When Endurance Matters

Most of the time, when it comes to fuel we’re only concerned about range. How far can we go and still arrive with adequate reserves? We routinely do this calculation and are pretty comfortable with the process.

The other dimension of the fuel equation is endurance. How long can we stay in the air? Endurance really matters when ATC gives you an indefinite hold and you’re in IMC.Also, when we arrive at our destination only to find our time to land is uncertain, we care a lot about endurance. For such situations we need to know how long we can hang around before we have to divert to an alternate.

How the Problem Occurs

Most of us are pretty conservative when it comes to fuel management. We fly short legs, always start with the tanks topped, and avoid really nasty weather. We need to be careful that conservative planning doesn’t give us a false sense of security. Conditions can changeduring the flight and when they do, we need to be prepared. When the weather unexpectedly goes sour, we have to decide whether or not to hang around for conditions to improve. I’ve had it happen to me. In one case the forecast was for good VFR and no reason to expect otherwise. I arrived at my destination to find a thunderstorm parked over the airport. ATC asked for my intentions?Hold until the storm moves off or divert?Another situation that can get dicey is arriving IFR at an uncontrolled field to find you’re just one of many planes queued for the approach. This happened recently at Tullahoma when the field was IFR and there were multiple simultaneous arrivals. Several planes had to divert because they could not hold long enough for their turn for the approach. IFR traffic moves very slowly at uncontrolled fields and really getsbacked up when a pilot forgets to close his flight plan after landing. In situations like these, fuel can quickly become a cause for concern.

Stretching Fuel

In the unlikely eventthat we need more time in the air, we need to have a simple procedure to size up our options. Complicated calculations don’t work. Just when you need to do this calculation is when you also have your hands (and mind) full with ATC and flying the plane so it needs to be dead simple. The POH for my A36 has a nice set of graphs for endurance at various power settings but I don’t find them very useful. Fortunately all that’s needed is a simple calculation.

Fuel Remaining / agal
Fuel to Alternate / - b
Fuel for 45 min Reserve / - c
Fuel Available to Loitering / = dgal
Loiter Burn Rate / ÷ egph
Loitering Time / = fhr

While it is a simplecalculation, there is some preparatory workwe need to do. Let’s take it one item at a time:

(a) Fuel Remaining How many gallons of fuel do we have left right now? This isthe most critical piece ofinformation when we’re trying to figure our remaining endurance. It’s not hard to estimate provided we know the numbers for our airplane: how much fuel we started with, how much we burn in taxi and climb, and our burn rate per hour in cruise.

For my Bonanza I usually start with full tanks, 74 gal usable. I allow 5 gal for taxi and climb, and I burn just under 13 gph cruising at 155 KTAS. So if I’m 2:30 hrs into a flight, my fuel remaining is 74 – 5 – 32 or 37 gal. My aircraft happens to have a digital fuel flow meter so I don’t have to do even this simple calculation. I cango to the Aux page of my Garmin 530 at any time and directly readmy fuel remaining in gallons. Even without this sophistication, the calculation is not difficult provided we know the numbers for our aircraft.

(b) Fuel to Alternate This may or may not be to the alternate we filed on our flight plan. It can be any airport we like. We are not bound by what’s on our flight plan. What makes a good alternate is a subject in itself. Personally, I never pick an alternate close to my destination even if it’s legal. I assume that if the weather should tank at my destination it probably will also for airports nearby so I look for alternates 50 to 100 miles away. I also try to have several alternates in mind and, as my flight progresses, I periodically ask myselfwhich one I would head for if I had to. It helps that I have data link weather in the cockpit, but a call to flight watch can also update us on current conditions. Many of my flights are 3-4 hours and I’ve noticed the weather can change a lot since the time I got my preflight briefing.

Let’s say the alternate of choice for this flight is Bowling Green, KY (BWG), roughly 60 nm from our destination, Smyrna, TN (MQurY). At a cruise of 155 kts and 13 gph it will take roughly 30 min to get there and we’ll burnabout 6 gals. Notice I’m using very simple math for this and making no allowance for wind. At this point in the flight, don’t count on having either the time or the mental capacity to do much better. Fortunatelya rough estimate such as this is close enough. If you believe the wind is a significant factor, add a gallon or two to be safe.

(c) Fuel for 45 min Reserve We already have this number at hand from our flight planning. For my Bonanza it’s 10 gals. This is our buffer for all the things that can go wrong. Don’t give this safety net away. I have a personal rule that I never plan to land with less than 10 gal of fuel on board. Never! Also, when the weather goes sour for you it will probably go sour for others as well and there is a good chance that they will head for the same alternate as you. If you get caught in a traffic jam at your alternate, you’ll be glad you have this reserve.

(d) Fuel Available to Loiter The net of (a), (b) and (c) gives us how much fuel we have for hanging around waiting for conditions to improve. In our example it’s 37-6-10 or 21 gals.

(e) Loitering Time The number we’re looking for is how much time we have before we need to head for our alternate. This is where we need to know the maximum endurance numbers for our particular aircraft. If all we’re doing is killing time, we want to do it at the minimum possible fuel burn rate. The numbers for this for your airplane might surprise you. Maximum endurance TAS for my Bonanza is pretty slow, 85-90KIAS depending on gross weight. Unfortunately I can’t cruise this slow. My POH says that cruise operation at settings of less than 20” MP and 1900 RPM are not recommended. At 20”and 1900 RPM I’m doing105 KIAS and burning 7.5 gph. This number comes from actual flight testing. I don’t rely on the POH numbers. Now when I have to loiter, I immediately pull the power back to 20” and 1900 RPM and lean aggressively to 7.5 gph. In our example flight, with 21 gal available, I can loiter for just short of 3 hours. Let’s call it 2:45 to be on the safe side.

Fuel Remaining / 37gal
Fuel to Alternate / 6
Fuel for 45 min Reserve / 10
Fuel Available to Loitering / 21gal
Loiter Burn Rate / 7.5gph
Loitering Time / 2:45hr

Last step is to make a note on our flight pad of when this loiter time is up. Now we can relax and enjoy the ride in the hold. When we’re approaching the end of our loiter time, 2:45 hrs in our example, we call ATC and tell them we have a fuel problem and need clearance toour alternate now!

Summary

The bottom line for getting the maximum endurance out of our airplane is to know our numbers. Forget the POH. Go do some flight testing to determine the actual fuel burn rate for your specific airplane for the configurations you generally use. Armed with this and some simple math you can quickly estimate how long you can hang around waiting for things to improve.

Acknowledgement

Special thanks to Chris Parks, Andy Lee and members of the general aviation flying community for much of the wisdom in this article.

Peter Cassidy

12/21/05