Knowing your true airspeed for fuel management

I have a healthy fear of running out of fuel and I do everything I can to be sure there is fuel left in the tank when I land. I measure the fuel in my tanks before every flight, I always land with ample reserve, and I also measure or otherwise verify fuel used and fuel remaining after the flight for my own flight debrief. I have even gone so far as to drain the tanks and fill them 5 gallons at a time to verify the fuel gauges against the fuel in the tanks.

Am I overly cautious? Maybe, but running out of fuel could mean death to me, or to my family, or to my bird, and none of those options are very palatable. So I check and recheck and verify that I am okay. When it comes to fuel, I don’t mind saying I am afraid to run out, especially when there are hundreds of places to stop for fuel if I need to. My system may not be perfect, but it works for me.

Fuel gauges getting low

Avoiding a low fuel situation takes planning and some skepticism.

I am writing this article to explain how I keep track of fuel burn during a flight in our airplane. Recently in Air Facts, I made a case for our Piper Cherokee 140, described how we decided to buy it, how we use it, and some of the improvements we have made. We use our airplane for family trips and often fly fully loaded 400 nm. We can do this in one leg with a light headwind, but correct fuel management is then very important. One of the best improvements I made had nothing to do with the airplane at all, but instead was a cheat sheet for quickly finding True Airspeed to trim the airplane and to determine engine fuel flow. Yes, the ASI can also be used to determine the fuel flow of your airplane!

If you look in your POH, there should be a graph describing True Airspeed, Density Altitude and Engine Power. It takes so much power to push an airplane through the air at a given speed. The faster you cruise the more power it takes, and the power demand increases much more then the airspeed does. Engine Power produced is proportional to fuel flow, so if you know your speed, then you can also know the fuel flow for your type aircraft. The trick here is to know your TRUE airspeed, because the ASI lies to you as you go higher. This means you must correct your airspeed reading to true airspeed while you are flying the plane, and trim the airplane for the true airspeed desired to properly manage fuel flow.

For the first few years, we flew 86F with open wheels and cruised along at around 100 kts on our long trips. We nearly always burned 7.5 GPH on these trips, but even a slight headwind meant we would be stopping for fuel. At first we stopped anyway, just to be safe. Over time I was able to confidently extend our range safely, so we completed a 400 nm leg and landed with our desired 1-hour fuel reserve left in the tanks. The airplane’s performance was predictable and reliable.

When we added wheel pants to 86F a few years ago, we were able to cruise faster airspeeds due to the reduction of drag but determining fuel flow became a bit of a problem. I was flying the airplane the same way, only faster, thinking I was burning fuel about the same or slightly more. At first this worked okay, and then one day we cut too deep into our reserve fuel. What I was doing was not giving me enough information to trim the plane properly to maintain our fuel reserve.  I needed a better way. 86F has a fixed prop, fixed gear, manual carburetor, and no engine analyzer or fuel flow meter. How could I have a better idea of fuel flow without spending a lot of money?

We all learned to use an E6B for various tasks in flight planning. Some of us can even make these calculations while we are flying the plane, but I can’t do so while bouncing around in the cockpit and maintaining my altitude and heading. Besides, I have never been real good with the E6B for true airspeed calculations, and this is one case where accuracy is really important. So I got FlyBy E6B, an electronic version of an E6B flight computer, and loaded it on my Palm Z22 (Geezer Alert!) and now I have a copy on my new smart phone. (Yes, I finally broke down and got a smart phone!)

E6B app on iPad and iPhone

With a little E6B work, it’s easy to calculate very accurate fuel burn numbers.

I initially tried to use my Z22 in the plane while I was flying, but I was only successful in smooth air. That just wasn’t good enough. I needed to know my TAS quickly, even during a bumpy ride! So I came up with a cheat sheet for TAS that was easy to use. It is printed on one half-size page, front and back. I keep a laminated copy in the airplane with my checklists, and refer to it periodically as I fly. Through trial runs and after flight analysis, I determined about how much fuel 86F burns at different cruise airspeeds when fully loaded. I added this information to the bottom of the sheet. The interesting thing is 86F burns about the same fuel at the same TAS independent of altitude. I know that disagrees with accepted theory. My theory is when fully loaded 86F is underpowered above 6500 and climbing higher takes more time and consumes more fuel, thus offsetting the gains of cruising higher.

Another interesting thing is that between 100 and 115 TAS the fuel burn varies approximately with the square in the change of airspeed. This also goes against the theory that fuel burn varies with the cube of speed, a proven theory widely accepted in the marine propulsion industry. I believe the difference here is due to induced drag, or the power required to keep the airplane flying, which diminishes as airspeed increases and increases as airspeed decreases. This works inversely to profile drag and causes the change in fuel burn to be somewhat moderated at normal cruise speeds.

So let’s look at the cheat sheet I created for 86F. The sheet has TAS tables at four different cruise altitudes with four calibrated airspeeds at each altitude. The back of the sheet has the same information for four more cruise altitudes, thus providing information for VFR altitudes from 2500 to 9500 feet. My airspeed indicator is accurate enough to use indicated airspeed interchangeably with calibrated airspeed on this chart.

hodges chart

To use the chart, read the outside air temperature and the airspeed indicator, then align those two in the columns under the current cruising altitude. For example at 6500 feet with OAT of 20°C and with 100kts indicated airspeed, the TRUE airspeed is 113 kts. That would put the fuel burn of 86F at about 9.5GPH. (113/110)² X 9.0 = 9.5. At 75% power the fuel burn is 8.4 GPH and 9.5 is right much more. That might be okay on a short trip when you have lots of extra fuel or if you want to stop and refuel anyway, but if you need more range then decreasing from 100 to 95 indicated brings the TAS to 108 and the fuel burn down to 8.7 GPH. (108/110)² x 9.0 = 8.7. Now check your fuel burn against time to destination on your GPS. Still need more range? Trim to 97.5 indicated for a TAS of 105 and burn only 8.2 GPH, much better.

Knowing your True Airspeed has other advantages as well. You can subtract TAS from the GPS ground speed and know exactly how much tailwind or headwind you are flying in. Now determine gallons of fuel per 100NM based on your TAS, GS and fuel burn. At 105TAS 86F burns 8.2 GPH. 100NM/105GS x 8.2GPH = 7.8 Gal/100NM. Still got 300 to go? You need 3 x 7.8 = 23.4 gallons plus reserve. In a 20 knot headwind, 100NM/85GS x 8.2GPH =9.2 Gal/100NM. Now you need 27.6 plus reserve.

I preflight with 9 gallons fuel reserve for all of our long flights. During the actual flight I am willing to accept a 3 gallon loss of fuel reserve if it means I can make the trip in one leg instead of two, but any more than that I hunt down a gas station. I usually make my initial decision to stop or not stop at about half way, and then again at the 3/4 point. I always verify how much fuel was burned after the flight to check my calculations. If I ever land with less reserve than I expect there should be, I find out why. It is this after flight analysis that led me to make the TAS chart in the first place.

The TAS data in the tables is accurate to the nearest ½ knot for an altimeter setting of 29.9-30.4. To correct for other altimeter settings, look at the ALT column on the left. 29.9-30.4 is in BOLD type, indicating the chart is generated on these numbers. If the current altimeter setting was 29.10, that number is included in the ALT setting listed two lines above (28.9-29.3), so ADD 2 knots to the TAS in the chart. If the current altimeter setting was 31.10, that number is included in the ALT setting listed two lines below (31.0-31.5), so SUBTRACT 2 knots to the TAS in the chart.

I first generated this chart long-hand and did not include an ALT correction, I simply accepted that at other altimeter settings some error would be introduced. It took me a while to figure out how to add an altimeter correction to the chart that would be easy to use. This chart was generated on an Excel spreadsheet, but the actual TAS calculations were done on the FlyBy E6B. I simply added TAS data to one cell for each altitude, then the rest of the cells are filled out according to a simple formula like cell-X + 1 or cell-X + 5. Then I spot checked the cells for errors and corrected them as required until the entire chart was correct.

This TAS sheet works for me and my airplane because I have spent the time to verify actual fuel used for various true airspeeds on long trips. I try to use every tool in my possession to make sure that I don’t run out of fuel or cut too far into my reserve. So far I haven’t run out even though I have pushed fuel to the reserve limit on several of our long flights. This system may not be perfect but it works, and it can work for you too. We, as pilots and navigators, should know and fly our aircraft well enough that we never risk running out of fuel, especially given all the tools at our disposal. I hope I have now added yet another tool to your flight bag.

4 Comments

  1. Keith Bumsted says:

    Pete,

    With all due respect, you’re making this way too hard! There’s this marvelous new invention called a fuel monitor that will tell you all you need to know about usage and amount available at a glance — accurate to a tenth of a gallon and available at a number of airplane supply stores for a few hundred dollars. Such devices will tell you what you need to know far more accurately than your spreadsheet and in real time. Check with your maintenance facility about installation time and costs.

    There is a certain amount fiction in POHs about tank capacities and burn rates that is there to help pilots err on the plus side. I once owned a very nice Bonanza that had four tanks, two in each wing, consisting of independent main tanks and two interconnected auxiliary tanks that were utilized together. To add to the mental gymnastics, there was only one fuel gauge to service both main tanks and one for the auxiliary tanks together. As was my practice at the time, I routinely ran both the left main tank and both auxiliary tanks dry before switching to the right main tank to access the final fuel left on board (about two hours worth). In following this procedure over 3,000 hours, I discovered that the POH said each main tank held 22.5 gallons whereas the capacity metal stamped on the fuel caps said the mains held 25.0 gallons. Alas, when refueling the left main tank after running it dry on virtually every flight, the line person could always squeeze in 26.0 to 26.1 gallons! Who’re you going to believe, me or your lying eyes?

    All of the ambiguity and uncertainty about fuel used and fuel left on board is wiped away with a inexpensive fuel monitor and it will free you to spend your piloting time on the more important chores of managing the flight. Happy trails!

  2. Duane says:

    Pete – I agree with Keith that you’re over-analyzing and overthinking fuel management. You don’t even need to purchase a fuel monitor, either. But you do need, and must get if you don’t already have it, a GPS. The GPS will do all of the number crunching for you in flight.

    The deal is, your engine, if properly maintained and with mixture properly controlled for your cruising altitude (preferably using an EGT gage, which many but not all Cherokees are equipped with), will always burn the same gallons per hour at the same throttle setting (as measured by engine RPM and/manifold pressure) and altitude.

    You derive your fuel available at any point in flight by measuring how much you started with (a dipstick calibrated for your airplane is a cheap but necessary tool for your preflight checks), multiply your known fuel burn rate by hours flown, and subtract your fuel burn from initial fuel. Better to think of fuel as “hours of fuel” rather than “gallons”. Your GPS can and will calculate that number and give you fuel remaining in both nm and hours, continuously … provided you program the GPS with the proper fuel burn rate for your aircraft.

    If you don’t have a panel GPS, by all means get either a cheap handheld aviation GPS, or use one of the pad computer apps on any of the common pad computers.

    It’s a good idea to always fill your tanks on long cross country trips, so the starting fuel is highly accurate and reliable. If you start with partial tanks, a calibrated dipstick is going to be accurate to within one gallon or less per tank, which is much more precise than your panel gages. Every time you refill your tanks, log the gallons purchased and periodically review your logged numbers to re-verify your fuel burn rate. Any significant change in your fuel burn rate should be communicated to your mechanic, as it may indicate an engine issue or potential fuel leak.

    Finally, I don’t think it’s a good idea to always be pushing your limits on range, even with one hour of fuel planned as reserve. As you know, headwinds can seriously limit your miles per gallon and give you a serious case of pucker factor on a long flight … and besides, why would a pilot flying a personal airplane, and not on a “company schedule”, try so hard to avoid another landing and takeoff? That’s the fun part of flying!

    Make that midpoint fuel stop, and then you can stop worrying about your fuel burn. Besides, maybe you’re young now, but as you get older it gets a lot more challenging to postpone those bathroom visits :-). A stop every two hours or so is a pretty reasonable flight plan for most of us. It also keeps you “fresher” as the pilot in command.

  3. Louis Sell says:

    I am really pleased to hear some really do concern themselves with fuel state / consumption. I’ll fly with a driver like you anytime.
    I also agree with those who consul you are overdoing it.
    I think Duane has good suggestions. My system is to know what your airplane burns per hour–you said 7.5 gal–round that up to 8. Now if you start with tanks full, and checked, you know how many hours you can fly. Also stated by Duane is not to push your fuel state. Landing at 1/2 tanks or just under, depending on weather ahead is the mark of a wise and safe pilot. (not to mention one with a empty bladder) (decisions always seem to be easier on an empty bladder).
    Long term I think you will notice your fuel burn per hour will be very consistent, assuming normal cruise setting. Variation in fuel burn rate should set off alarms in your head to start looking for the reason.

  4. William "Pete" Hodges says:

    Maybe you have missed the main points of the article: 1. True airspeed and fuel burn are related in a precise way. 2. A cheat sheet for finding true airspeed in flight quickly gives you the fuel burn rate for your aircraft as it is flying. 3. Learning how to do this makes you more familiar with your airplane and you become a better a better pilot as a result. And 4. it’s free