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Distribution of equipment over several buses, in a single alternator aircraft

In the TB series you have the supply (alternator+battery) which splits over 3 buses.

Various equipment (which does not go through the avionics master switching) connects to these 3 buses, often via fuses (which are virtually inaccessible).

Then you have 3 relays and the other side of these you have 3 avionics buses, with various avionics coming off these. These 3 relays are energised via the avionics master switch.

In later TBs, operating the starter motor disconnects all 3 relays – just like you had/have in cars. This is to protect the avionics from the ~200V starter motor transients.

Is there any accepted wisdom as to which equipment should go on which bus?

There are not many failure modes which would bring down a whole bus anyway, given that almost everything coming off these avionics buses has its own CB.

There are subtle aspects e.g. all equipment which comes off before the avionics master must be capable of withstanding the starter motor spike. This then becomes relevant. However I know of equipment which is required by STC to be connected to the primary bus yet I know for a fact that ~50V will blow it up… Such equipment needs to be wired in contravention of the STC, via a switch which per-startup-checklist is OFF during engine start.

Administrator
Shoreham EGKA, United Kingdom

Are you sure the starter transients are a problem? In the experimental world most people wire all the equipment to the main bus directly without any issues. There is quite a bit of debate about it – eg http://www.aeroelectric.com/articles/avmaster.pdf

EGEO

I know Bob Nuckolls and he’s a very clever guy (incidentally, many years ago, he designed the B&C alternator and voltage regulator product line) but there is plenty of counter data. One guy I know (TB21; he has posted here occassionally) blew up two radar altimeters this way. Another (TB10) blew up his entire centre stack.

The IEC load dump spec is there for a good reason, at least historically

200 volts.

I know of two items, both very well known bits of avionics for which I know the input circuit. I am not going to mention them One uses a 7805 regulator (and the on/off switch on it is a fake in that it switches the output of the regulator!) and the other uses a DC-DC converter. Both are unprotected and both rated at 36V max. I reckon both will blow up at around 50V, especially the former.

The reasoning behind it is simple enough: when the starter motor current is interrupted (when the engine starts) the energy in the inductor has to go somewhere. So a huge arc will appear across the starter relay contacts as they open. This does 2 things: the arc generates massive RF which will go everywhere in the aircraft wiring, and the energy being dumped has to go somewhere. Hopefully it get soaked up by the battery (IF the battery has a low enough source impedance) but this will depend to some extent on how far the battery is (in terms of the inductance of the wire going to the battery).

I can sell see that

  • a lot of avionics are designed to cope (any designed by a real hardware engineer will be ok)
  • on some (or many) aircraft installations the wiring is such that it doesn’t matter

However

  • most homebuilts are 12V so the problem is much smaller
  • anyone even half careful flying a homebuilt which they spent 100k on is not going to be starting the engine with the avionics on
  • homebuilts tend to be relatively simple, with few of them having panels packed with various individual instruments of old design
  • cars have turned off all non-engine items during starting, since for ever, and presumably again for a good reason
  • the cause can never be proved, because you cannot tell if it failed at turn-off or at turn-on

However the main part of my Q was what is the thinking behind splitting it up over 3 buses and are there any rules for it? I can see that it might make sense to have COM1/NAV1 on bus 1 and COM2/NAV2 on bus 2, but I still can’t see what the 3 buses give you IF each item is protected by its own CB.

Administrator
Shoreham EGKA, United Kingdom

That’s fair, can’t argue with any of that.

Re 3 busses vs 2, I can’t think of any reason other than possibly current limits in the relays? Even then, 3 vs 2 isn’t a huge help.

Multiple relays along with the fail safe mechanism in the TB do make sense from a single point of failure perspective (the relays close when the control circuit is open, so pulling the “Avionics Master” circuit breaker closes the relays, giving a secondary control if the switch fails closed).

EGEO

The use of multiple busses may also relate to load-shedding requirements in the event of an alternator failure. Maybe worth reviewing the Emergency procedures section of the AFM/POH to see if there are any specific shedding procedures to ensure you keep the required 30 minute battery duration.

Avionics geek.
Somewhere remote in Devon, UK.

other than possibly current limits in the relays?

The relays used by Socata are the Cartier relays here which are rated at 20A. The whole plane draws around 20A with everything on (excluding the gear pump and flaps, which anyway come off before the avionics master). Admittedly on a strict load analysis the plane probably draws about 50A, with stuff like a KX radio “drawing” 7A but actually it doesn’t unless you are driving a 4 ohm speaker with a continuous sinewave at full blast.

use of multiple busses may also relate to load-shedding requirements in the event of an alternator failure

How would having the 3 buses help with this, compared to just turning off avionics, or (if everything was on one avionics bus) pulling the respective CBs?

The generic TB GT circuit is here (with Socata one can never be sure because they do “working drawings”)

Incidentally, how would one wire up a scheme whereby one radio (it would be a GTN650 by the time this is done) can be turned on with a toggle switch, to save the battery while waiting for a departure clearance at some busy airport? I can think of obvious ways (most people just connect a switch between the battery and the radio ) but I want the starter motor protection as well, so a relay would have to be involved which is opened when the starter is energised, just like happens in the above diagram with the three avionics bus relays. I wonder if there is some smart way to do this.

The other way to deal with starter motor muck is to put a clamp somewhere, say 36V, but then you have nasty failure modes to deal with if the alternator goes haywire and sends say 50A into the clamp, which will then dissipate best part of 2kW One could have some protection on the clamp… I wonder if anyone has done this?

Administrator
Shoreham EGKA, United Kingdom

The plan for my RV build is related to this, but uses two independent busses:

A main bus, with a big battery, the main alternator, the starter and an avionics master relay powering the audio panel, COM/NAV2, autopilot, and some duplicated EFIS components eg magnetometers.

A secondary bus with a smaller battery, a second alternator and no avionics master. This will power COM/NAV1. Its is not connected to the starter.

Most EFIS components will be powered by both devices with diode-isolated inputs (this is a feature of Garmin’s G3X system).

For starting you will engage the secondary bus, which will bring the PFD, engine analyser, COM/NAV1, a few auxiliary EFIS components and the interior flood lights online. This gives you radios to call for start, and about 3 hours of battery life if you have to wait (overkill, but a small LI-ION battery has a surprising amount of juice).

Then you engage the main bus but leave the avionics master off. You start the engine, avoiding any risk of electronics rebooting if the voltage drops too much. Finally, you bring the main bus avionics online.

If the main bus or avionics master fail, you lose COM/NAV2, the autopilot, the right hand EFIS screen, some backup devices (magnetometer #2, ADAHRS #2) and the audio panel defaults back to COM1. Airframe systems (flaps, trim, pitot heat, external lights) would also be unavailable.

This also gives you a simple load shedding mechanism – if the main alternator fails you just shut down the main bus avionics and kill the lights/pitot heat. At that point you’re pulling about 0.5A for the main contactor, so the battery should last for ages.

If the secondary bus fails, you lose some backup devices (magnetometer #1, ADAHRS #1, OAT, interior flood lights), but most things just carry on.

Load shedding here is trivial – just shut down the second bus.

EGEO

Socata use the “diode-OR” thingy for the transponder, taken from bus1 OR bus2 (see circuit diagram above).

Interesting they don’t do it for the radio – one would think comms is more important

OTOH any arrangement which does the radio also needs to do the intercom (audio panel) so the headsets are alive, etc. That’s unless one also connects one headset directly to the radio… complicated and messy.

Your second battery is a neat solution but not possible on a certified plane.

Administrator
Shoreham EGKA, United Kingdom

Reading some stuff on this, it would appear that the reason for having three (or some other similar number) of avionics buses is to guard against a failure of one of the relays, with it being stuck in the open position.

One could guard against it with a bypass switch, as shown in Bob Nuckolls’ paper above

but for some reason nobody seems to have done this in a production plane.

Curiously he connects the autopilot to the unprotected bus!

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

but for some reason nobody seems to have done this in a production plane.

That, as well as the ability to load shed in the event of dual gen failures etc, is essentially done in a lot of jets with an EMER bus.

EGTK Oxford
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