Engine fuel pump not working at altitude

T

tommaness

New member
If Jerry Jr. is still on line please let me know if it is common for a 2004 8KCAB fuel pump to lose pressure at altitudes from 12 to 14000 feet. We do a lot of mountain flying and as we approach 11 to 120000 feet the pump pressure will drop down from around 24 to red line or below. If we switch on the dash fuel pump it goes back to normal(around 24). Jerry, is this anything to worry about or is it normal at these altitudes.(Can be hard to find a spot to land in the mountains if i lose my fuel pressure). Thanks in advance. Tom
 
No, that is not normal.
BTW, anyone who wants to chime in, the fuel pump system is pretty much the same since Decathlon inception. For that matter, any injected Lycoming.

So, when you decend, does it come back to normal?

Trying to gather more data here. Something seems amiss.

A Lycoming expert might know things too, since this is the engine driven pump we are talking about.
 
When we descend the fuel pressure will return to normal. It will return to normal pressure around 11000 feet. Thanks Tom (8KCAB-491MA)
 
Well I'm no expert on the Decathalon so I'm not real sure but troubleshoooting correctly is no problem. I'd start with the simple things, check your lines make sure they are not pinched or damaged, check your fuel vent.The electric pump probably has a lot more ability than your mechanical pump.The mechanical pump is nothing more than a pair of check valves and a diapharm. The inlet check valve allows fuel in and keeps it from returning back into the inlet line as the diapharm flexes the other way fuel is forced out. The problem occuring at altitude would tend to make me think that external pressure is affecting fuel flow and maybe starving the mechanical pump. Are you running 100LL fuel? I run auto fuel all the time one of the draw backs is you really never know just what you are getting. I'm sure there are some guy's on this group that can probably tell you just what the problem is. Anyway hope this helps .


Woodie
N29763
 
Woodie, I run 100LL aviation fuel only. I will go to altitude once more and get back with you on exactly when I lose pressure and at what altitude it returns. thanks Tom
 
I just noticed that this a a 8KCAB which means it's fuel injected also means there is probably a "flop" tube in the tank for inverted flight which is something else to look at. These fuel injection systems on these engines amaze me how well they work to really be so simple. Impact air hitting the servo is basicly what tells the flow divider to open and allow more fuel as the throttle vale is opened. Do you notice any difference in how the engine runs as the fuel pressure drops? I can't remember who makes and repairs the injection system componets, but you can look in Trade a Plane and find who it is ,give them a call and see if "anything" in the injection system will cause these symptoms.


Woodie
N29763
 
Woodie, as the fuel pressure dropped to red line and below I did not notice any loss of power. I might add that I did not leave it there long before I turned on the dash pump. As soon as I hit the switch the pressure returned to normal. While still at altitude I swiched off the aux pump and the pressure slowly dropped back to low pressure. When that happed I did not wait to see if I lost power in that I was a good distance from a landing site. Thanks Tom
 
Well I'm one of those persons that just has to know so I did a little research and asking. The mechanical fuel pump you have is probably not an altitude compensating pump. According to a local mechanic as the ambient pressure drops the mechanical pump would have to compensate for the pressure drop or your fuel pressure will drop. Have you looked in the POH or maybe Jerry could find this out for you . If this is the case I wonder what would be involved in putting another type pump on. In most cases these aircraft never see the 12k' flight even though the service ceciling is much higher. It might give the type pump in the TCDC I'm not sure.Anyway let us know what the verdict is for sure.

Woodie
N29763
 
Just to clear it up, there is no flop tube/plate/whatever for the inverted fuel. Its all in the header tank design. Basically, a spigot in the center of the header. Only thing that might happen at altitude is a decrease in static head pressure in the tanks......basically a reduction so small, would be hard to measure.
(8KCAB - Space Ship One) :p

Anyway, the fuel pump that it on there is hidden within the Lycoming parts manual. There is nothing here that I can look at that will determine that. If there is a difference in pumps, there is the posibility that Lycoming put in the wrong one. Unlikely, but posible.

Does the pressure just all of a sudden drop?
or is it a slow steady decline?
Are you leaning it when this happens?

Do you have a contact for a Lycoming tech rep?
 
Woodie and Jerry Jr. Just got back from 14,500 and the pressure stayed pegged on 23-25. I live in the Houston Tx. area so it took a while to get there. The only thing I can think of that was different from the other day was I may have leaned it more but I am not positive about that. Anyway, it performed perfectly today. Jerry, I don`t recall if it loss pressure quickly or slowly. We`re heading for Co. the middle of Sept. so I`ll continue to take it up to altitude before then and let you know if I have anymore problems. Thanks to both of you for your time and expertise. This is my second American Champion(1`st was a Scout) and I wouldn`t own any other 2 seater. Thanks again Tom
 
One last question guys. Ihate to admit this but after returning from 140000 ft. I leveled off around 8000 to do some rolls and inverted flight to keep everything greased. While inverted(and not paying attention to the horizon) I let myself get into an inverted dive. Well to not bore you with my stupidity, I let the airspeed wrap all the way around before recovery(way past yellow line). Although the G-meter only went to 3.5 g`s, do you fell I need to have it looked over by an A&P before I resume my fun and games? I looked over the outside and nothing is out of place or wrinkled. Thanks again. Tom
 
So, did you exceed Vne?
When you say past the yellow, I am assuming between 160 and 200.

If you capped out at 3.5 while staying below Vne, everything should be fine. You are within the limits of the airplane's certification.
 
You must log in or register to reply here.
Back
Top