Turbine Surge

[FN FAL]

Just get the dam thing check out by a qualified technician (mechanic). It should not be surging

Are you refering to the surge in general?

 

[KeroseneSnorter]

Ok then. Here's a real life scenario for you.

We're about 1.5 hours into the flight when #4 "surges". Personally I observe, all four thrust levers moving slowly *towards* idle. I personally observe #4 N1 decreasing and I personally observed a slight roll to the right and the autopilot make a correction.

Naturally this all takes about one second.

The next thing I observe is the FE applying Max Continuous Thrust on all *four* engines and #4 N1 has recovered.

The FE thinks the auto throttles were making an uncommanded roll back towards flight idle. The Capt is asking for more information as he was checking a chart and just caught the roll back when the plane rolled.

I don't know what to think. I thought the autothrottles were making a normal adjustment. We were in level cruise so I wouldn't consider a compressor stall and certainly not a bird. Besides that the engine recovered and continued to operate normally for the next six hours.

Were you on the center tank or wing tanks? If on wing tanks were you still only on #2 and #3 main or feeding from #1 and #4 also?

My suspects would be:

Ice? or other contaminates but you made no mention of filter ice lights coming on.

Water? You know how bad that center tank is if the bozos didn't sump it.

Cold fuel from the # 4 tank? but only 1.5 hours into a 7+ hour flight I doubt that you were to that tank yet.

Boost pump "burp"? You must be running GE's since you made no mention of EPR's, a boost pump did something weird maybe? Max grav feed altitude is what...about 280 to 310 or so on the GE's? Not supposed to happen with other pumps backing it up but dunno how the FE had it plumbed at the time.

Of course it could always have been Grimlins.........I hate those little buggers.....always show up and wake you up from your transatlantic snooze.

Of course as always it could have been a wore out engine......still doesn't explain why the auto throttles would roll back though.

 

[avbug]

My guess is no. He referred to "the Rt. engine" (singular) and "both engines", as in two. I don't think that description would fit the case of the KC-135. He also referred to "the left [fuel] system", another term inconsistent with the StratoTanker. It has (I'm stretching my memory a bit here) at least nine separate tanks from which to choose.

Thanks. It's fairly evident I didn't read the question too thoroughly .

 

[mar]

Kerosene Snorter; Tony C.

I'm not a very good reporter. The autothrottle movement (I think) was just coincidental to the "burp" on #4. But it's sort of a Red Herring because the FE (who's very good and *was* awake) mistakenly thought (in my opinion) that there was a malfunction in the autothrottle system and that's why he advanced all four.

The engines are CF6 so no EPR. I did see the N1 roll back. And I heard and felt the loss of thrust.

Clear night; no ice.

Tank configuration. Hard to say because that's all behind me but I'm assuming we were burning out of tanks 2 & 3 that early into the cruise. As for center fuel tank, I'm sure there was some but I honestly can't say how it was being used.

Interesting theory about cold fuel from the #4 reserve tank but I doubt that one for a few different reasons (too early, mixing in #4 main, fuel heaters, etc).

Fuel pump burp above FL280? I suppose, that's not impossible.

Just one of those things....

 

[TonyC]

I'm not a very good reporter. The autothrottle movement (I think) was just coincidental to the "burp" on #4. But it's sort of a Red Herring because the FE (who's very good and *was* awake) mistakenly thought (in my opinion) that there was a malfunction in the autothrottle system and that's why he advanced all four.

The engines are CF6 so no EPR. I did see the N1 roll back. And I heard and felt the loss of thrust.

Clear night; no ice.

That leaves the second of my hypotheses, the airspeed indication. A brief erroneous high airspeed indication would have resulted in a signal to the autothrottles to retard the thrust. I doubt anyone was looking at the airpseed during that brief time since your other senses (sight, sound, vestibular acceleration, seat pressures, etc.) assumed that to be constant. Perhaps you witnessed the first hiccup of an Air Data Computer.

Also, do you have FADEC where each throttle is individually fine-tuned by the autothrottles, or does someone have to "fine-tune" or adjust the individual throttles once the autothrottles have moved the "bank" of throttles to the target "region." If it's the latter, it's possible that #4 was lagging behind the other three and therefore more susceptible to the deceleration when the autothrottles received the retard signal.

Who ever said monitoring the automated systems was easy, huh?










.

 

[KeroseneSnorter]

I'm not a very good reporter. The autothrottle movement (I think) was just coincidental to the "burp" on #4. But it's sort of a Red Herring because the FE (who's very good and *was* awake) mistakenly thought (in my opinion) that there was a malfunction in the autothrottle system and that's why he advanced all four.

The engines are CF6 so no EPR. I did see the N1 roll back. And I heard and felt the loss of thrust.

Clear night; no ice.

Tank configuration. Hard to say because that's all behind me but I'm assuming we were burning out of tanks 2 & 3 that early into the cruise. As for center fuel tank, I'm sure there was some but I honestly can't say how it was being used.

Interesting theory about cold fuel from the #4 reserve tank but I doubt that one for a few different reasons (too early, mixing in #4 main, fuel heaters, etc).

Fuel pump burp above FL280? I suppose, that's not impossible.

Just one of those things....

Yea I doubt it was the #4 reserve temp problem, especially since you were running the CF6's with heat exchangers, more of an issue on the pratt powered airplanes.

I am inclined to go with your thoughts about it not being an autothrottle issue, but not ruled completly out.

That early in the cruise I am leaning toward your water theory since there is a better than average chance that you were still on the center tank, or getting close to switching to the mains, possibly it was getting low and one of its pumps unported briefly or ran a shot of H2O to the number 4.

Who knows what evil lurks in the plumbing of a 30 year old 74!!

 

[pilotyip]

Aeration?

ATP Cliff's answer. It was most likely aeration in the fuel due to the loss of a fuel boost pump. They were probably climbing at a T-5 limit in the 3K-4K per minute range. With warm fuel and a rapid climb rate through the low teens, 10K-14K, warm fuel will give off bubbles like a can of Coke being opened. The boost pump keeps a head of pressure on the fuel to prevent the bubbles from coming out. With the loss of a boost pump the bubbles will appear in the line and cause air to be introduced to the engine with the engine driven pump pulling from the tank. Fuel flow for both engines was probably higher than the output of one boost pump therefore when the crossfeed was opened pressure dropped on both sides and the aeration problem reappeared. When power was reduced, the boost pump supplied more fuel than the engines needed, pressure built up in the crossfeed line and the engines then stabilized. If I remember right this was a classic problem discussed in aviation fuels school 35 years ago. This is also why crossfeeding during climb is not a good idea.