Sunday, August 10, 1997

An Interesting ASRS...

ASRS Report, dated 10 August 1997

I've owned my Tiger for 2-1/2 years and have flown it over 400 hours. I typically cruise at 6500 or 7500 feet MSL for best speed and fuel consumption, 130+ knots averaging 10.5 gph with 51 gallons of fuel useable. Most of my trips are within a 2-hour range of my home field (BDR - Bridgeport CT) so fuel usage has never been a concern. I always used my watch as a fuel gauge, switched tanks religiously, and always re-calculated and mentally updated my fuel consumption after every flight.

This past week I attended our type-club convention in Bowling Green, KY (BWG), a 670-nm trip from BDR. I flew with a group of 2 other Tiger owners, and our one-fuel-stop trip out VFR to BWG was uneventful. The performance of my Tiger was on par with my history of it, with average speed and fuel consumption were right where I expected it.

The return trip, however, was not so. A warm front was moving in from the southwest threatening to block my return home for the next several days, and there was a line of mixed Level 1 to 3 rainstorms to our immediate northeast, my intended direction home. Our group had been expanded to include a fourth airplane that needed to be dropped off in Mount Joy, PA (N71) for service, and the pilot needed a ride home. After discussing the weather situation with briefers and amongst ourselves, we decided that our flight-of-four would break up for the return trip, and my compatriots would file IFR home.

Unfortunately, I am not yet instrument rated, although I am in the process of getting the rating and I have over 55 hours logged instrument time, some in actual IMC (with an instructor, of course). After further reviewing the weather, I felt comfortable that I could reasonably expect a break to the due east, whereas I would weave through the line of rain into much better VMC and then turn northward to CT. The prognosis looked good for minimum-to-marginal VFR conditions, worsening as the day progressed. If the weather didn't work out as expected, I was fully agreeable to turning around and returning to Bowling Green to wait out the weather.

We all launched, with my friends assigned to IFR cruising altitudes of 7000 and 9000 MSL. We maintained contact with each other on the common air-to-air frequency, and they kept me updated on the weather ahead.
This plan worked very well, and I was able to maintain VFR all the way through Kentucky and southern Virginia, never lower than the minimum safe sector altitudes as depicted on the VFR charts. My friends helped me to find the best routes to VMC along the way. Once clear to the east I was able to climb to 5500 MSL and continued cruising at speed.

My original intention was to fly about 3.5 hours and stop for fuel. This would have put me somewhere in central-to-northern West Virginia/Virginia for lunch and a top-off with plenty of fuel to make it to BDR. I mentioned this to my friends and asked their opinion. Their response was that since we were picking up such a nice tailwind we could make it all the way to PA, stop for lunch near where the fourth airplane needed to be dropped off, and then continue home. That new goal was determined to be Chester County Airport in Coatesville, PA (40N).

Using my VFR GPS and my watch, as well as my experience with my Tiger, I decided that 40N was possible, and I would get there with my 30-minute VFR fuel reserve intact. I agreed to the destination, and pressed on.

I have absolutely no faith in the fuel gauges in my Tiger. I use my watch exclusively to monitor fuel consumption, and I use my experience with the gauges as a general guideline to account for any unexpected fuel leakage. My technique for switching fuel tanks is to use the first tank for 30 minutes, then switch tanks every hour thereafter. This keeps the fuel balance left-to-right, and assures me that both tanks are always within 30 minutes of each other.

After my initial departure from BWG, I was so busy reading the maps and watching the weather that I neglected to switch the first tank until 40 minutes. This did not concern me at the time. Each subsequent tank switch happened exactly 60 minutes later, and I kept a log of all tank changes. After I diverted to 40N I decided that when I switched to the first tank for the last time I would drain that tank, leaving me 30 minutes in the other tank, and I would immediately land.

With 25.5 gallons useable in each tank, and a history of fuel consumption at 10.5 gph, I was calculating being able to use each tank for a minimum of 145 minutes, for a total flight time of 4.8 hours. I was fully expecting to drain the first tank at 145 minutes. Once this first tank was depleted, I would have 25 minutes of fuel available in the other tank.  With a high-cruise speed of 130 knots I had a 55-nm radius to cover myself; this would be my "safety valve" if I had miscalculated. There were plenty of airports in the general area, and the weather at this time was very good VFR. Based on the winds, I calculated that I would arrive at 40N with 4.3 hours total flight time, so I fully expected to land with 30 minutes of fuel remaining in one fuel tank.

Both of my friends landed ahead of me, one an hour ahead, the other 20 minutes. About 15 miles from my destination the engine sputtered with the left (first) tank run dry. I switched on the electric fuel pump, and flipped over to the right tank; the engine caught immediately. However, I become very concerned; I had expected that left tank to be useful for 145 minutes, but a check of my logs showed that it had only run 130 minutes!

Even worse, my logs showed the right tank was at 120 minutes, leaving me only 10 minutes of fuel!!

I immediately pulled the throttle back to best economy cruise speed and leaned the engine out to within an inch of its life, while querying the GPS for the nearest airport. I relayed my situation to my friends ahead at 40N. Then, the fuel pressure gauge dropped and the engine sputtered.

I had just become a glider.

At that point I was at 5000 AGL and still 10 miles from 40N. There were no other closer airports downwind from me, and we were experiencing a good 10-knot tailwind component; I decided to continue towards Coatesville. I called a Mayday on the 40N CTAF, maintained pitch for best glide, and looked for an area to land. Fortunately, there were numerous open fields in the area, so an emergency landing was not going to likely cause much damage or injury. I shook the airplane around to get any remaining fuel into the sumps, and I also switched the tanks back and forth to keep the engine running.

Someone was smiling on me, because with all of the above I managed to keep the engine sputtering until I arrived over the airport at 3000 AGL when the engine quit for the last time. I made an uneventful dead-stick landing and taxi'd right up to the fuel truck. The tach showed 4.3 hours, exactly as calculated.

This airplane claims to have 51.0 gallons usable. We pumped 51.5 gallons of fuel into it.

My friends and I were dumb-founded; what had happened? They had each put in 42 gallons of fuel into their identical airplanes, yet I managed to run my tanks dry. There were no fuel stains anywhere. Then it hit me: I had spent at least one-third of this trip down on the deck scud-running at full-bore to try and keep up with them. A quick check of the Pilot's Operating Handbook showed that my airplane was able to make that 135 knots at 2700 rpm while using as much as 13 gallons per hour! While running low I still had that 10.5 gph number stuck in my head and had neglected to reference the POH to see how it would affect my fuel consumption. The trip averaged almost 12 gph.

My past history of casualness with fuel consumption by having the luxury of a 51-gallon capacity had come back to bite me. Up to this point I had never explored the limits of the range of this airplane, therefore I had never felt the need to review the POH for normal, let alone unusual, circumstances. This luxury and casualness lulled me into a dangerous situation, that could have potentially ended up in damage and serious injury.

I've learned from this experience, and I will never be so casual about fuel consumption again.

By the way, I read all the aviation safety magazines, and I had always wondered how someone could get into a low fuel situation. I never could imagine how it would ever happen to me.

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