Crossfeeding with fuel pumps on?

[neema]

It's been a minute since I got my initial training in the 42-VI (8 years). I recall my instructor mentioning a limitation using crossfeed while the fuel pumps are on.

Dusting off some of the cobwebs, I was trying to find something in the 42-VI or 62 AFM that supports that and couldn't find anything. In fact, found the opposite in some of the 62 AFM. A couple examples attached.

Intuitively, this doesn't make a lot of sense since one of the two pumps is running at all times anyways. Not sure I understand the fuel diagram enough to see where running both pumps may be an issue in crossfeed.

Could very well be my memory has distorted some info over the years. Appreciate any comments

[CFIDave]

I seem to recall the limitation of "not using crossfeed with fuel pumps on" applied only to the DA42NG (before the -VI was introduced in 2012). If so, that's why you remember it only from many years ago!

[FlyingPenguin]

With my DA42NG training, I purchased John’s ebook. Excellent book BTW, highly recommend it.

In it, it states:

“WARNING: Fuel pump switches must be OFF before selecting CROSSFEED to prevent damage to the high pressure fuel pump.”


Excerpt From
The Concise Guide to the Diamond DA42
John Robert Ewing
https://books.apple.com/us/book/the-con ... d917120323

[MarkO]

It says the same in his DA62 book.

[FlyingPenguin]

From the DA42NG AFM.

Pump off, before crossfeed.

[neema]

The 42NG (not -VI) AFM seems to be the only one that suggests pumps off before crossfeed. What changed, if anything, in the -VI or 62?

[FlyingPenguin]

The 42NG (not -VI) AFM seems to be the only one that suggests pumps off before crossfeed. What changed, if anything, in the -VI or 62?

Hm..I believe the NG and VI run off the same checklist? On my VI docs, it also says pump off.

[neema]

Ah, I think you’re right. This tidbit in the back of the 42-VI AFM in chapter 7 description.

DA62 AFM doesn’t have the sentence suggesting pumps off

[Colin]

Perhaps when the valve operates it momentarily blocks the flow, which is bad for the electric pumps. So the procedure is for making the switch, pumps should be off when you move the valve. After that you can turn them back on?

That makes a little sense to me.

[FlyingPenguin]

Perhaps when the valve operates it momentarily blocks the flow, which is bad for the electric pumps. So the procedure is for making the switch, pumps should be off when you move the valve. After that you can turn them back on?

That makes a little sense to me.

Well, I did some digging. Turns out it’s an excessive fuel pressure issue. Keep in mind that each engine has one high pressure rail, and two low pressure fuel pump. At any given time, one low pressure pump is always on.

In the case of a crossfeed, the fuel line is already pressurized by TWO low pressure pumps already, before getting into the high pressure rail. If one was to turn on the boost pump on the engine, you now just increase the fuel pressure into the high pressure rail (three low pressure pumps on now).

See AFM warning. Hope that makes sense.

So, this is for the DA42. DA62 fuel system is setup slightly different. It has a Bypass valve in the fuel line next to the low pressure pump, maybe that’s why the DA62 is different?