Engine problem during descend

[turbotrk]

Hi!

Yesterday while coming back home base on a cross country flight on a 252 on a hot day I started descend form FL 80, I had almost finished the fuel on my left tank and the low fuel light was on while I had about 14 USgal on my right wing. I always plan to land with not less than 12Gal in at least one of my tanks as I want to make sure that in case of go around fuel can be sucked properly (mooney did not demostrate take off with less than 12gal). Fuel remaining was sufficient to go to the alternate wich was pretty close and wait for about 45 minutes. 

I was set with 20mp and 2200 rpm and mix leaned to peak. Passing 3500ft I changed to the right tank and just afer a few seconds my engine started to run rough, I put the low boost pump on and it stabilized but as I advanced the throttle the engine wanted to quit. Same problem when trying to set rich mixture. So I switched back to left tank and problem solved but in it I had just enough fuel to land at destination.

Afer landing checked remaining fuel quantity and everithing was as espected, 14gal on right wing and 2 on left. I turned the engine on to try again on the ground , selected right tank and set different power settings but problem did not replicate.

The aircraft had flown a previous flight of one hour plus 1:10 when it had the problem, preflight checks were done properly

 and fuel was drained. After take off I selected left tank and flew for on hour without changing.

Could it be the fuel selector? 

Any similar experience?

[Mooneymite]

Just a wild guess, a mud dobber has partially obstructed your right tsnk vent.

[ArtVandelay]

Just a wild guess, a mud dobber has partially obstructed your right tsnk vent.

If thats the case shouldn't you hear the telltale hiss of air when you open the fuel cap?

[nels]

After landing, did you sump the tank looking for water?

[Mooneymite]

3 hours ago, teejayevans said:

If thats the case shouldn't you hear the telltale hiss of air when you open the fuel cap?

Not necessarily.  A partial block can be pretty sneaky.

[turbotrk]

I went flying today to perform some checks to understand the situation. I eventually came to the following conclusions:

Much of the flight was performed on the left tank, a little bit of air must have formed in the right fuel line and when the tank was selected near the end of the flight the air caused the engine to run rough. The activation of the Low boost pump immediately solved the problem but pretty quickly it caused the opposite problem (too much fuel) but with the same apparent simptoms. 

It was difficult to understand it because everithing happened pretty rapidly and to restore the situation i switched back to the previous tank and turned off the pump. 

This is the point: although the manual does not specify this in the vapor suppression procedure if you turn low bust pump on the engine is likely to quit just as soon as the vapor is removed due to overrich mixture. According to POH this problem occurs with high boost pump wich is forbidden to use unless mechanical pump is broken.

I did the following tests to replicate the problem:

20" mp 2200 rpm lean to peak (descent) low bust on - engine quits

25"mp 2500 rpm lean to peak (cruise) .    Low bust on- egt decreases ff increases by 8gph but engine still operates

25" mp 2500 rpm full rich .                        Low bust on - engine quits.

Basicly you have to lean the mixture quite significantly to get the engine to operate normaly when low bust is selected on unless you are at high rpm and at least 25"mp lean to peak wich means that it is very easy to misunderstand what is happening. If vapor builds you activate the pump, you solve the problem but in a few seconds you are in the opposite situation!

Low boost is not safe guarded , but it is almost as dangerous as high boost!

This is vapor suppression procedure according POH:

Low fuel boost pump ............ON

Fuel flow............. Monitor

Low fuel boost..................OFF

No warnings at all!

 

[carusoam]

Nice review of the situation, TT.

Do you have a fuel pressure gauge and FF indication?  They can be helpful to know.  But, trying to fly while comparing to old memories is a huge cognitive challenge!

 

A review of the possibilities that have been brought up so far...

1) no or not enough fuel...

   - blocked vent.  Vacuum sound when opening cap.  Wing skin crushing like an oil can.

   - blocked fuel line.  Dirt/rubber/rust particles from upstream.

   - air leaking into the selector switch/valve. How old are the seals/ OH on the valve..?

   - water in fuel.

   - empty tank.  Fuel level is different than what indicators are saying.

 

 

2) too much fuel...

   - control with mixture knob.  Sweep from full rich to full lean.  

   - Often the only time we experiment with this is during primary flight training.  Simulated stuck throttle, use the mixture to find a place the engine will run at less than full power.

 

3) TCM engines also have a fuel return line that goes back to the selector valve (or tank).

   - pressure and FF are on the line to the engine. 

   - hot start procedure, mixture out, runs fuel back towards the tank.

   - review your POH for how the fuel line return works.  The M20R's goes back to the valve, and returns fuel to the same tank that is selected.  Other TCM systems return fuel to only one tank.

   - for reliable information, make sure your instruments are only reading FF and pressure being supplied to the engine. (Make sure they Don't include the recycle stream)

   - Os didn't get a FP gauge for troubleshooting while flying...

 

Sharing PP ideas only, I am not a mechanic...

 

Thank you for sharing your experience.

Best regards,

-a-

[turbotrk]

22 minutes ago, carusoam said:

Nice review of the situation, TT.

Do you have a fuel pressure gauge and FF indication?  They can be helpful to know.  But, trying to fly while comparing to old memories is a huge cognitive challenge!

 

A review of the possibilities that have been brought up so far...

1) no or not enough fuel...

   - blocked vent.  Vacuum sound when opening cap.  Wing skin crushing like an oil can.

   - blocked fuel line.  Dirt/rubber/rust particles from upstream.

   - air leaking into the selector switch/valve. How old are the seals/ OH on the valve..?

   - water in fuel.

   - empty tank.  Fuel level is different than what indicators are saying.

 

 

2) too much fuel...

   - control with mixture knob.  Sweep from full rich to full lean.  

   - Often the only time we experiment with this is during primary flight training.  Simulated stuck throttle, use the mixture to find a place the engine will run at less than full power.

 

3) TCM engines also have a fuel return line that goes back to the selector valve (or tank).

   - pressure and FF are on the line to the engine. 

   - hot start procedure, mixture out, runs fuel back towards the tank.

   - review your POH for how the fuel line return works.  The M20R's goes back to the valve, and returns fuel to the same tank that is selected.  Other TCM systems return fuel to only one tank.

   - for reliable information, make sure your instruments are only reading FF and pressure being supplied to the engine. (Make sure they Don't include the recycle stream)

   - Os didn't get a FP gauge for troubleshooting while flying...

 

Sharing PP ideas only, I am not a mechanic...

 

Thank you for sharing your experience.

Best regards,

-a-

Thanks, I do have a fuel flow indicator but being quite low altitude And 10 miles from home base I just tought to reset everything and get the plane down. Then once on the ground to identify the cause of the problem.

[carusoam]

Always best to be on the ground safely when an unknown issue like that occurs...

Be reluctant to fly if a resolution is not discovered.  The problem might return, at a less than friendly altitude...

Best regards,

-a-

[Guest]

6 hours ago, turbotrk said:

I went flying today to perform some checks to understand the situation. I eventually came to the following conclusions:

Much of the flight was performed on the left tank, a little bit of air must have formed in the right fuel line and when the tank was selected near the end of the flight the air caused the engine to run rough. The activation of the Low boost pump immediately solved the problem but pretty quickly it caused the opposite problem (too much fuel) but with the same apparent simptoms. 

It was difficult to understand it because everithing happened pretty rapidly and to restore the situation i switched back to the previous tank and turned off the pump. 

This is the point: although the manual does not specify this in the vapor suppression procedure if you turn low bust pump on the engine is likely to quit just as soon as the vapor is removed due to overrich mixture. According to POH this problem occurs with high boost pump wich is forbidden to use unless mechanical pump is broken.

I did the following tests to replicate the problem:

20" mp 2200 rpm lean to peak (descent) low bust on - engine quits

25"mp 2500 rpm lean to peak (cruise) .    Low bust on- egt decreases ff increases by 8gph but engine still operates

25" mp 2500 rpm full rich .                        Low bust on - engine quits.

Basicly you have to lean the mixture quite significantly to get the engine to operate normaly when low bust is selected on unless you are at high rpm and at least 25"mp lean to peak wich means that it is very easy to misunderstand what is happening. If vapor builds you activate the pump, you solve the problem but in a few seconds you are in the opposite situation!

Low boost is not safe guarded , but it is almost as dangerous as high boost!

This is vapor suppression procedure according POH:

Low fuel boost pump ............ON

Fuel flow............. Monitor

Low fuel boost..................OFF

No warnings at all!

 

The  low fuel boost pump is not running at low setting when the switch is selected to low.  You should be able to have the pressure checked per the manual.

Clarence

[turbotrk]

13 minutes ago, M20Doc said:

The  low fuel boost pump is not running at low setting when the switch is selected to low.  You should be able to have the pressure checked per the manual.

Clarence

Tought this as I possibility, on the ground I activated low and high boost and I could hear the pump running at different speeds, (high faster).

[turbotrk]

I wrote a couple of mails to Mooney support to ask them about my problem. Of corse they suggested to check LOW BOOST output pressure to be max 8psi but they did not want to reply to my question:

Is it possible that a normal operating (8psi) low boost pump can cause the engine to quit at at high power settings if mixture is not immediatly leaned? If so should the pilot be warned about this danger as the only warnings are given about high boost pump?

Got no reply to this question wich was asked twice. On the first time they told me that low boost is for emergency only ( wich I know) and it is not supposed to be used at idle (not my case ) and that TSIO 360 can be flooded pretty easily.

I think that there is a serious safety issue here, the checklist should specify to control fuel flow with mixture to avoid engine to quit when pump has to be turned on and then back off. The metering system of tsio 360 does not compensate for extra pressure added by the engagement of any auxiliary pump. The output of mechanical plus electrical pump will summate changing totally the air to fuel ratio. I have flown tsio360 for years and never had to use such pumps but the first time I did the result was not what I was expecting.

Can anybody of you try to engage the low boost on the 231 or 252 during flight to see what happens?

 

 

 

 

 

[Guest]

It seems strange that the "low" pump pressure setting would be higher than the unmetered pressure required in SID97-3G.

Clarence