Sunday, November 20, 2016

Takeoff Loss Of Power Experienced - After Fix Video

Earlier last week I was planning on making a trip to the furthest airport in my phase one fly off area. The air was cool and crisp, clear skies and very light winds …. a perfect day to begin working on seriously attacking the flight test cards.

After getting the oil temperature up and performing the preflight run-up, took to the active runway and decided on a 65 – 70 knot climb out. The engine was running good down the runway and developed over 5200 RPM as the climb out began. At about 250 feet off the deck I caught a whiff of fuel and a moment later there was a partial loss of power. I had lost approximately 1,200 RPM … I quickly throttled back some then advanced the throttle a little but the engine seemed “flat” so it was throttled to keep the engine around 4300 RPM where the engine was running OK and seemingly smooth , decided it best to abort the flight so called the tower for clearance to return to terra firma to conduct a “little FBI Double-O-Seven-type investigatin” to coin a lyric from a Jr. John song.

One of the really nice features of the Dynon SkyView is there is a built in flight data recorder that constantly records a plethora of parameters …  input from all of engine and fuel sensors, GPS, transponder, autopilot, airspeed, altitude, attitude, ect., ect. By default, the sampling rate is 16 times per second (this can be changed in the setup menus) which provides approximately 2 hours of very detailed data capture. If the sampling rate is lowered, then much more than 2 hours of storage can be obtained … as an example, a sample every 10 seconds will result in 150 hours of flight data storage.

Decided it best to make a data dump of the SkyView's log to a flash drive and try to analyze what happened ... this is done by saving the SkyView's data logs onto a flash drive plugged into the USB port. But before delving into that, I should mention the USB port under the instrument panel base connects to one of the USB ports on the back of the Dynon SkyView. The under the panel USB port is used for a plethora of tasks such as upgrading the SkyView’s firmware and allowing for the monthly updating of navigational data bases. In addition, third party chart software can also be utilized by the SkyView to display FAA sectionals, IFR approach plates and detailed airport diagrams called “Safe Taxi”.  Plus, the USB port allows flight plans to be loaded into the SkyView or saved from the Skyview along with allowing the pilot to make backups of all the SkyViews internal settings and the biggie for me at the moment, exporting of the SkyView’s user data logs. All of this can coexist on one flash drive ... it just requires plentiful storage space on the flash drive.

Some of the above features such as FAA sectionals, IFR charts and “Safe Taxi” airport diagrams require leaving a flash drive in the under the panel USB port so the SkyView can access the data as needed. This can present a problem, because a typical flash drive would extend down from the bottom of the instrument panel and could very easily be snapped off from contact with the knees or an arm. So one of the many niceties the DOG Aviation procurement department has recently purchased consists of two tiny PNY 32 Gig Elite-X USB flash drives. They are small  in stature (yet big on available storage space) … they are just long enough for the fingers to get on them to pull them out of the USB port. At 32 Gig, there is plenty of storage for the software revisions, monthly updates, third party charts, flight plans, saving internal settings and the biggie at the moment, saving the user data log recordings … with plenty of room left over. Two of the flash drives were purchased so one flash drive can be at the house to get updated with the latest maps, ect, then taken to the airport and swapped for the one in the SkyView.
As one can see in the above photo, compared to a rivet, the PNY 32 Gig Elite-X Fit flash drive is tiny and will not be subject to breakage from the knees because it barely extends beyond the USB port.

OK, back to the user data log dump … after making a safe landing and taxing back to the hangar, the flight data logs were saved onto one of the PNY flash drives to take home for analysis. Saving the user data logs to a flash drive plugged into the USB port is accomplished by pressing and holding buttons 7 & 8 on the SkyView to enter into the Setup Menu. The first item in the Setup Menu list is System Software … selecting this will bring the user to a sub-menu that has an entry of Export User Data Logs. Selecting Export User Data Logs will save the data logs onto the flash drive plugged into the USB port on the bottom of the RV-12’s instrument panel. The files are saved as a .csv file which is a computer file standard meaning … comma separated values. The Microsoft Excel spread sheet is capable of opening and displaying the contents of this type of file.

A tip for those builders using Excel to view the SkyView’s data log:  Do yourself a favor, after opening the data log file with Excel, click on the View tab on the top of the window and click on Freeze Panes … then select Freeze Top Row. What this does for you is freeze the top row which is where the name for each column is located … such as RPM, EGTL, EGTR, Fuel Pressure, Fuel Flow, ect. Freezing the top row allows the spreadsheet to be scrolled to where the event being analyzed takes place without losing the name for what the column represents. The user has to analyze what the numbers mean and form a picture in “your mind’s eye” as to how the numbers correlate with one another. So a lot of back and forth scrolling is needed … but there is a better way.

Getting high tech … deciphering the SkyView data log can be taken to the next level by using a service provided by a company named Savvy Analysis. Savvy Analysis offers a service for pilots that allows the Dynon SkyView’s user data log to be uploaded and converted into a graph chart which is visually much more meaningful …. various parameters can be selected by the user to be charted simultaneously, very cool!! And best of all, the service is free … unless you want them to analyze the data log results for you. Below is a copy of the Savvy Analysis chart showing the “event”… once I had the chart on the computer screen, I converted it into a .jpg file to post here on the Blog.
Uploading the Dynon SkyView’s data log to Savvy Analysis for conversion into a chart will yield an easy to look at chart of the event… which greatly aids in analysis of the event. Here the sudden loss of RPM can be seen accompanied with a cooling of the left cylinder’s exhaust gas temperature (EGT) while the right EGT remain unchanged. Looks like the carburetor for the left side went rich, probably due to the carburetor’s float needle valve getting stuck on a piece of debris.

As can be seen in the above chart, the Rotax 912ULS is turning out a steady 5,200 RPM during the climb out then instantly drops to roughly 4,000 RPM the exhaust temperature on the left side of the engine also takes a drop suggesting there was a rich mixture coming from the left carburetor. Also of note, at the time of the “event” the fuel pressure dropped from roughly 5.5 PSI to around 5 PSI which, to my way of thinking, would correlate with a stuck needle valve in the left carb reducing the fuel pressure because of the momentary excess flow of fuel. It is not a fuel pump issue because the minimum pressure for the Rotax 912ULS engine is 2.2 PSI so that is not an issue.

There are only a few things that would contribute to a carburetor going rich … excess fuel pressure (not the case here) choke being applied (it was full off) which leaves … floats sinking in the bowl, debris preventing the needle valve from closing, bad needle valve or float level adjustment being off … all of which require removing the bowls from the carburetors. So the bowls on both carburetors were removed for inspection. All the floats appeared to be floating nicely. Just to be sure, each float was weighed at 3 grams and each pair weighed 6 grams … so they are spot on weight wise. The left carb bowl (suspected problem one) had just one little speck of debris in it. I decided to turn on the master switch and use the electric fuel pump to flush out the float needle valves while moving them up and down with the float bracket. Next a check was made to see if the needle valves would stop fuel from flowing when closed … all seemed well. Also checked the arms on the float bracket which should parallel the body of the carburetor when in the closed position … both float brackets appeared to be correct. Interestingly, the right carburetor had much more debris in the bowl than the left carb did and also had a very small flake of what appeared to be a flat black paint or coating that appeared to come from the float brackets. This got me off on a short tangent thinking perhaps the wiring for the EGT probes got switched or the probes were attached to the wrong port on the engine monitor… I used a heat gun on the right exhaust pipe near the EGT probe to verify the Dynon SkyView data log file recorded the heat on the correct sensor … fortunately, it did so the “event” was a left carburetor event for sure.

After the carburetor bowls were placed back onto the carburetors, decided to make a test flight with a shallower climb out of 80 knots. The test flight reveled no engine problems. Engine RPM remained constant around 5,250 on the climb out until I pulled back on the throttle … so for the next few flights, will plan on staying in the traffic pattern for a handful of takeoffs and landings so I can incrementally steepen the climb outs to make sure the issue has been resolved. Below is a couple of videos of the takeoff and landing after working on the carburetors. The videos are heavily edited to meet the under 100 Meg requirements of Blogger. Unfortunately, the camera switched files just as I landed so there is a little skip in the video at that point as I landed with a slight thump. Also will have to make improvements on mounting location for Mike’s camera to get it a little sturdier so the camera does not rattle.

A takeoff video made after working on the carburetors all seemed to go well … had a steady 5,250 RPM on climb out although it was a shallower climb out than when I had the “event”.

During the landing, I kept the RV-12 purposely high until on final then slipped a bit to lose the excess altitude which can be seen during the period of time when the horizon looks canted while on final to the runway.

I would have flown a few more loops around the pattern but needed to get on the ground to install a winter heating system that requires temperatures around 70 degrees to cure the epoxy properly … and there are only two warm days left before the winter plunge. More on the  heating system in the next post.