Hiccups Part 1

Hold your nose and drink a glass of water. Take a deep breath and hold it. Have someone scare you. There are lots of “cures” for the human hiccups. Don’t try these when your airplane hiccups.

If you could choose, would you rather deal with something that breaks and stays broken, versus something that seems to happen randomly – and never when the shop has the plane? Neither is very welcome, but sometimes there are troubleshooting clues in the engine data. This month we’ll take a look at hiccup events and see if we can determine what caused them.

We’ll start with data from a Bonanza 33 with a Continental IO-520 and data from a Garmin G3X with a 1 sec sample rate. The request noted intermittent oil pressure fluctuations during recent flights. Rather than show my standard ranks of EGT, CHT, FF and Oil temp, let’s start with oil pressure isolated.

The arrows on the left are partially hiding the trace, but it’s a little steadier there than later. There are three noticeable jumps in the trace – at 10 mins, at 20 mins and a short blip at 2 hrs. I wondered if they were coincident with changes in power, so I added RPMs and MAP. Here’s that screenshot.

Yes there’s a big power change at 10 mins – takeoff – and another small power change at 20 mins – leveling for cruise. So far so good. The cursor is on the MAP change just before the last oil pressure blip, and they don’t sync up. So on a hunch I displayed altitude. Here’s that screenshot with MAP and ALT and the cursor in the same spot. They’re similar because RPM and ALT are both green – but the patterns are very different.

To my mind it came down to a choice of —

1 – it’s an engine problem because two of the three jumps are timed with power changes – or

2 – it’s a sensor problem because the first jump is timed with the vibration of the takeoff roll, the second jump is timed with the increase in speed as the pilot changes his angle of attack in anticipation of leveling off, and the third jump at the two hour mark happens as vibrations increase during initial descent.

We’re not ruling anything out but I think the case is stronger for it being a sensor problem, and that’s where we suggested to begin the troubleshooting. For the sake of argument, if the initial request had said there were fluctuations AND the engine was surging during those fluctuations – different story > different conclusion.

Next up is data from a Mooney M20TN with a Continental TSIO-550 and data from a Garmin G1000 with a 1 sec sample rate. Senior analysis Paul Kortopates worked this request. Paul is our Mooney specialist and our turbo specialist and this issue checks both boxes. Here’s the whole flight with EGTs, CHTs and FF.

The client reported that EGT 4 took a brief dive, that he then turned on the fuel pump and that’s why FF got lower. Let’s zoom in for that.

The event is right in the middle at 15:06. I just moved the cursor off to the right to get it out of the way. The client reported that this happens frequently but this is the first time he saw it in the data. Knowing that the fuel pump went on at that point helped Paul understand the drop in FF. Note to Santa — it would be helpful to diagnosticians if engine data monitors tracked on/off events events like fuel pump, autopilot, pitot heat, etc. Rant over.

Paul considered a momentary sticky valve but there wasn’t supporting data in previous or subsequent engine starts. He also considered fuel but with the engine well rich of peak, a momentary loss of FF would increase EGT and that’s not happening. But in a turbo a momentary lack of air can also make mixture richer. He added “Unlikely the intake valve didn’t open all the way (not without bending something), but exhaust not fully closing can also cause this.”

The client flew again and had a similar event. “Same scenario – climbing through 16,700, full power, low boost on this time. Seems isolated to full power climb teens and up. Never a problem running cruise power.” And this time there’s no drop in EGTs – not even a little one like the one above. This gets Paul thinking…

“Based upon some momentary roughness without a noticeable change in EGT or CHT – we typically see this from various forms of fuel contamination of the following types:

1- Air from either vapor lock or air leakage from the fuel selector. Boost pump usually readily clears this up , otherwise it occurs in climb typically continues till cruise. Its worsened by altitude or less likely to clear up in high altitude cruise. It’s most discernable from fuel pressure but can be bad enough to see fluctuating FF too. 

2- Water – small amounts of trapped moisture can have a more pronounced momentary roughness when the moisture passes through into a cylinder in the form of ice or water. They are usually more of the type that the engine feels like it missed a beat and then all is back to normal. The aux pump is not effective and FF and FP remain regular. Perhaps of the 3 this meets the symptoms the best. 

3- Oil contamination from the Turbo – This really isn’t an example of no EGT symptom since it typically causes a drop in EGT and rise in CHT and can lead to detonation at high power. Oddly it doesn’t usually appear till cruise power either. But also never seen the symptoms stop without pulling back power significantly either. Don’t think this one is at all relevant here. 

A little more on water contamination from tapped moisture. Besides water getting into the tanks it can also come from humid air in the tanks which is most susceptible in the summer time with hot humid surface temperatures and cold air aloft up high. The moisture condenses out at altitude as the air cools. Then aloft at higher altitude we can pick up some moisture in the fuel. So at first, trapped moisture may not seem plausible since its takes till higher altitude to exhibit the symptoms, but the higher altitude is an argument for this theory since the necessary condensation won’t occur until reaching cooler air aloft. 

This may not be it but if its a possibility, a bit of dry Isopropyl alcohol can absorb the moisture and is approved in the POH. Doesn’t take a lot to do the job. 

Generally speaking fuel delivery issue through the fuel pump, Throttle Mixture Control, fuel divider/manifold effect fuel flow to the engine entirely in a EGT noticeable way while an injector will affect a single cylinder but also affecting EGT.”

Paul suspected surging due to an over-rich mixture and recommended leaning slightly in the climb to 36 GPH. The client tried it and it worked – no hiccups. Book FF for the Mooney Acclaim is 33.3-35.0 GPH. Savvy likes to see 0.5 – 1.0 above Continental’s high number – so in this case 36.0 GPH. They say you can’t be too thin or too rich. Maybe you can.

I have more hiccups but I’m already at my word limit this month. So see you in October with Son of Hiccups.