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MarkHurst

Standby Boost Pumps - am I confused?

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Two separate questions, although it may be that they are related...

(1) I understood from reading the manual that the No.2 (standby) boost pump should come on if the No.1 (main) pump fails. However, if I switch the No.1 boost pump off I get both caution lights (No.1 and No.2) and then the engine stops shortly after. If I start the No.2 pump manually all seems okay. Have I misunderstood the operation of the standby boost pumps?

(2) If I set the bus tie switch to OPEN, switch the battery to OFF and then switch off the LEFT generator, I first get caution lights for boost pump No.1 (on both engines) but shortly after that the engines stop. I take this to mean that the No.1 and No.2 boost pumps are powered by the same bus (left DC bus), which doesn't seem right.

Help me out...?

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You have two maintanks, the forward and the aft, when we forget about the optional wingtanks.

Each tank has 2 pumps:

Boost pump 1 FWD

Boost pump 2 FWD

Boost pump 1 AFT

Boost pump 2 AFT

Each Boost pump has a caution light.

Each light is energized by a pressure switch in the delivery line of the corresponding boost pump if the fuel pressue downstrean of the pump falls below 2 Psi.

In normal operation the pressure from the No. 1 boost pump in each tank actuates the No.1 pressureswitch, which extinguishes the No.1 Boost pressure caution light.

It also renders the No.2 boost pump and the No. 2 boost pressure caution light inoperative.

Should the No.1 boost pump fail, and thus the No.1 fuel pressure switch activates (No. 1 Boost pressure caution light will illuminate) due to low fuel pressure, this will turn on the No.2 boost pump automatically.

Should this not happen automatically it can be done by switching the STBY BOOST PUMP EMER switch from NORM to Up position.

Boost pump 1 FWD is powered by the Left DC bus and feeds the Right engine

Boost pump 2 FWD is powered by the Right DC bus and feeds the Right engine

Boost pump 1 AFT is powered by the Left DC bus and feeds the Left engine

Boost pump 2 AFT is powered by the Right DC bus and feeds the Left engine

Also...

Boost pump 2 (FWD/AFT) will only activate if Boost pump 1 (FWD/AFT) fails, or the STBY BOOST PUMP EMER switch is in the up position, or the Boost pump switch is set to TEST (springloaded).

But I see that if the Boost pump 1 (FWD/AFT) fails due to no electrical failure of their feeding DC bus, then this doesn´t happen, which is a bug.

Try this hotfix and see if it makes more sense (slightly corrected electrical dependencies):

See new Hotfix below !!

Unzip and overwrite (after having backed up TW_Sys1.xml) into:

\..\Microsoft Flight Simulator X\SimObjects\Airplanes\Aerosoft_DHC6_300_WHEEL\Panel\TW_System\

Finn

TW_Sys1.zip

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Try this hotfix and see if it makes more sense (slightly corrected electrical dependencies)

Thank Finn, I will try it tonight and report back.

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Try this hotfix and see if it makes more sense (slightly corrected electrical dependencies):

Hi Finn, this doesn't seem to make any difference. I can see the changes you added and tried following the logic in the XML file but I gave up! (I don't know about XML but it looks kind of like FORTH!)

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So Mark....

Here is a new Hotfix test version:

TW_Sys1.zip

As always...

Unzip and overwrite (after having backed up TW_Sys1.xml) into:

\..\Microsoft Flight Simulator X\SimObjects\Airplanes\Aerosoft_DHC6_300_WHEEL\Panel\TW_System\

But do note !!!

Both the AFT Boost pump no. 1 and FWD Boost pump no. 1 are powered by the Left DC bus.

The AFT Boost pump no. 2 and FWD Boost pump no. 2 are powered by the Right DC bus.

This means that with the Fuelselector in NORM, the Boost pump switches in ON, and the Stdb boost pump switches in down position, the following will be true....

If You turn OFF the Battery, Open the Bust tie switch, and turn OFF the Left generator, then both the FWD no. 1 and AFT no.1 pumps will fail and the Std by pumps (FWD no.2 and AFT no,2) pumps will automatically switch on, thus restoring fuel pressure.

If instead You turn OFF the Right generator, then nothing will happen to the Boost pump operation (FWD no. 1 and AFT no. 1will keep running), since Left DC poer is still available.

But since the Bus tie switch normally is set to NORM, the Left and Right DC busses are connected, so if one of the engiens and/or it´s associated generators fail, then the other engines generator will supply both busses.

Best regards

Finn

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This means that with the Fuelselector in NORM, the Boost pump switches in ON, and the Stdb boost pump switches in down position, the following will be true....

If You turn OFF the Battery, Open the Bust tie switch, and turn OFF the Left generator, then both the FWD no. 1 and AFT no.1 pumps will fail and the Std by pumps (FWD no.2 and AFT no,2) pumps will automatically switch on, thus restoring fuel pressure.

Yes, that now works for me! However, I still don't understand why I get Caution lights for BOTH No.1 and No.2 pumps (and then engine shutdown) when I just switch off the main pump. Honestly it's not a real problem, I'm just trying to understand how it works :)

Thanks for the fix!

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Note that the teo "main" boost pump switches aren't only for the FWD no.1 and AFT no.1 boost pumps, but turns on the no.1 pumps and "arms" the no.2 pumps to automatically start shoul the no.1 pumps fail.

So if You turn the "main" boost pump switces off, nonr if the pumps will run and since the fuel tanks are under the floor of the Twin Otter, the engine driven pumps are not sufficient to maintain adequite fuel pressure and the engines will starve.

To turn on the no.2 boost pumps You must turn on the stdby boost pump switches.

Finn

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"main" boost pump switches...turns on the no.1 pumps and "arms" the no.2 pumps to automatically start shoul the no.1 pumps fail.

Okay, that makes sense now. Thanks for the fix!

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Interesting note, the twin otter should be able to draw fuel from the belly tanks using the HP fuel pump (engine driven) all the way up to 8000ft. If we have a failed boost pump and we are limited to 8000 ft and below even if the #2 stdby is working.

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Interesting note, the twin otter should be able to draw fuel from the belly tanks using the HP fuel pump (engine driven) all the way up to 8000ft. If we have a failed boost pump and we are limited to 8000 ft and below even if the #2 stdby is working.

I did not know that....

I guess 8000 ft still is a safe margin, so maybe engines shouldn´t quit before before over 10.000 ft ?

It is possible to add the code for that if needed.

Finn

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Hey Finn,

That is so that if the Automatic Change Over System (ACOS) fails or the stdby pumps fail, the engine can still draw fuel up. the ACOS functions by detecting a loss of fuel pressure and automatically starting the second pump if the main pump is on but fuel pressure is <2PSI. The plane is certified to 8000 feet with a pump MEL'd and the MEL states only 1 pump is reqd per tank but ops are limited to 8000 and below. There have been stories of twin otters flying up to 16000 with NO pumps on and not flaming out, but they are largely unconfirmed.

//crash

edit: here is what the manual says about the twin otter

"BOOST PUMP FAILURE NOTE

Failure of a No. 1 boost pump (BOOST PUMP 1 FWD PRESS or BOOST PUMP 1 AFT PRESS caution light illuminated) will automatically activate the No. 2 boost pump, and engine operation will be unaffected.

Boost Pump CB: .... PULL

CAUTION

Simultaneous failure of the No. 1 boost pump and the automatic change over system could cause a flameout. The applicable BOOST PUMP 1 AFT PRESS or BOOST PUMP 1 FWD PRESS caution light does not illuminate with the AFT BOOST PUMP or FWD BOOST PUMP switch OFF."

and
under the engine shut-down in flight

Boost Pump Switch: .............. OFF & LIGHTS ON

Check BOOST PUMP 1 AFT PRESS and BOOST PUMP 2 AFT PRESS or BOOST PUMP 1 FWD PRESS and BOOST PUMP 2 FWD PRESS caution lights illuminate.

NOTE

If the boost pump caution lights do not illuminate immediately, a fuel boost pump pressure switch failure has occurred. This will prevent the automatic switching on of the standby boost pump and may be the cause of the engine flameout. An engine re-light can be attempted after restoring the fuel supply by selecting the appropriate STDBY BOOST PUMP EMER switch ON to energize the #2 boost pump independently of the automatic changeover system. If it can be determined that the engine flame-out resulted from fuel starvation due to a pressure switch failure, the engine may be restarted."

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Hi Crash

Not sure if I fully understand the description above.

The only issue You see, is it that engines shouldn´t flameout below, let´s say 12.000 ft with both the No.1 and No.2 fuelpumps off ?

I think the logic for the Fuel pressure caution lights should be correct as they follow the checks in the checklist very closely.

Finn

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Hi Finn,

I can only tell you what my Ops spec and Aircraft operating manual say. They both say with one pump inop limit flights to 8000 ft or below because if the second pump fails there is no guarantee that in all flight conditions above 8000 fuel flow will not be interrupted to the affected engine.

The light logic should be fine.

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As a matter of curiosity, are these changes included in the update of the post called:

[FSX] [P3D V1] DHC-6-300 Wheels Alternative FDE and Engine simulation

I am just afriad if I add these hotin this post, it is going to break my latest update as per above post..

Since You posted this:

Bummer ok, I will go fly the A2A planes where developers are making GTN750 available for 172 and 182 etc etc etc, and please note; in real live in 99.9% of cessna's the 750 is also not installed but that does not mean the developer now refuse to have it installed - to force us to fly it like in Real live. ( Exact same as in capt crash circumstances. Just becuase it is not installed in his fleet, we should now rather "fly it like he is doing it". - nothing against Capt Crash - I am merely using it to illustrate a point.)

Was good but it seems the fat lady gave her last song.

as a principle. I uninstalled - > and is moving on.

I think a reply is no longer needed.

Finn

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As a matter of curiosity, are these changes included in the update of the post called:

[FSX] [P3D V1] DHC-6-300 Wheels Alternative FDE and Engine simulation

I am just afriad if I add these hotin this post, it is going to break my latest update as per above post..

You written publically that you would de-install this add-on, what did make you reconsider? You seemed to be very determined because we did not do what you wanted.

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