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

[MarkA]

A graph of fuel flow (blue line) and fuel pressure (green line) is included below for the same flight.

It is a little hard to read but you can clearly see there are drops in fuel pressure corresponding to the fuel flow spikes. In this case, the booster pump was not on and almost for sure no changes were being made (mixture, RPM, throttle, etc.) around the last three spikes. The earlier spikes happened during the climb to 13K while I was leaning as the altitude increased. The plot below shows fuel flow vs Altitude MSL.

It's interesting to note the early spikes all seem to happen exactly at the point I leaned the mixture a bit during the climb. As if when the mixture was adjusted, the fuel flow sometimes spikes for a few seconds after the adjustment. Again, I don't think any mixture adjustments were made after the "big pull" at 13K which leaned down to about 7.2 GPH.

Maybe it just takes a while for the fuel flow to settle down after a mixture adjustment? Could the fuel flow spike cause a drop in fuel pressure or could a drop in fuel pressure somehow cause a spike in fuel flow?

[Rich]

It looks to me like the fuel pressure drops occur slightly ahead of the fuel flow increases when the drops are greater than some specific magnitude. Can you zoom in on that area?

[MarkA]

The "fuel flow vs fuel pressure" and "fuel flow vs MSL" plots for the return flight from KHLN -> KHIO at 12K are included below. The same leaning approach was used on this flight as before - lean incrementally during the climb then a "big Pull" to go lean of peak at the cruise altitude. On this flight there were no fuel flow spikes for the entire ~4 hour flight.

This seems to indicate at a minimum it can't be easily or reliably reproduced. Sigh...

[MarkA]

Yep, the fuel pressure drops right before the fuel flow spike happens. I assume the fuel pressure drop occurs when the mixture leaning starts. See the plot below of a zoom in for two of the fuel flow spikes that happened while leaning during the climb.

Rich, I sent you a PM with a link to flysto.net where you can access the KHLN flight directly. Let me know if that link works for you.