Avionics use while engine is off

nj-pilot

nj-pilot

Pre-takeoff checklist
Joined
Aug 17, 2015
Messages
240
Location
Maine
Display Name
Display name:
josh_me
I know some people who get atis and copy ifr clearance before starting the engine (sitting in cockpit with master + avionics on). I've been conditioned to believe that this is very bad and could quickly drain the battery (and thus always do these things with engine idling).

As a renter, I'd prefer to do these things without running the engine.

Thoughts?
 
As a renter, I do it with the engine off as well.
 
Isn't the Hobbs tied to the master?

Sent from my SGH-M919 using Tapatalk
 
in NY you might need that battery juice on cold days. In summer/warmer days, I think you would be OK with Wx/clearance with battery on
 
One time I lost the alternator in Mexico. Flew home through Tucson and on to Colorado. Radio and Transponder worked for more than three hours. I don't think a few transmissions will drain a battery in decent shape.
 
JoseCuervo said:
What is the Hobbs meter hooked to?
Click to expand...
Wikipedia summarizes it:

there are several ways in which the meter may be activated:

-- It can measure the time that the electrical system is on. This maximizes the recorded time.
-- It can be activated by oil pressure running into a pressure switch, and therefore runs while the engine is running. Many rental aircraft use this method to remove the incentive to fly with the master electrical switch off.
-- It can be activated by another switch, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twin engine planes). In these cases, the meter only measures the time the aircraft is actually flying. Metrics such as Time In Service and Turbine Actual Runtime are kept to monitor overhaul cycles, and are usually used by commercial operators under Federal Aviation Regulations Parts 135, 121, or 125.
-- It can be activated when the engine alternators are online (as in the Cirrus SR series).

​
 
ejensen said:
One time I lost the alternator in Mexico. Flew home through Tucson and on to Colorado. Radio and Transponder worked for more than three hours. I don't think a few transmissions will drain a battery in decent shape.
Click to expand...

It won't. But it really sucks to find out the battery in your rental is weak when you're just about to leave from a remote stop. And it's Sunday and the one mechanic is playing golf (BTDT, with an alternator failure).

It's also a real good idea to test your handheld, every flight. So, if you need it, it's not doing an aviation flashlight imitation (flashlight = holder for dead batteries). Backups don't work as backups unless they are known good.
 
I don't know how much juice a couple minutes of COMM would use. But if you do that, make sure to turn off the radio master before you start the engine. I don't know if it's bad on the radios or not (I've heard different things), but as a courtesy just in case.
 
I prefer to use a portable radio, especially if it's going to be a cold start,with low outside temp.
 
JoeFromKS said:
Isn't the Hobbs tied to the master?
Click to expand...

Usually, but the good rentals have an oil pressure sensor so the hobbs runs only when the master is on AND there is oil pressure, giving true engine running time.

But I know many have it just on the master so if the master is on the meter is running and you're paying. Hence, the preference for a handheld to pick up ATIS and clearances ahead of time.
 
JoseCuervo said:
What is the Hobbs meter hooked to?
Click to expand...

Either the master (bad FBO) or the oil pressure sending unit (good FBO).

Personally, I can't see the radio receiving draining much if that is the only thing on. I believe the transponder drains a lot more.
 
If you go the portable route then make sure you have a way to hook to an external antenna. A connector under the panel you can reach and a piece of coax to get to the handheld. A headset adapter is very nice to have also.
 
I've seen airplanes with Hobbs meters that worked off oil pressure, an air switch, and the squat switch. I'm guessing that on rental singles its usually tied to oil pressure. The air switch might be too easy to defeat, since it was only a tab which moved when air was flowing past it at a sufficient speed. People might be less willing to cheat with the squat switch since you would need to fly the airplane gear down to defeat it...
 
If a few minutes use of the comm radio is going to make the difference between the engine starting or not starting then I think that needs to be addressed before you go flying.
 
Everskyward said:
I've seen airplanes with Hobbs meters that worked off oil pressure, an air switch, and the squat switch. I'm guessing that on rental singles its usually tied to oil pressure. The air switch might be too easy to defeat, since it was only a tab which moved when air was flowing past it at a sufficient speed. People might be less willing to cheat with the squat switch since you would need to fly the airplane gear down to defeat it...
Click to expand...

When I was renting, a customer took the plane and turned off the master (and hobbs) for at least half the flight. Might have got away with it but then he turned in the gas receipts.
 
I do it often, unless there is some reason to suspect that power for starting might be limited, like weak battery or cold weather start. I am often instructing so it is even easier to discuss the information we just received with the engine not running. Additionally it is a great time to find out if a radio, headset or intercom isn't working, so you aren't troubleshooting an issue with the engine running.
 
ejensen said:
When I was renting, a customer took the plane and turned off the master (and hobbs) for at least half the flight. Might have got away with it but then he turned in the gas receipts.
Click to expand...
:eek: :D
 
ejensen said:
When I was renting, a customer took the plane and turned off the master (and hobbs) for at least half the flight. Might have got away with it but then he turned in the gas receipts.
Click to expand...

Reminds me of when I took an Avanti on a driveaway service from Palm Beach to the Hamptons. Disconnected the speedo cable from the tranny when I got up there so I could cruise a couple days and impress my buddies. Then forgot to reconnect it for delivery and had to come up with some bull that gave me a chance to do that away from the owner. Hey, I was young ... and stupid.
 
For cash money I can teach you how to beat the Hobbs and Tach. Or was that Calvin and Hobbs. I'm getting old. I forget things.
 
I can't speak to aviation systems, but generically speaking the reason you typically turned off electronics prior to starting in the past was due to the voltage drop that occurs when you have a high current draw on the system from the starter. The voltage drop causes a current spike to already running equipment which can potentially result in blown fuses or damaged electronics. (think brown out in your house)
That being said, I can't imagine an avionics system that doesn't have a voltage regulator in place to prevent a current spike such as this.
So, my thoughts would be that it's no big deal other than the power drawn from the avionics which you could probably figure out with a slide rule and a few toes.
I built an inverter once to run a 500W PC off of a car battery and it would run for about an hour. I just googled the current draw of a G1000 and it is 250W so I suspect it would run for well over an hour on battery, but depending on the overall battery capacity it could likely effect starting capabilities after 10-15 minutes.
 
Review the plethora of threads about the vagaries of starting engine with avionics powered up. Then give some thought to how long it takes to grab the info before the lighting the fire.

Then, ask yourself why clearance ends with "advise when ready to taxi"

Hmmm... could it be that they know you're calling up before you've started the engine? Is your headset is so awesome they can't hear the engine running?

Without going through it all, the people I've spoken to at Dynon and Garmin say go ahead and crank it with our units powered up. I don't know if I've saved a dollar in gas, but my legs are never tired from holding the brakes waiting to get started.

I don't remember what a Calvin and Hobbs have to do with it. I fly 3 planes, 2 charge by the Tach and the other is mine alone.
 
nj-pilot said:
I know some people who get atis and copy ifr clearance before starting the engine (sitting in cockpit with master + avionics on). I've been conditioned to believe that this is very bad and could quickly drain the battery (and thus always do these things with engine idling).

As a renter, I'd prefer to do these things without running the engine.

Thoughts?
Click to expand...

The ATIS only takes a moment and in most (all?) cases can be done while the engine is warming up before taxi.
 
I rent.

Does it really take folks *that* long to get the ATIS once the engine is running? Seems like doing the handheld is just one other thing to have to work in. Startup, get the ATIS, get moving. Not a big deal.

I guess over 30 flights *maybe* you've saved enough for an hour of rental time. But that's more hassle than it's worth IMO.

Edit... yeah. What Aztec said.
 
frfly172 said:
I prefer to use a portable radio, especially if it's going to be a cold start,with low outside temp.
Click to expand...

Nothing wrong with not wanting to drain the battery, but when it's that cold outside what is the advantage of using a portable before you start up? You are going to have to spend at least a few minutes waiting for the oil temp to come up, so you aren't saving any time.
 
rolivi said:
Then, ask yourself why clearance ends with "advise when ready to taxi"



Hmmm... could it be that they know you're calling up before you've started the engine? Is your headset is so awesome they can't hear the engine running?
Click to expand...

No, not at all.

I get that most of the time clearance and ground are using the same freq and I always call after engine start.

They know that many people like to review their clearance and set up radios/warm up engine...etc and typically need a moment before taxi.
 
The Hobbes is supposed to be connected to a small fuse that feeds directly off the airplane's battery, though the oil pressure switch. That's to prevent cheaters from stopping it by turning the master off. Some are smart enough that they might find that fuseholder and remove the fuse for a leg or two of a flight.

The Garmin and Dynon equipment, being newer, probably have internal surge/spike suppressors. Lots of older radios don't, and the spike is generated by both the starter contactor and the starter itself when the starter is released. It has nothing to do with the voltage drop of starting and everything to do with the inductive reactance of the magnetic coils in those components.

Some airplanes don't have avionics master switches, so no separate bus, and the altitude encoder is one of those things that's on the bus and gets the spike because you can't turn it off like you can the radios. We used to suffer encoder failures until the avionics shop put radio masters in the airplanes.
 
The only aircraft I ever rented with Hobbs on the master was a Turbo Arrow. That was deliberate: it allowed the renter to run the engine for free during cool-down so they wouldn't be tempted to cut that short and toast the turbo.

I would have preferred a squat switch, though!
 
I start the engine first. The electrical gyros spin up with the master on besides just the COMM radio. I think it is easier on the equipment this way.
 
I generally turn on my handheld and catch the wx while I preflight. I'm VFR at a class D so I don't need to get any clearances.
 
nj-pilot said:
I know some people who get atis and copy ifr clearance before starting the engine (sitting in cockpit with master + avionics on). I've been conditioned to believe that this is very bad and could quickly drain the battery (and thus always do these things with engine idling).

As a renter, I'd prefer to do these things without running the engine.

Thoughts?
Click to expand...

Radios use very little juice in receive mode. Much less than in vacuum tube days.

My dad taught me that when a car starter turns weakly on a cold morning, turning the lights on for a couple minutes can actually help. A little current flow will warm the battery slightly and increase its output.
 
Dan Thomas said:
The Hobbes is supposed to be connected to a small fuse that feeds directly off the airplane's battery, though the oil pressure switch.
Click to expand...

There's no SUPPOSITION of what it is supposed to be connected to unless the manufacturer of the aircraft mandates it. Hobbs can be connected to anything that will provide power to it to run. In fact, on my plane, the Hobbs is connected to the gear switch which pretty much gives the FAA definition of time in service (I don't have a recording tach).
 
You must log in or register to reply here.
Back
Top