Flying from the Mid-Atlantic to the PNW, California, and back again

[Steve]

From our trip about 12 years ago...

[dant]

Glancing through your great photos I noticed you were flying at 12,5 without your fuel pump off - do you not have issue with vapor lock at altitude?

[mhoran]

Glancing through your great photos I noticed you were flying at 12,5 without your fuel pump off - do you not have issue with vapor lock at altitude?

When level it wasn't much of an issue but I did have to turn it on in the climb. I've a habit to keep it off unless absolutely necessary as we've had to replace the pump twice already.

[dmloftus]

Glancing through your great photos I noticed you were flying at 12,5 without your fuel pump off - do you not have issue with vapor lock at altitude?

Have you had an issue with vapor lock? Like Matt, I do not typically turn on the fuel pump at altitude. I probably should have in multiple flights over the Grand Canyon but I have never experienced any problems in any higher altitude flights up to 15,500. I would be very interested to hear the symptoms and recovery.

[danno2000]

Glancing through your great photos I noticed you were flying at 12,5 without your fuel pump off - do you not have issue with vapor lock at altitude?

I have had the fuel flow dips and spikes associated with vapor lock on a fairly regular basis, most often at 7,000 to 9,500 feet and particularly on warm days. Interestingly, flying much the same cross-country route eastward at 11,000 or 11,500 feet, I don't remember any problems.

When it happens to me, the engine feels different but the RPM/MP indications don't change too much in my experience. My usual remedy is to turn the fuel pump on for 30-60 seconds which is usually enough to take care of it. On a troublesome flight, I might have to repeat that process once every 10 minutes or so.

best,
dan

[dant]

I've a habit to keep it off unless absolutely necessary as we've had to replace the pump twice already.

Do you have the Welden pump?

I usually leave it off until the flow warning dings which brings it to my awareness, then I leave it on until I descend below 10k or so. I suppose it's possible that I could get away with danno2000's technique of just bumping it ever 10 minutes or so but since it's a welden I haven't worried about it.

[mhoran]

I've a habit to keep it off unless absolutely necessary as we've had to replace the pump twice already.

Do you have the Welden pump?

I usually leave it off until the flow warning dings which brings it to my awareness, then I leave it on until I descend below 10k or so. I suppose it's possible that I could get away with danno2000's technique of just bumping it ever 10 minutes or so but since it's a welden I haven't worried about it.

I have the CJ pump (which I believe is Dukes?) My procedure is pretty similar to yours but I leave it on until my switch tanks alert goes off (15 minutes) and then shut it off again. But I didn't have any issues over the Rockies. Since performing RSB 40-094 I've had a lot less issues with vapor lock on the ground and in the air. Prior to installing the new insulation I would sometimes get low fuel warning when shutting off the switch shortly after takeoff. That didn't really happen much on this trip. Just a couple of times I had to turn the pump on but turning it off sometime later was fine.

For me whenever I get the low fuel warning there is no noticeable power / performance change nor do I hear the engine struggling in any way. It almost seems like a bad reading but I turn the pump on anyway and the alert goes away.

[Rich]

The only fuel condition anomaly I've ever seen is very occasionally when high and hot. It'd be a short duration substantial increase in indicated fuel flow with no accompanying change in indicated fuel pressure, MAP, RPM, EGTs, CHTs or perceived engine power. It'll be a sudden indicated increase and slowly (period of maybe 10 seconds) ramp back down to normal. It never shows a dip below normal FF, only an increase.

I attribute the difference I observe to some others' observations to any one (or all) of these possibilities:
1. The VM1000 FF sensor may react differently than the G1000's to whatever transient fuel condition (bubbles?) is triggering it. This may also be true of the fuel pressure sensor.
2. Differences in the ways that the VM1000 and G1000 are programmed to respond to their respective FF sensors. Remember you're seeing what the black box chooses to display. So it's possible that the actual condition is more momentary than the indicating fields would suggest.
3. My plane has not had the RSB about the firesleeve done, but somehow has the additional large FS installed over part of that fuel line segment. I don't know how it got there. Factory installed?

[MarkA]

Rich, I have seen this same behavior in my DA40 XLS - short duration fuel flow spikes in cruise that settles back down to normal without changing anything.

These fuel flow anomalies show up as spikes in the G1000 logs. As you point out, there is no change to RPMs, MAP or CHTs when they happen. I suspect the CHTs don't change due to the thermal mass of the cylinder heads and the short duration of the spikes.

However, there is a corresponding slight dip in EGTs that shows up in the logs when a spike occurs so I think the fuel flow really is increasing at those points. See the attached FlySto.net plot of fuel flow and EGTs from a recent 3.5 hour flight from KHIO -> KHLN (13,000 ft, ISA +13, wide open throttle, 2400 RPM, lean of peak @ 7.2 GPH). Short fuel flow spikes occurred 6 or 7 times early in flight then didn't happen again for the rest of the flight. In my case, I do have the fire sleeve RSB (RSB 40-094) installed.

One thing that catches your attention right away when a spike occurs is the G1000 fuel range rings suddenly appear on the MFD for a few seconds indicating you don't have enough fuel to reach your destination

[Rich]

Mark, this all makes perfect sense. It's data I can't really determine from my ancient EIS. If fuel actually is flowing at a higher rate, what the heck would cause that? Can you add fuel pressure to the graph? It seems logical that fuel pressure would be part of the equation. Then we'd be left with what mechanism that normally limits fuel pressure is acting up.