircraft during start-up and found nothing exciting. That would suggest that either devices have the appropriate clamping elements or simply that the battery was able to suck up any excess current. This would render the whole discussion of inductive spikes into the theoretical realm, but, even then, given that it don't cost **** to leave the radio firmly in the off condition until after start, why not do it?
 
gismo said:
Because I often suspected that most concerns WRT "voltage spikes" during engine start, alternator turn on, etc were pretty much OWTs I actually made some measurements last year looking for excessive voltage on the main bus with a digital oscilloscope under various conditions. What I found was that none of the usual suspects including engine starts caused more than a few volts disturbance. Of course this is just one airplane and the batteries were relatively new (3 year old Concord RG-XCs) but I was surprised at how little cranking the engines affected things.

That said, I did manage to damage a radio once when I accidentally attempted to start an engine with the master off while connected to a 60A external supply and I still start with avionics off most of the time out of habit.
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Did that airplane have an ACS igntition switch that has been properly addressed by the AD? That system has to have a diode across the starter contactor to suppress the spike to protect the flimsy contacts in the switch so they don't burn out prematurely.

I used a 'scope to measure a spike across a contactor's coil when it was released. 600 volts.
 
My thoughts - The difference between automotive and aircraft systems is the presence of a battery disconnect relay (the master). If it chatters or stumbles, the starter inductive kick will be dissipated in any connected electronics. A low battery or bad contactor can cause this.

Also, anything that enables a charging alternator to be suddenly disconnected from the battery from this master relay has the potential for generating a major negative (I think that is the case) pulse in the remaining electrical system.

Yes diodes can protect against some of this if they are fast enough. The fastest fuse though is a transistor junction.
 
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It has to do with transients. Transients are high voltages produced by turning on and turning off switches (near)instantly. On to off with inductance makes high current and high voltage. Off to on with capacitance makes for high current, high voltage. Transients can also produce sparks. Anyway, follow the procedures in the manual for the airplane. The manual KNOWS best for this for sure!
 
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Transponder quit working.

Took it to avionics shop.

They replaced an internal fuse.

Asked if I forgot to turn it off before shut down or starting.

I had.


Sent from my iPhone using Tapatalk
 
pigpenracing said:
I always leave my avionics on.
Do you turn your radio off everytime you start your car and turn it off also?
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Yes you do unless you ran your own feed from an unstitched source. It's a function built into the key switch circuitry.
 
Dan Thomas said:
Did that airplane have an ACS igntition switch that has been properly addressed by the AD? That system has to have a diode across the starter contactor to suppress the spike to protect the flimsy contacts in the switch so they don't burn out prematurely.

I used a 'scope to measure a spike across a contactor's coil when it was released. 600 volts.
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Both starter contactors plus the master contactor have diodes across the coils to absorb the inductive energy when the current to the coils is stopped. But they don't prevent voltage spikes on the main bus, they protect the associates switches from arcing.
 
coloradobluesky said:
It has to do with transients. Transients are high voltages produced by turning on and turning off switches (near)instantly. On to off with inductance makes high current and high voltage. Off to on with capacitance makes for high current, high voltage. Transients can also produce sparks. Anyway, follow the procedures in the manual for the airplane. The manual KNOWS best for this for sure!
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Close but not quite. Open the circuit on an inductor that was carrying current and the voltage across the coil reverses and can climb to a high voltage but the current decreases immediately and eventually goes to zero shortly after.

Applying voltage to a capacitor does cause a current transient but no voltage transient. FWIW, switching incandescent landing lights on also causes a large current transient. Neither is likely to cause a problem with any modern avionic device.

You also get voltage transients any time you turn off a high current load because the alternator/regulator can't react fast enough. This is called a "load dump" transient and is probably the most common cause of voltage spikes.

In any case as long as the battery is in the circuit it should absorb most of the energy in any transient unless the battery is on its last legs and has high internal resistance.
 
It's risky to make generalizations based on experience, or even measurements, in one plane. Starter motors can generate huge voltage spikes, and diodes across the contactors can fail with no warning or symptoms, leaving avionics exposed.

I wouldn't be too concerned with unexpected shutdowns, but I would never crank the starter with radios, transponders, EFIS, or any other electronics sharing the buss.

I had an amateur transceiver in my last truck and forgot once. After the engine started, I noticed that the display showed random digits and the VFO knob would not enter a frequency. It was fixed by powering it down and back up, but that was a close call.

If you can hear a high-pitched whine in your intercom when its muted, and the whine varies with engine RPM, you're either about to lose a brush in an alternator, the internal resistance of the battery is getting high, or there's a high-resistance ground (or ground loop) somewhere. Any of these cases leaves your electronics vulnerable anytime they're on the buss with the engine running.

If the aircraft is an experimental with a 12V system, a good safety precaution is to install an inline noise filter, like the ones designed for car sound systems, in the positive avionics buss. They use both an inductor and a capacitor to block alternator while, but also effectively crush spikes coming in from the power system, and respond more quickly than diodes. If you're worried about a cheap consumer auto part failing, add an emergency bypass switch.
 
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gismo said:
Both starter contactors plus the master contactor have diodes across the coils to absorb the inductive energy when the current to the coils is stopped. But they don't prevent voltage spikes on the main bus, they protect the associates switches from arcing.
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I run across an awful lot of airplanes that don't have starter contactor diodes, and some don't have master contactor diodes, either. The diodes don't even show up in the wiring diagrams. It's dangerous to assume that they are all protected.
 
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