Long XC time vs fuel

[midwestpa24]

I've noticed a lot of these "hypothetical" scenarios that are sometimes posted here could, or should, have been avoided with proper planning and procedures. The point of careful planning, preparation, and training, is to avoid placing yourself in these binds in the first place. The point I have always been taught and passed along to my students is to always have a Plan A, Plan B, and a good idea of what Plan C may look like. If you have to abandon Plan A and go to Plan B, you better be figuring out the details for Plan C. Never put yourself in a position where there is only one path to take, always leave an out...or two or three.

To the original scenario posted. I never like landing with less than an hour of fuel on board, especially IMC or in remote areas. Any airplane I will typically cross country in, I know exactly how many hours of fuel I have on board, how many hours it should take to make my destination, and once airborne a pretty accruate idea of my ground speed and ETE from the GPS.

 

[Bill Watson]

Then you have "guidelines," not "minimums." There's a difference. My next comment is only intended to illustrate the difference.

"Just like IFR mins." Do you mean "once in the air on rare occasions," you go below published approach minimums a bit "due to actual wx"?

No.

But with that said, my personal mins are probably best described as guidelines because for the life of me, I just can’t come up with a single set of minimums other than those specified in the FARs for IFR flight. And those personal guidelines are getting more conservative by the year.



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[LongRoadBob]

Maybe it can theoretically work, but I was really puzzling over the idea that with headwinds slowing you down, flying for max distance, even slower, would make up for lack of fuel.
How is that supposed to work? Sure, more time flying with the same fuel, but also you are slower, and still have the same headwinds, and now a slower ground speed. Isn’t that slower speed going to eat up the advantage?

More time to get there, but almost the same amount of fuel left when you do, because...more time to get there?

 

[midlifeflyer]

Maybe it can theoretically work, but I was really puzzling over the idea that with headwinds slowing you down, flying for max distance, even slower, would make up for lack of fuel.
How is that supposed to work? Sure, more time flying with the same fuel, but also you are slower, and still have the same headwinds, and now a slower ground speed. Isn’t that slower speed going to eat up the advantage?

More time to get there, but almost the same amount of fuel left when you do, because...more time to get there?

Flying slower means a lower power setting, which in turn means using less fuel and expanding range and endurance. The actual enroute time will depend on the winds, but whether headwind, tailwind, or none the differential between them should be the same.



The tables or graphs for this are in the manual. I forget what you fly, but in, say a Cessna 172 with 40 gallons usable on board and a 45 minute reserve, at 4,000 msl, zero wind, range at 75% power is about 435 NM' at about 115 KIAS. At 55% power, it's about 530 NM, 100 miles more, although at a slower 96 KIAS. Endurance profile for the same parameters are about 3.8 hours and 5.4 hours, a bit more than an hour and a half aloft. You can use those numbers with your wind-corrected groundspeed, to calculate the differential and figure whether it makes sense. Does that extra hour fuel really buy you enough to be wortwhile.

 

[asicer]

Maybe it can theoretically work, but I was really puzzling over the idea that with headwinds slowing you down, flying for max distance, even slower, would make up for lack of fuel.
How is that supposed to work? Sure, more time flying with the same fuel, but also you are slower, and still have the same headwinds, and now a slower ground speed. Isn’t that slower speed going to eat up the advantage?

More time to get there, but almost the same amount of fuel left when you do, because...more time to get there?

Let's look at a couple of somewhat real-world examples.

Example 1:
35kt headwind in a Piper Cub. Throttled back to 35kts cruise, it won't go anywhere. It will just hover at 0kts ground speed. It will spend an infinite amount of fuel at that power setting. Any higher power setting will burn less fuel to go any distance.

Example 2:
35kt headwind in a Cessna 182. Burns 13gph at 135kts or 10gps at 115kts. With 65 gallons on board, that's 5 hours at 135kts or 6.5 hours at 115kts. At higher power settings, you get 100kts ground speed * 5 hours = 500 nm range. At lower power settings, you get 80kts ground speed * 6.5 hours = 520 nm range.

 

[chemgeek]

Maybe it can theoretically work, but I was really puzzling over the idea that with headwinds slowing you down, flying for max distance, even slower, would make up for lack of fuel.
How is that supposed to work? Sure, more time flying with the same fuel, but also you are slower, and still have the same headwinds, and now a slower ground speed. Isn’t that slower speed going to eat up the advantage?

More time to get there, but almost the same amount of fuel left when you do, because...more time to get there?

The power required to overcome drag increases by the CUBE of the velocity. So throttling back dramatically reduces drag, thereby increasing fuel efficiency, and therefore range, using the same amount of fuel. In addition, reduced fuel usage also reduces the amount of fuel required to maintain your timed reserve capacity. You can actually figure this out using the performance charts in the POH. For my plane, a simple calculation of range with 1-hour reserve at 2600 rpm, 117 kt, is around 400 nm; At 2400 and 103 kt (painfully slow, but...) range is around 460 nm. Headwinds will of course change the real-world calculation a bit, as will adding in additional fuel burn for taxi and climb to altitude, but it is possible to calculate fairly accurately if a reduced cruise setting will allow a non-stop trip with adequate fuel reserves. Certainly, strong headwinds will reduce the advantage of throttling back.

I've done the reduced power thing once when my calculated fuel reserves at my normal cruise rpm would result in violating my fuel reserve minimums for a non-stop trip. Reducing cruise power allowed me to make the trip without a stop, albeit VERY slowly with the anticipated headwinds. On the other hand, the non-stop trip was still faster than cruising faster and stopping for fuel. It's fairly easy to estimate fuel consumption for a particular trip at 2 or 3 different cruise power settings and see what kind of reserves you will have left. It should be easy enough to put this kind of calculation into a spreadsheet if desired.

 

[Skyrys62]

What did we expect everyone to say here?

Do I do something very risky like fly into night IFR using quick mental math about a low fuel situation while experiencing poor communications?
Or, do I do something much safer? (and even get to fuel up with some daylight so I can see what I'm doing)

Of course everyone will agree.

Matter of fact, if they don't, I wouldn't be listening to them anyway

 

[chemgeek]

What did we expect everyone to say here?

Do I do something very risky like fly into night IFR using quick mental math about a low fuel situation while experiencing poor communications?
Or, do I do something much safer? (and even get to fuel up with some daylight so I can see what I'm doing)

Of course everyone will agree.

Matter of fact, if they don't, I wouldn't be listening to them anyway

Yes, and the real lesson has been mentioned a few times, namely, the purpose of proper pre-flight planning is to try to avoid putting yourself into the dire situations of these hypotheticals. There is always a probability you could get yourself into a fuel pickle on any flight, but the idea is to make that probability as low as possible by anticipating the most likely adverse events and having enough reserve to cope with them safely. A good pilot will frequently evaluate the fuel bingo time to see if it is still safely beyond the estimated arrival time. I've had more than one flight were winds aloft were changing that calculation in and out of the safety window, necessitating changing altitude for better winds, reducing fuel flow, or simply capitulating and landing for fuel at an intermediate stop (one or more of which were already identified pre-flight.) If you are up there with increasingly critical fuel reserves and not near an intermediate fuel stop, something went wrong in pre-flight planning. You are not supposed to get caught that short, not even in a little flib. And especially IFR.

Fly safe out there.

 

[N1120A]

Land. It is even easier with transponders that measure your overall flight time. If you know your plane has 4.5 hours of range, and you are up against the 3.5 hour mark, you know it is time to land. In a low wing, you can count your 30 minute tank changes too. My E33 has about 5.5 hours of range with the tanks full, cruising at 13 gph and 152 knots TAS. Garmin Pilot does the math for me when filing my flight plan. Briefing allows me to double check. If I have less than 1 hour of reserves, alternate considered, I plan a stop. If I have done 8 tank changes/flown 4 hours on my handy GTX375 - it is time to start thinking about where I am and where I'm going. Easy.

What? Did I miss something or did I really just see 100% consensus of replies?

On POA. Hard to believe, eh?

Land.
Phone call.
Credit Card meets fuel pump.
Arrive 40 minutes late.

- VFR Pilot so maybe missing something? Seems like you are saying "Should I follow all the rules and kill myself or break a rule, live and make a phone call". Its hard to imagine this would result in any FAA action because winds changed on you after you received a authorized brief.

They wouldn't even give a phone number over this. "Say reason for diversion?" "Longer than expected flight challenging my personal minimums." "Alright, cleared to PoDunk via direct." That's it.

 

[denverpilot]

You’re on a long XC, having a really good argument with the center controller over a nitty point of an FAR interpretation, when you see you’re gonna need fuel. But to get to the pump, you need to execute a DME arc......

But did you take off from a treadmill?

 

[bflynn]

That is, I would never plan a flight where I arrive with less than 10 gallons just as I wouldn’t plan a flight with less than IFR reserves. But the reserve is included exactly for such situations and I’ll use part of it in the air if it makes sense to me

I disagree with you on using reserves. They are not there to make up for performance problems, nor for the convenience of not having to stop. They’re there to cover the time where you’re wrong about how much fuel you have and/or not being able to land at your planned destination.

The plane I soloed in died that way. Pilot overflow a class C when he had about quarter tanks. Arrived at the next class C at 4am with 400’ ceilings and minimum fuel, then went missed. While recycling for another try, the engine stopped. He broke out over a forest with one small clear patch directly in front of him and is lucky to be alive - any place else and he would have going into the trees. Why is that he flew into his reserve to get to his refueling point.

 

[Witmo]

I disagree with you on using reserves. They are not there to make up for performance problems, nor for the convenience of not having to stop. They’re there to cover the time where you’re wrong about how much fuel you have and/or not being able to land at your planned destination.

The plane I soloed in died that way. Pilot overflow a class C when he had about quarter tanks. Arrived at the next class C at 4am with 400’ ceilings and minimum fuel, then went missed. While recycling for another try, the engine stopped. He broke out over a forest with one small clear patch directly in front of him and is lucky to be alive - any place else and he would have going into the trees. Why is that he flew into his reserve to get to his refueling point.

And the regulations support your opinion. To operate under IFR you must have the IFR fuel reserve at all times. Dipping into the reserve to avoid having to stop for fuel violates the regulation because you would be operating under IFR with less than the required reserve. That IFR fuel reserve is there for emergencies, not for convenience or for any other rationale outside of an emergency situation. If you plan a flight and find that you will have the bare minimum of a reserve at your intended destination, you should declare "minimum fuel" if ATC tries to alter your plan airborne and it would cost you extra fuel. If you haven't included fuel for any holding at the destination and ATC gives you holding instructions and an expect further clearance time, you need to inform them that you are minimum fuel and if you're going to have to dip into the reserve to comply with their instruction--emergency fuel.

 

[steingar]

I had a really good habit in my old Cherokee, which had fairly reliable fuel gauges. If i hit quarter tanks on both sides I landed at my nearest, even if my nearest was behind me. No question whatsoever. I did that once about 10 minutes from home, but it just didn't matter. My thought is and was that if I did that every time I'd never run out of gas, which is still leading cause of aviation accidents. The fuel gauges in the Mooney suck, but I usually start a long flight with full tanks. 4.5 hours on the clock and I'm done, no questions. I pushed it to 5 once completely by accident trying to get into Oshkosh. Still, when I saw the time I veered off and landed immediately. I pumped more gas into my airplane than before or since, but I didn't bust minimum fuel. Still had an hour's worth.

Problem with trying to push it is you don't know everything. You really don't know how much you're burning, could be your mixture is set wrong or your engine is thirsty that day. You really don't know how much gas is in the tank, could have ported some overboard on the ground, could have a leak.

 

[bflynn]

Smart.

In ground school, a very wise and very old aviator / flight engineer / pilot / A&P taught us that reserves are not to be touched even in VFR. If you ever fly into the reserves, you've messed up. Once I landed with about an hour and 5 minutes left, so I'm good so far.

 

[iamtheari]

And the regulations support your opinion. To operate under IFR you must have the IFR fuel reserve at all times. Dipping into the reserve to avoid having to stop for fuel violates the regulation because you would be operating under IFR with less than the required reserve. That IFR fuel reserve is there for emergencies, not for convenience or for any other rationale outside of an emergency situation. If you plan a flight and find that you will have the bare minimum of a reserve at your intended destination, you should declare "minimum fuel" if ATC tries to alter your plan airborne and it would cost you extra fuel. If you haven't included fuel for any holding at the destination and ATC gives you holding instructions and an expect further clearance time, you need to inform them that you are minimum fuel and if you're going to have to dip into the reserve to comply with their instruction--emergency fuel.

This is one of those places where the regulations are poorly written, although the intended meaning seems clear:

For IFR, FAR 91.167(a) says "No person may operate a civil aircraft in IFR conditions unless it carries enough fuel ..."

For VFR, FAR 91.151(a) says "No person may begin a flight in an airplane under VFR conditions unless..."

"In IFR conditions" and "under VFR conditions" are the dumbest phrases in the FARs. The way the regulations are written makes it sound like you can accept an IFR clearance with only VFR fuel reserves and no alternate, regardless of en route or terminal weather, as long as your takeoff is in VMC. That's clearly not what they wanted to say, but interpretations of the law do have to start with the law as it is written.

Even applying the assumption that 91.167 was meant to say "under instrument flight rules" instead of "in IFR conditions" and 91.151 was meant to say "under visual flight rules" instead of "under VFR conditions," though, the smart thing to do is to set personal minimums for fuel reserves, both a takeoff reserve and an in-air reserve. Far too many crash sequences begin with air in the fuel lines to be interpreting the fuel reserve requirements leniently.

 

[N1120A]

I had a really good habit in my old Cherokee, which had fairly reliable fuel gauges. If i hit quarter tanks on both sides I landed at my nearest, even if my nearest was behind me. No question whatsoever. I did that once about 10 minutes from home, but it just didn't matter. My thought is and was that if I did that every time I'd never run out of gas, which is still leading cause of aviation accidents. The fuel gauges in the Mooney suck, but I usually start a long flight with full tanks. 4.5 hours on the clock and I'm done, no questions. I pushed it to 5 once completely by accident trying to get into Oshkosh. Still, when I saw the time I veered off and landed immediately. I pumped more gas into my airplane than before or since, but I didn't bust minimum fuel. Still had an hour's worth.

Problem with trying to push it is you don't know everything. You really don't know how much you're burning, could be your mixture is set wrong or your engine is thirsty that day. You really don't know how much gas is in the tank, could have ported some overboard on the ground, could have a leak.

These are great rules of thumb. Even newer electronic engine monitors rely on old, crapbag floats and wires. Doing the basic flight planning math is still your best friend. Time/Fuel/Distance.

 

[Bill Watson]

These are great rules of thumb. Even newer electronic engine monitors rely on old, crapbag floats and wires. Doing the basic flight planning math is still your best friend. Time/Fuel/Distance.

Well, modern fuel flow transducers take the guess work out by measuring the actual fuel burnt. I’ve been flying with one for 9 years (RV10) and can rely on it right down to a plus/minus a gallon.

It’s backed up with floats which are good to plus/minus 3 gallons/side. But without the fuel transducer to both calibrate and back the floats up, it always feels iffy. So I guess that makes your point.


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[N1120A]

Well, modern fuel flow transducers take the guess work out by measuring the actual fuel burnt. I’ve been flying with one for 9 years (RV10) and can rely on it right down to a plus/minus a gallon.

It’s backed up with floats which are good to plus/minus 3 gallons/side. But without the fuel transducer to both calibrate and back the floats up, it always feels iffy. So I guess that makes your point.


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And only if the K Factor is done correctly.

 

[Witmo]

This is one of those places where the regulations are poorly written, although the intended meaning seems clear:

For IFR, FAR 91.167(a) says "No person may operate a civil aircraft in IFR conditions unless it carries enough fuel ..."

For VFR, FAR 91.151(a) says "No person may begin a flight in an airplane under VFR conditions unless..."

"In IFR conditions" and "under VFR conditions" are the dumbest phrases in the FARs. The way the regulations are written makes it sound like you can accept an IFR clearance with only VFR fuel reserves and no alternate, regardless of en route or terminal weather, as long as your takeoff is in VMC. That's clearly not what they wanted to say, but interpretations of the law do have to start with the law as it is written.

Even applying the assumption that 91.167 was meant to say "under instrument flight rules" instead of "in IFR conditions" and 91.151 was meant to say "under visual flight rules" instead of "under VFR conditions," though, the smart thing to do is to set personal minimums for fuel reserves, both a takeoff reserve and an in-air reserve. Far too many crash sequences begin with air in the fuel lines to be interpreting the fuel reserve requirements leniently.

I can see the FAA drawing a fuel reserve distinction for IFR aircraft operating in VMC vs IMC because the former allows the pilot to cancel (outside of class A airspace) and proceed under VFR with its less restrictive fuel requirement. I agree it could be worded more clearly.

 

[Bill Watson]

And only if the K Factor is done correctly.

K Factor? No idea what that is.


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[steingar]

Well, modern fuel flow transducers take the guess work out by measuring the actual fuel burnt. I’ve been flying with one for 9 years (RV10) and can rely on it right down to a plus/minus a gallon.

It’s backed up with floats which are good to plus/minus 3 gallons/side. But without the fuel transducer to both calibrate and back the floats up, it always feels iffy. So I guess that makes your point.

You remind me of Mrs. Steingar. She always drives her car unit the little gas light comes on. Then she goes to get gas. I asked her what was going to happen when the light burned out.

Gadgets work until they don't.

 

[Salty]

K Factor? No idea what that is.


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It’s the calibration number for the transducer. I ran a few tanks measuring exact quantities used and then adjusted my k factor accordingly. My totalizer has been on the money ever since, but I still use the lowest value between it and the gauges (even though the gauges are always wrong)

 

[Bill Watson]

It’s the calibration number for the transducer. I ran a few tanks measuring exact quantities used and then adjusted my k factor accordingly. My totalizer has been on the money ever since, but I still use the lowest value between it and the gauges (even though the gauges are always wrong)

Thanks.

I have spent quite a bit of time working on my fuel system (experimental RV10). Originally my transducer was installed just downstream of my electric fuel pump. Due to fuel line turbulence and such it was somewhat inaccurate at low RPMs. Given the amount of time and fuel involved when operating at low RPMs, it really wasn’t a big issue. But I reconfigured it so that the transducer was between the fuel spider and the fuel metering unit. That eliminated the inaccuracy.

I have 2 independent fuel measuring systems; floats and the transducer. I calibrated both. They are displayed so that I see both indications in one graphic so they can be compared. It’s interesting watching the float numbers fluctuate depending on whether I have full, half or bingo fuel while comparing them to the transducer numbers which are dead nuts accurate throughout the flight.

Next to fuel quantities and burn rates, I have miles to go and time to go constantly displayed and updated.

Redundancy-wise, this information can be displayed on any of 3 different displays (I have had 2 failures over 9 years). Electrically I’m flying with 2 batteries, 2 alternators and 2 independent buses.

It takes several failures to be without accurate fuel remaining information on a given flight.


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[N1120A]

It’s the calibration number for the transducer. I ran a few tanks measuring exact quantities used and then adjusted my k factor accordingly. My totalizer has been on the money ever since, but I still use the lowest value between it and the gauges (even though the gauges are always wrong)

Yeah, the GPH measurements are still based on inputs based on usage measurements - they aren't measuring the actual fuel flow.