Which Radios to Switch On?
 

Do you teach your students to only switch on the radios they require and/or know how to use or do you require them to turn on ALL the radios for all flights?

For example, for a circuit detail do you teach them to turn the DME and ADF, as well as COM and TXPDR?

Can you list any reasons supporting your views please.

Finally, does switching, for example, the DME on and off on every flight significantly effect time before failure?

Thanks.

I was always taught (and hence I teach) to turn them all on - the reasoning behind this was that it warmed them through and helped get rid of any condensation that may have formed, thus helping prevent corrosion. I don't know how much truth there is in this - be good to hear a radio mechanics view.:suspect:

Can't help to have them on and set for local nav aids just in case you need them unexpectedly

Airport closure / divert

Rapid weather deterioration

Other ?

Apologies !

Please swap \"help\" for \"harm\" in the above and it should then make sense !

Given the absurdly low MTBF for most GA avionic displays, I think that it makes good sense to keep them powered up to dry them out during flight when the aircraft have to parked out in the damp at this time of year.

Interesting. I always leave radios off when I'm not planning on using them, but I've never really thought about it or tried to justify it.

I do make sure that transponder is always on Standby, even if only doing circuits. And I make sure my IMC students turn on, tune and identify all navaids, even if they're not planning to use them. But for a VFR flight, I don't bother.

Maybe based on the other replies so far, I should change that?

FFF
-----------------

I'm in the "turn 'em on, warm 'em up and dry 'em out" camp - but whether it makes any real difference ... :confused:
I also believe that a briefing on the avionics should be part of any type conversion, even if the bod only has a vanilla ppl.

Edited: hey, I've reached 150 posts - and on my birthday too. How exciting. :D

HFD

Just a suggestion:Why not?

Happy Birthday!

Isn't the compass swung with all radios and electrical equipment on. So there's another reason to switch them on and warm them up.

Also if you are going to switch them on you may as well check they work. Trying to get an RNAV out of PAR mode in the air is 20 times harder than on the ground (if you've never done it before).

Sounds like I am not the only one who has heard that turning them on dries them out!

I take it you turn on the pitot heat before T.O. (I also do this, but have flown with many PPLs who do not).

Why do you turn the Pitot Heat on before TO Foxmoth? Says it's optional on my checklist and I only use it if there is a risk of it icing. That said, my FIC instructor always switches it on... Interested to know the debate here.

As was said the compass is swung with all electrics on - including the pitot heat :hmm:

Leave them on, then you know if they still work.

Also quite like having a avionics master then you can leave them all switched on thus stopping the wear and tear of the on/off switchs getting fannyed about with every trip. I don't know if this is a good idea though as you might get funny eddie currents getting at the avionics back through the earth.

Must admit my ex CFI used to say leave the pitot heater off because it burns out the element. Never heard of a pitot heater burning out yet on a light aircraft and the engineers said it was a pile of !!!!e as well so you might as well have it on. Better to have it on than think later bugger i wish i had turned it on earlier.

If the weather is 25 deg. C and dry, then the Pitot heat won't usually be required unless you plan to go IFR, but if in doubt, then turn the damn thing on. If having it on burnt out the heating element, what the hell's the point of having one? I have to say I've never noticed, but what's the average life of the pitot heat anyway?
Pretty long I would assume, as long as you have some airflow over it and don't let it just sit there on the ground heating away.

With regards to radios, I always like them on at this time of year for the reasons already stated, but I also like to know that they all work before going off, just in case you need them.

Pitot heater
 

Ahhhhh...

I've had the 'Turn it on all the time because the check list says' comment (which never made sense because it was not backed with an explanation).

I was also told its designed to work in a 100 mile per hour wind and doesn't like being turned on for long times on the ground - which did make sense so turn it on at take off and off when you land.

One instructor once said 'since we are not going to get near 5000' on the trip we will only turn on the heater if the OAT says less than 10degrees'. That seemed to make good sense but then I remember my first comment about the compass.

So now after 700 hours the 'Turn it on all the time argument seems to make sense'. But I can now explain why.

Friends of mine owned a Partenavia which, being a twin didn't get a lot of use. Pilots who used it were always moaning about randon avionics faults which could never be traced, just cost the owners a mint in trying to get 'em fixed. Then they sold it and had to deliver the aircraft somewhare in Southern Spain. As they continued their long flight south the avionics problems disappeared one by one and they delivered a fully functional aircraft. Obviously it was not used to doing long flights and this had dried out the avionics. In my book therefore, all avionics must be switched on before departure. This will minimise the problem due to damp and if some of it doesn't work then it is up to the PIC to decide whether to continue with the flight. Furthermore, any non working avionic boxes can be reported.

P.P.

With regard to the DME. Pilots should keep the DME off when they do not require it i.e. in the circuit. The reason is that DME stations have a capacity limit and when that limit is reached they stop stations at the greatest distance (affected by power) from obtaining information. This means that while you blast round the circuit with the DME blazing out the distance from some VOR/DME, the poor sod at the limit of the range may not get a DME indication when they need it because you are.

When it comes to drying out the avionics - turn up the heater! Many avionics faults are fixed by re-racking the equipment - moisture on the contacts. Turning up the heat or taking the aircraft to a better climate will help the avionics even when switched off.

Now to the biggest load of cobblers.

Could the person who thinks that the compass will only read correctly when the pitot heat and every bit of electrical equipment is on please tell me how they can tell what direction the aircraft is pointing after an in-flight electrical failure?

The Airworthiness procedures require that - The most adverse combinations of electrical loads must not cause deviation in
excess of 2° (5° for light aircraft).

When completing a compass swing, electrical equipment, e.g. radio, instruments and pitot tube heaters, should be
switched on to ascertain that there are no adverse effects on the compass.

Thus having the pitot heat on or off will have little effect. Since most light aircraft pitot heats are powered by DC then even having the cable passing by the compass, there would be no change in effect by having the switch on or off.

Do those pilots who require the pitot heat to be on so that the compass is reliable ensure that the instrument pannel light in the compass is also on and serviceable? :D

Next time someone trotts out that kind of rubbish, ask them if they had 4 dummies wearing david clarkes in the C172 so that the efects of 8 magnets waving round the cabin was checked during the compass swing! :D :D

Regards,

DFC

Up here in Scotland the avionics need all the help they can get at this time of year! Especially if the a/c are left sitting for a while and you only do short flights anyway.

Good point about the DME though!

DFC:
The comment about DME was fair but some of the rest is harsh and a bit suspect.

I didn't understand the bit about: "Since most light aircraft pitot heats are powered by DC then even having the cable passing by the compass, there would be no change in effect by having the switch on or off." - one of us needs to re-read their electromagnetism books :O

I didn't know about the 5 degree deviation limit from electrical services but the deviation card WILL only be CORRECT when all the services are on. 5 degrees may or may not be significant based on what you're trying to achieve at the time. There's an old story:
Nav to pilot: turn 1 degree port
Pilot to Nav: I can't fly that accurately
Nav to Pilot: OK, turn 5 degrees to stbd
Pilot to Nav: steady
Nav to Pilot: turn 4 degrees to port. :)

The comment about permanent magnets in headsets is certainly true if they're very close to the compass but they're fairly well shielded and I seem to recall that the field decays as an inverse cube law (may be wrong), so at normal distances the effect would be negligible.

HFD

hugh flung_dung,

How do you get DC power to the bulb inside the compass, vary the brightness and switch it on an off while having no significant effect of the compass reading?

If I remember correctly the right hand rule applied to DC. However, if one has the positive and negative cables side by side the magnetic effect is according to the UK CAA "usually insignificant"

The compass deviation card should say if the swing was done with the radio equipment on or off.

If the engineer checked that the radios, pitot heat etc had no significant effect on the compass as required by the airworthiness procedures and then did the compass swing with everything off as allowed (recorded on the dev.card). Would you say then that all flights should be completed with the radios off?

Why do the radios get special treatment? - Simple - many sources of alternating voltages and currents as various frequencies. Some of which - DME, Transponder, WX Radar can be high powered.

But please do not turn on the wx radar during the compass swing while the engineer is out front unless you want to endanger them.

Regards,

DFC

right so I have to have all the electrics on all the time then HFD :uhoh:? Pitot heat, all radios, nav lights, landing lights, taxi lights, dome lights, instrument lights, strobes, beacons every damm switch on in the plane has to be on all the time so the compass will read correctly?? Sorry but I have to agree with DFC, the point of a compass swing is to make sure that it stays within limits when all the services are turned on on; but that doesn't mean that you have to fly with them on all the time to get max accuracy! If I flew with everything on in the old heaps I instruct on there's no way the battery wouldn't be able to hack it! (and thats if even if all the lights and nav aids work, which is a rareity :rolleyes: )

hfd - that navigator needs a bit of groundschool in mathematics. He/she just put the aircraft further off track.

Keygrip
What can I say :uhoh: - would you believe it was a deliberate mistake? 'thought not! :)

Benix
That's not what I'm saying. DFC told us that the electrically induced deviation had to be less than 5 degrees. I simply made the point that there could therefore be an error of up to 5 degrees. Try looking at the compass when you turn things on and off; it moves noticeably in many aircraft although I'd admit the most I've seen is probably 5-7 degrees.
Personally I don't switch on pitot heat as a matter of course for VFR trips but it's part of my line-up checklist for IFR departures.

DFC
The "right hand grip rule" illustrates the direction of the field around a current. The magnetic field around a pair of long wires with equal and opposite currents varies directly with both the current and the separation of the wires and inversely with the square of the distance between the centre of the pair and the victim. The answer to disturbance from leakage fields is to keep the wires very close (coaxial would be good), the current low and the distance maximised.

"Why do the radios get special treatment? - Simple - many sources of alternating voltages and currents as various frequencies" You seem to think that alternating currents cause more compass deviation than direct currents; this is not correct. The compass cannot respond to alternating fields above a few Hz so effectively only static or half wave rectified fields are an issue. Therefore any significant load may have an effect.

I'm surprised that it's legit to swing with electrics off, do you have a link to where the rules are defined?

HFD

The compass swing does not garrantee that the compass will give a true reading / deviation with all states of electrical load.

Again its this old cherry of read the POH it will specify what it is swung using. And usually there is 2 bulbs in a compass linked in series, one either side which cancel each other out. They are in series so it fails safe ie if one bulb goes it stops the other one working so the whole lot still gives the correct reading.

My day job type needs for you to get a resonable reading all sorts of stuff turned off. And normally sits with about 30 degrees error on it. And the most use it gets is for jamming a chart behined when the sun is in you face. And the stuff you do require to turn off means that you are flying on standby instrument power and you will have gone onto timed turns from ATC.

And the point on re-racking is to be honest bordering on dangerous. Re-racking even in the hands of an avionics engineer must be the quickest way of shagging sevicable equipment ever invented. Bent pins, wear and tear. Turn everything on and dry it out is by far the safest way to both yourself (less chance of slashing your hand open on sheet metal) and to the plane of not bending delicate connections.

The pitot heater is one of personal preferance persoanlly now I would have it on all the time. When instructing I had it on at night or IMC.

And the DME argument is very nice but. In reality most modern area nav equip planes will be using and speaking to 6 if not 10 different DME stations. And the likes of TLA or POL can accept over a thousand requests per cycle. A terminal ILS DME must be nearly 500. In the UK i wouldn't worry about it leave it on if you want. If the big boys use a DME slot when they arn't even using it for primary nav so can a SEP. If you don't get a reading inside the DOC NATS need to know so they can upgrade the installation as required. If they don't know they can't fix it.

Turn the whole lot on leave it on would be my advice and if you really don't want the DME just tune it to a freq which isn't used.

MJ

Thanks for all the replies - I never imagined my query would generate so many answers!

I have to declare I am in favour of switching all radios on after start.

Happy Christmas to all and safe landings in 2005.

DFC

I agree that it does not make sense to have things like pitot heat on when not needed (unnecessary electrical drain on system)

where I disagree is with your following comment....
When it comes to drying out the avionics - turn up the heater!

Carbon monoxide poisoning is a real and ever present threat so makes more sense to turn on radios and other low current essential services.

HFD,

The place to go for info is the CAAIP - CAP562

-------

Mad Jock,

If something is U/S. The Avionics Engineer can a) Re-rack the box or b) Replace the box or in some cases swap the #1 and #2 boxes. Each of the options involves removing a box from the rack and putting a box back into the rack. Replacing unserviceable items is not dangerous. Re-racking done properly is a common standard maintenence procedure.

-----

unfazed,

Ask yourself - if the radios kept the cockpit nice and toasty why would they have installed a heater?

If you never use the heater for fear of Co I would recomend that you get yourself one of those Co detectors and watch out for a black dot while you and your equipment enjoy the heat. :)

You are correct about Co but hypothermia can be just as deadly!

Regards,

DFC

re-racking is a con merchants way of getting the plane online again and very rarely actually fixes anything apart from allows the engineer to sign off the fault and usually means you re-start the deffered defect sheet the next flight or the next. The potential damage which can be done usually far out ways the benefit. And if its being done day in day out just to get the thing working it will lead to damage.

Replacing I have no problem with but re-racking is a easy sign off when you don't know or can't be arsed fixing it properly. Also the avionics might not actually have the moisture where the heater air can get to it. And the working temp of most avionics is way in excess of the heater air so the kit will dry out faster with use than with just cabin heat. If you re-rack once and the problem occurs again the next day its the seals on your connectors you need looking at not a constant re-rack.

MJ

You are correct about Co but hypothermia can be just as deadly!

If flying in the Arctic region I agree but at these meore temperate latitudes a warm jumper normally suffices

And if its being done day in day out just to get the thing working it will lead to damage.

Agree with that. Re-racking is not done as an isolated fix and some related checks need to be done. I have experience of a modern aircraft where two boxes had to swapped on a regular basis. This was an approved maintenance procedure until the software engineers could make a more long term fix!

Regards,

DFC

A lot of talk about keeping the damp at bay and I think that this has some merit at this time of year but the biggest "killer" of avionics is heat build up and therefore overheating, this is not my opinion but that of Bendix/King and is clearly stated in most of that companys instalation manuals.

So I have to ask BEagle if "his" aircraft have avionics cooling fans fitted ?

I've had more avionics problems due to corrosion and damp, than for anyother reason. In the U.K I've never had anything overheat, but I also wouldn't turn everything on if the ambient temperature was high.

Like most things in flying, just use a little common-sense, If the weather is cold and miserable, then turning on all the avionics will be a benefit, but during a hot summer when everything will be dry anyway, what's the point.

Horses for courses.

Re: Which Radios to Switch On?
 

As you say you have not had a radio overheat in the UK but you dont have to go to far south in the summer to get the radios very hot.

Put your hand on one of your radios at the end of an hours flying and you will find it walm , when one of your aircraft flys to the south of France or Spain in the summer the OAT on the ground may well be up towards +40C.

Add the heat that you are feeling from an hours flying and the radio could hit the +55C that most avionic companys state as the upper limit for the radio.

I have no doubt that the radio will continue to work at that heat level but that is when the damage is done that surfaces months later, perhaps mistaken for damage from damp conditions ?.

The cost of a radio cooling fan would be covered by the first radio repair that you DONT have to do. In all of my aircraft that have more than a NAV/COMM & transponder I have radio cooling fitted and the low level of radio problems is I think entirly due to the fact that the radios dont suffer from heat build up but not swiching on some of the equipment is simply not an option in some airspace enviroments that my aircraft fly.

Re: Which Radios to Switch On?
 

DFC.....

I know the answer to this question.

" which direction the aircraft is pointing in the event of a total electrical failure"



...............forwards.

Sorry

Re: Which Radios to Switch On?
 

I would say that you are getting the benefit of both the heat and the extra airflow from the cooling fan, giving even better elimination of any damp.:hmm:

Re: Which Radios to Switch On?
 

FWIW (which may not be terribly much) I recall from a previous existence that the thermally-induced failure rate of electronic components was given by the Arrhenius equation. The broad result is that the failure rate will roughly double for every increase in temperature of 10 degrees (the detail is far too tedious to type here but a quick Google will probably reveal chapter and verse if you are feeling masochistic). The bottom line is that a fan is good if it's outside the equipment and you can tolerate the noise but bad if it's internal because it has a large impact on the MTBF.

HFD

Re: Which Radios to Switch On?
 

[B]Noise.. What Noise ?

I can hardly tell if the avionic cooling fan is running when the engine is not so noise in flight or for that matter when taxing is not an issue.

If it is electrical noise that you are thinking of that is not a problem with the Ameri-King unit that I have fitted.

I cant see a down side to fitting avionic cooling, In my opinion the reason that cooling is not fitted is the same as the reason that DME remote channeling & GPS/Encoder interface is not fitted ...... That is the avionic companys have entirely to compeat on cost and the price of a cooling fan and some of the other functions that a lot of the owners dont understand will blunt the avionic shops compeditive edge.

The irony is that six months down the line the owner wishes that he had all the functions of the equipment fitted but now it will cost twice as much to fit the equipment to enable all the functions.

Re: Which Radios to Switch On?
 

A&C: When I mentioned fan (acoustic) noise I was thinking generally, I agree that it's not an issue in aviation.

Re: Which Radios to Switch On?
 

Different aircraft have different requirements (electrically speaking) to get the most accurate reading on the standby compass, so there is no hard and fast rule about that.

Regarding switching pitot heaters on, remember that venturi effect will provide cooling. It costs nothing to switch it on (unless you're stationary on the ground for hours while the stude goes through the run-up checks), and will cost lots if it gets iced up in the air. Besides, it's a good habit to get into.

Finally, on the use of the transponder, if you turn it on, for heavens' sake make sure you're squawking Mode C. Nothing gets the heart rate up if you're trogging along at altitude and you get the "Traffic - Traffic" callout and no Mode C info to tell you where to look. I have known instructors who tell studes to sqawk only Mode A on the grounds that they can't get done for straying into controlled airspace. And yet they call themselves professionals! :bored: