Backup battery switch

by Poul Stromberg in 915iS Technical Questions 5 years ago

Confused!
How do you restart an engine in the air if the ALT A & B is dead?
if as I did read in this forum:
- Caution: Do not turn ON the Back-up switch during engine start as this will defeat some of the ECU self-checks.
-
If I like to restart the engine do I need to switch on "Start Power" or can I only switch on "Backup Switch" and restart the engine
or
Do I switch on "Start Power", start the engine and then go over to "Backup". Feels a little bit tricky.
Hard to find in Rotax documentations about backup and operating procedures.

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Re: Backup battery switch

by Rob Seaton » 5 years ago

Rob Seaton

3 1.32k 240

For a emergency re-start turn the Back-up switch ON and hit the starter. (If the engine is windmilling you do not need the starter).
You do not need to turn on the Start-power, cycle the Lanes or Master.
The "- Caution: Do not turn ON the Back-up switch during engine start as this will defeat some of the ECU self-checks" is referring to a normal, everyday start.

Thank you said by: Poul Stromberg

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Re: Backup battery switch

by Joe Gores » 5 years ago

Joe Gores

0 6 0

A Kitfox wiring diagram uses one DPDT switch for both Battery Backup and Start. The switch can be in only one position at a time, either backup or start, but NOT both at once. Suppose that all electrical generators fail on takeoff when 200 feet above the end of the runway. The engine quits and is not windmilling. There is not enough altitude to dive. If that switch is moved to "START", will the engine start? Will it even crank without the backup switch being closed? Should the engine be wired according to the Rotax manual using two switches instead of Kitfox's single switch?
Thanks, Joe

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Re: Backup battery switch

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

Either position should work.

The engine Cranks and Starts in the Start position on every normal engine start. Neither generator is functioning until after the Start.
The Start Power connection (X3-2) connects to the ECU power Bus through a Relay internal to the Fuse Box.
Under normal operation, once the engine starts and the ECU confirms "Power Good" from the "A" Generator, the ECU transfers the relay from "Start" power to "A-Gen" power.
If, as in your hypothetical situation, Both Gen's have failed, the "Power Good" signal will never appear and the engine will continue to run on Start power.

"Backup" would be the preferred choice in an Emergency situation.
The Backup Power connection (X3-1) connects directly to the ECU Power Bus.
It bypasses all the relays and cannot be disabled by an intermittent Generator.
This is why you should Not use Backup for Normal Starting.
The Gen Power Good Tests will not be valid.

You can confirm all this by disconnecting the cables to the two generators.
Start the engine with the Backup/Start switch in both positions.
It should Start and continue to run Run just fine
Both Lane Warning lamps will be ON, however.
Ground test only, Do not Fly until both generators are connected and working.

Footnote: The engine ECU initially starts using the B-Generator.
Once it is Confirmed to be making power, The A-Generator is switched to the ECU Bus.
Once the A-Generator is verified, The Start Power is removed.
- - - - -

Please confirm that your DPDT Backup/Start Switch is a Center-OFF switch. >>> Backup/OFF/Start <<<

One switch is functionally OK
You never Need to turn both functions ON at the same time. Plus they are essentially the same thing.
However, if that One switch should ever fail there is no redundancy in the circuit.
But then, what are the Odds that BOTH Generators AND the Switch will ALL fail on the same flight???

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Joe Gores, Christopher Peet

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Re: Backup battery switch

by Joe Gores » 5 years ago

Joe Gores

0 6 0

Thanks Bill,
Yes, the DPDT switch is center off. But the start position is momentary and will return to center off when released. The Backup position is maintained. In the above posted failure mode, the pilot could start the engine, then quickly move the toggle switch from Start to backup. The plane belongs to a friend of mine who will be wiring his Kitfox soon. He will be happy after reading your response.
Joe

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Re: Backup battery switch

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

For an emergency start, select "Backup" and just start cranking.
If you select "Start" and then toggle to "Backup", you risk resetting the ECU as the switch passes through the OFF position and power is temporarily lost.

Verify that there is another "Start" Button/Switch in addition to the "Start" Switch
There should NOT be just the one momentary Start switch.

The NORMAL start sequence should be...
Enable Start Power.
Wait for Lane lights to go dark. (ECU Reset Complete)
Engage the Starter. Cranking!
Confirm Start.
Release Starter.
Set Idle RPM.
Confirm Lane Lights are Dark. (Generator Power Good)
Disable Start Power.

The EMERGENCY Start sequence should be...
Enable Backup Power.
Engage the Starter. Cranking!
Confirm Start.
Release Starter.
Set RPM.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Joe Gores, Christopher Peet

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Backup battery switch

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