bpattonsoa
Well Known Member
Here is an interesting fuel pressure issue I saw the other day in the -10. Looking for input.
Flight from KSBP to 7S5, cruse altitude 14,500, good tail winds up at altitude. Aircraft was fueled the day before and sat outside with an OAT approaching 95. I assume the fuel temperature was high. Spent 15 minutes on the ground after engine start so plenty of time for things to heat soak, very conservative tower, used to fly out of there. Takeoff about 1630 with an OAT of 80 (had been over 90 earlier) rising to 91 six minutes after takeoff. Takeoff with fuel pump on, fuel pump off at about 1,500'. With the high OAT and a long flight ahead, I decided to do a slow climb, around 145 knots. Going thru 5,000' (much higher PALT) I leaned it down to about 15 GPH.
Looking at the data on Savvy Analysis during the climb, the fuel pressure was wiggling about .1 - .2 PSI and slowly dropping. I did not notice until I got a low fuel pressure alarm (at 17 PSI) 11 minutes into the flight at a pressure altitude of 8,500', Turned fuel pump on and it immediately recovered.
Continued on (plenty of airports around) another two minutes and then turned fuel pump off at about 14 minutes. Dropped in about 15 seconds to alarm and fuel pump went back on. Now it had my attention! Thoughts of Lefffler's issue running thru my head, a long way from home.
Switched tanks and waited about 5 minutes and turned fuel pump back off. Pressure did a quick drop to around the alarm and the fuel pump went on again. OAT was now 55.
Case of get home today, so I continued until getting to 14,500, OAT around 49. Turned the fuel pump off 30 minutes into the flight. Pressure went to normal, about 28.6, but looking at the data reveals that it was bouncing around about .1 - .2 PSI for at least another 40 minutes. It then stabilized into a straight line with and occasional bump for the rest of the flight.
For the first 15 minutes at cruise altitude, it did not seem to want to lean normally. GAMMI spread (Skyview system) was normal, around .7 GPH, but TAS would not remain stable after leaning. I tried several times and then it finally remained steady at its regular 172-3 knots. Rest of flight was normal.
I have a tunnel thermocouple, tunnel temp was 132 during the weird behavior and 113 the rest of the flight. The fuel system is normal Vans design except the Red Cube is between the injector and the Spider. (Solved the strange behavior with the fuel pump on)
I am hoping that I was seeing the effect of hot fuel, not a mechanical pump problem.
Comments
Flight from KSBP to 7S5, cruse altitude 14,500, good tail winds up at altitude. Aircraft was fueled the day before and sat outside with an OAT approaching 95. I assume the fuel temperature was high. Spent 15 minutes on the ground after engine start so plenty of time for things to heat soak, very conservative tower, used to fly out of there. Takeoff about 1630 with an OAT of 80 (had been over 90 earlier) rising to 91 six minutes after takeoff. Takeoff with fuel pump on, fuel pump off at about 1,500'. With the high OAT and a long flight ahead, I decided to do a slow climb, around 145 knots. Going thru 5,000' (much higher PALT) I leaned it down to about 15 GPH.
Looking at the data on Savvy Analysis during the climb, the fuel pressure was wiggling about .1 - .2 PSI and slowly dropping. I did not notice until I got a low fuel pressure alarm (at 17 PSI) 11 minutes into the flight at a pressure altitude of 8,500', Turned fuel pump on and it immediately recovered.
Continued on (plenty of airports around) another two minutes and then turned fuel pump off at about 14 minutes. Dropped in about 15 seconds to alarm and fuel pump went back on. Now it had my attention! Thoughts of Lefffler's issue running thru my head, a long way from home.
Switched tanks and waited about 5 minutes and turned fuel pump back off. Pressure did a quick drop to around the alarm and the fuel pump went on again. OAT was now 55.
Case of get home today, so I continued until getting to 14,500, OAT around 49. Turned the fuel pump off 30 minutes into the flight. Pressure went to normal, about 28.6, but looking at the data reveals that it was bouncing around about .1 - .2 PSI for at least another 40 minutes. It then stabilized into a straight line with and occasional bump for the rest of the flight.
For the first 15 minutes at cruise altitude, it did not seem to want to lean normally. GAMMI spread (Skyview system) was normal, around .7 GPH, but TAS would not remain stable after leaning. I tried several times and then it finally remained steady at its regular 172-3 knots. Rest of flight was normal.
I have a tunnel thermocouple, tunnel temp was 132 during the weird behavior and 113 the rest of the flight. The fuel system is normal Vans design except the Red Cube is between the injector and the Spider. (Solved the strange behavior with the fuel pump on)
I am hoping that I was seeing the effect of hot fuel, not a mechanical pump problem.
Comments
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