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Putting A Stop To The Fuel Guessing Game
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By Ian Sutcliffe
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Recently a Mooney crash landed 400 feet short of our local airport runway. Luckily there was no fire and everyone aboard walked away. The reason there was no fire was the same as the reason for the short landing - there was no fuel left in the tanks. The NTSB accident reports for any recent year list over 100 accidents due to fuel starvation. 10% involved fatalities. The NTSB attributes these accidents to such human performance issues as an incorrect measurement of fuel onboard, a failure to calculate fuel burn properly, or the decision to fly past known fuel range limits.
Fuel state calculations rely on estimates and judgments by the pilot: how much fuel is on board at start up; how much fuel is used during climb; how effectively we have leaned the engine during cruise; how close to book performance have we achieved during the flight. Factors such as air temperature, aircraft load, altitude and air density impact the actual fuel flow and may or may not be included in our calculations. In many cases we go by our experience and rough rules of thumb to assist our judgments and most of the time this works pretty well.
Every year though many aircraft are damaged or destroyed due to errors in these judgments as to how much fuel we have burned and how much will be left on arrival at our destination. Given the frequency of fuel starvation accidents, it is clear our estimates are at best educated guesses.
As a case in point, flying from Regina to Calgary in the summer of 1986 in our 1955 Cessna 170B rapidly turned from a routine flight to a nail biting experience. Our flight plan called for us to cover the 360 nautical miles in about 3.5 hours leaving us with 1 hour of fuel on arrival in Calgary. We checked our position and progress every half hour and for the first 3 checkpoints everything went according to our flight plan.
The weather was CAVOK with small cumulus clouds at about 10,000 feet lined up into the wind across the prairie. We stayed low to avoid the headwinds higher up and had to climb periodically to maintain 2,000 feet above the ground as the land was slowly sloping up toward the Rockies in the distance. We became a bit complacent and didn’t bother to check our progress at the next half hour checkpoint.
There are not many towns between Regina and Calgary to start with and even fewer with an airport and fewer still with fuel available to the public. Most airports with a gas pump are not manned and require a call into town to get someone to come out and unlock the fuel pumps.
This makes it a hit and miss proposition. You could land and phone but may not reach anyone and have to carry on to the next town. This uses up valuable time and fuel and if fuel is not available, reduces the options available. As a result there is significant incentive to be able to make the leg in one hop.
In our case the headwinds began to increase and at our fifth checkpoint, 2.5 hours into the flight, we were estimating 45 minutes of fuel in the tanks on arrival at Calgary. This was a little short of our target of 1 hour but still a provided a reasonable margin of safety.
By the next checkpoint the winds had increased even more and we had lost another 15 minutes of our buffer. If we continued on we estimated we would have about 30 minutes of fuel on landing at Calgary. Any further increase in wind, any deviation from our plan or any hold on arrival into Calgary would leave us in a very marginal situation.
Our calculations were all predicated on the amount of fuel we estimated we were burning. If our leaning was off or if our actual burn was higher than we thought we would be in real trouble. The fuel gauges on the vintage Cessna are the old cork style in the wing root. The pundits say that they are only accurate when the tanks are empty.
The headwind was now up to 42 knots and was not expected to increase further. The forecast was for a maximum of 20 and with we decided to push on into Calgary.
We landed safely and fueled at the local Esso FBO. We took 32 gallons which meant we landed with 5 useable gallons in the tanks – or about 36 minutes at our cruise burn of 8.3 gallons per hour.
In many ways this experience was a confidence boost. It demonstrated that we could fly for just over 4 hours and land with a safe margin of fuel. It demonstrated that good piloting and reliance on the published performance of the aircraft resulted in a safe and successful flight. Practically speaking we had done everything right. We were within the rules for fuel on board, we had checked the weather and we kept track of our progress.
In reality we were lucky. It would have only taken a small increase in the headwind or a minor miscalculation to have added our ship to the accident statistics. Five gallons of fuel in two tanks meant that either tank was within 18 minutes of dry. I had spent the last 30 minutes of the flight nervously watching the fuel gauge bounce up and down near the empty mark, wondering – and worrying - if we would make the airport.
Fifteen years later, I have another Cessna 170B and another 1,200 hours of experience under my belt. In the intervening years I have been more conservative and have rarely planned or flown legs much greater than 3 hours.
The new 170B has something that has become standard equipment in most new cars - a fuel computer. This instrument has eliminated much of the guessing – and related worry – from my cross country flying and if used by more pilots would likely reduce fuel starvation accidents.
The fuel flow indicator provides accurate monitoring of actual fuel consumption, cumulative fuel burned and fuel remaining. It calculates flight time remaining to dry tanks at current burn rate. When interfaced with the GPS, it provides projected fuel remaining at the next waypoint in gallons based on current ground speed and rate of consumption.
Having this information takes the guess work out of fuel and leaning decisions. Instead of estimating fuel burn during climb and cruise, and guessing at the impact of leaning, the instrument tells you exactly how much you are burning and what you have left. As long as the unit is properly calibrated and you have correctly entered your fuel at start up, the instrument will provide an accurate picture of your current and projected fuel status. You should also monitor the fuel gauges as the instrumentation will not recognize a fuel leak or fuel being siphoned off by a missing fuel cap.
With inexpensive fuel flow indicator instruments available there is no longer any reason to rely on guestimates to determine fuel status. This is especially true when connected to a GPS device. Of course, these are not infallible solutions and like other instruments, are prone to human error. Care must be taken to enter the right amount of fuel after each fill up, and it is important to periodically check that the flow calibration is accurate. I double check fuel purchased against what the fuel computer says was used at every fill up. Selecting the correct GPS waypoint also becomes important.
Another guessing game that relates to fuel management is the leaning process. Using the fuel flow indicator together with a CHT/EGT instrument, one can much more accurately determine proper and safe leaning and the true impact on fuel flow. By checking cylinder head and exhaust gas temperatures and actual fuel flow against book performance and allowable ranges enables accurate leaning. The CHT/EGT gauge provides information on each cylinder, making leaning more science than art.
I encourage more pilots to take advantage of current technology to reduce the guess work involved in fuel calculations. If they do they will worry less about running out of fuel and, if the tanks do run dry, at least it should not be a surprise.
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The details:
The fuel computer used is a JP Instruments Fuel Scan 450 Fuel Flow Indicator and is connected to a Garmin 296 GPS. The Fuel Scan 450 sells for about $750 and is readily available through avionics supply companies. Installation was straight forward and took my local FBO approximately 15 hours to complete.
The GPS is a Garmin 296 which has a NMEA output that connects to an input on the Fuel Scan 450. You need to purchase the optional data/power cable from Garmin to make the connections. This will work with most GPS units that output a standard GPS NMEA sentence. I had the data cable hardwired into my panel so that I can easily connect and power the Garmin unit which sits on a yoke mounted bracket.
The CHT/EGT gauge is an International Electronics US-8A Engine Analyser with 6 CHT & 6 EGT probes, plus probes for Carburetor Heat & Oil Temperature. The unit cost about $1,200 including all the probes and is readily available through avionics supply companies. Installation at my local FBO took approximately 10 hours.
The aircraft is a 1952 Cessna 170B registered in Canada (C-FFYL). We have owned it for 5 years and frequently fly long cross country trips exploring Canada and the USA.
I have used this combination of modern electronics since 1999 (previously with a Garmin 295) and all three have performed flawlessly.
About the author: Ian Sutcliffe is an active private pilot and regularly flies cross country in his Cessna 170B and in his LS8-18 racing sailplane.
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