Cross Country Planning

SkyChaser

SkyChaser

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SkyChaser
I am planning my first cross country right now, and was wondering - is it a good or bad thing to include a bit of margin in the calculations?

Say, fuel burn. It is 8.8 gph in cruise at 75% power. Is calculating for 9 gph a good or bad thing? Or say, TAS calculations. At 75% power, would using a TAS of 120 mph as opposed to the exact value of 124 TAS a good or bad thing?

I like knowing there's a little bit of margin in there, but am curious if this is a good practice. Opinions or thoughts?
 
NO!!!!!! Must be exact number. No fudging!!!!!!!! ZOMG!!!!

Oh, wait I always fudge for a higher fuel burn and lower airspeed. :D

Actually, I used to do two plans - sort of. One to be safe, and one to see how close I could get to the actual vs planned. Usually used the one with the buffer, to be safe.

Then after a while, I got an airplane that has a 7 hour range, and my bladder only goes about 3.5 hours. So my planning is about as lazy as it can get. Top it off and fly till I have to pee.
 
SkyChaser said:
I am planning my first cross country right now, and was wondering - is it a good or bad thing to include a bit of margin in the calculations?

Say, fuel burn. It is 8.8 gph in cruise at 75% power. Is calculating for 9 gph a good or bad thing? Or say, TAS calculations. At 75% power, would using a TAS of 120 mph as opposed to the exact value of 124 TAS a good or bad thing?

I like knowing there's a little bit of margin in there, but am curious if this is a good practice. Opinions or thoughts?
Click to expand...
like warm muffins straight from the oven, my dog's tail wagging when i come in the door and a fire in the fireplace on a chilly night planning for a higher burn and a slower TAS is a good thing.
 
The margin you should be leaving is your fuel reserve. Legally, it's 30 minutes for VFR, but many pilots plan on 1 hour reserve.

Plan using the book fuel consumption and TAS, unless you know better from prior experience and real data. Of course, you will plan your actual GS using the forecast winds aloft at the desired cruising altitude, and add the appropriate extra fuel consumption for taxi and climb. Don't forget to adjust your GS for the initial climb to altitude. It will lengthen your trip slightly compared to computing the whole trip based solely on your cruise speed. Then you will re-evaluate in flight as you go along. Are you doing better or worse than planned? If worse, how much into your reserve, if any, are you eating into to make it to your destination? If you find in-flight you are eating into your inviolable reserve, you will land and get fuel. That's how is works in practice. For a 30-60 minute cross-country on full tanks, you will be hard-pressed to eat into a 1-hour reserve, but you should get into the habit of properly planning and monitoring fuel consumption so you don't become a member of "empty tanks club" during a longer range XC.

Here is how it works in my plane: I plan on cruising at 2550 rpm, which is right at 8.0 gph and 115 kt TAS by the book properly leaned at 6500 feet, a typical cruise altitude. I add one gallon for taxi and one gallon extra for departure climb. (For each leg.) If I start with full tanks and refuel upon returning, I'm never off by more than one gallon based on my stopwatch time, cruise gph and 2 "extra" gallons taxi/climb for one leg. If I'm off more than than that I have a fuel leak somewhere.

During flight on a near maximum-range flight, I will start comparing my expected time of arrival in total "fuel time" to my total available fuel minus 1 hour reserve, based on real-time flight conditions. (Sometimes those darn headwinds are not as predicted, or ATC vectors you all over creation, or you are diverting weather.) My plane has 4+30 fuel at 8.0 gph with full tanks, so if my destination ever gets more than 3+30 away in total"fuel time", I start looking for potential fuel stops. If it stays that way when I'm at 3+00 "fuel time" I'm definitely stopping for fuel ASAP.

So plan by the books, and re-evaluate in flight. And when you refuel, compare your estimates to the real thing to hone your planning. That's how I arrived at my "2-gallon" rule for taxi and climb. And you have been taught how to properly lean for cruise, yes? If you don't lean properly, you won't get book performance. Most aircraft will get scary close to book performance if properly operated and maintained.
 
I would say you plan by the numbers for you to show your CFI and mostly to fly your XC by, but you also mention reasons why you round up your fuel burn for safety reasons (could be because of headwinds, etc...). but fudging your TAS doesn't make much sense, you are trying to hit your waypoints based on the time you calculated. then if you are off by much, you adjust your times in flight.
 
Unfortunately there are two answers to your question. One answer is what you should do for your student pilot training and checkride. The other answer is what you should do when you are not in a training environment. In theory, they should be the same, but generally they are not. If you want to know what you should be doing right now, make sure you discuss this with your CFI. Your CFI also probably knows what to expect from your local examiners, although the FAA does have some guidance in the ACS and elsewhere. If you want to know what pilots really do, this is a good place for that.

Keep in mind, good cross country flight planning and tracking of fuel reserves during flight is important for developing these skills because if you ever start flying IFR it could become super important unexpectedly. When ATC suddenly vectors you around some city you were planning to overfly, and then tells you to start holding, you need to know if you have enough fuel to comply or if you need to request an expedited clearance.

For the types of planes we generally fly early in our careers, a rough but slightly conservative "gallons per hour" figure can be multiplied by your time flying (add a gallon or whatever it is for taxi and takeoff, and consider if there was a lot of climbing) to figure out how much you've burned.

If you ever work on your commercial certificate, you will be expected to be fairly accurate in predicting your time to way points and determining how adjustments will affect your estimated arrival time at the destination. For private I don't think they are as strict.
 
EdFred said:
NO!!!!!! Must be exact number. No fudging!!!!!!!! ZOMG!!!!

Oh, wait I always fudge for a higher fuel burn and lower airspeed. :D

Actually, I used to do two plans - sort of. One to be safe, and one to see how close I could get to the actual vs planned. Usually used the one with the buffer, to be safe.

Then after a while, I got an airplane that has a 7 hour range, and my bladder only goes about 3.5 hours. So my planning is about as lazy as it can get. Top it off and fly till I have to pee.
Click to expand...

Just a different kind of planning. Nothing like getting to 3 hours and realizing it is another hour to the nearest pee stop.
 
There are two extreme answers to this. One is to never carry a drop of extra fuel than needed because you are wasting fuel by carrying extra fuel, and it is ok to land with less than the legal reserve because legal reserves only applies for takeoff. The other extreme is to always takeoff with full tanks, and the only time you have too much fuel is when you are on fire. Where you fall in between this is going to depend on your personal judgement.
 
SkyChaser said:
I am planning my first cross country right now, and was wondering - is it a good or bad thing to include a bit of margin in the calculations?

Say, fuel burn. It is 8.8 gph in cruise at 75% power. Is calculating for 9 gph a good or bad thing? Or say, TAS calculations. At 75% power, would using a TAS of 120 mph as opposed to the exact value of 124 TAS a good or bad thing?

I like knowing there's a little bit of margin in there, but am curious if this is a good practice. Opinions or thoughts?
Click to expand...

I do the higher fuel consumption to add a little bit of extra safety margin. When I did my checkride I had the correct calculation and the round up on my plan and explained to the DPE that it was an added safety measure.
 
Remember that book fuel consumption numbers are for a properly leaned engine. I get the impression that many instructors don't spend much time on this, or actively discourage leaning. Not leaning could lead to an unpleasant surprise.

Of course you should always build in a safety margin in your planning. What if the plane doesn't perform exactly as the book says? What if winds are different? What if you have to divert?
 
SkyChaser said:
Say, fuel burn. It is 8.8 gph in cruise at 75% power. Is calculating for 9 gph a good or bad thing?
Click to expand...

Does your plane have a gauge that measures real-time fuel flow? If it does, you'll know how much you're really using during the flight.
 
The trouble with "fudging" your fuel burn up and TAS down is that you are going to run into your reserve fuel much sooner, unnecessarily restricting your safe range. This can be a factor if planning longer legs near maximum range with reserves. For short hops it won't matter.

PLAN accurately. OPERATE the plane properly (leaning). VERIFY your results after the flight. REFINE your future planning. During the flight, periodically EVALUATE how you are doing relative to your plan and reserves and ADAPT as necessary. The original plan is obsolete the minute you take off. In-flight conditions will likely change.
 
For practice do it to the numbers, and when you and your CFI review it you can add a buffer. But in real world zi always add a buffer. Fuel I add an extra 2 galleons an hour.
 
I prefer to plan as accurately as you can then add in a comfortable amount of EXTRA fuel. The more confidence you build in your ability to accurately plan what you should burn, the less your EXTRA fuel will be over time.

My fuel planning might look something like this.

TRIP ---- 1:30 13.2g <Planned fuel burn
RSV ----- 0:30 04.4g <FAR Reserve fuel
-------------------
MIN T/O - 2:00 17.6g <Minimum fuel required for takeoff
EXTRA --- 0:30 04.4g <Extra fuel
--------------------
TAXI ---- 0:10 03.0g <planned taxi-out fuel
RAMP ---- 2:30 25.0g <Fuel at engine start
REMF ---- 1:00 08.8g <Fuel remaining at destination
 
Don’t forget start, taxi, & takeoff, all different than cruise FF. I’d found up some, land after a reasonable ‘time’ flying.
 
NHWannabe said:
I do the higher fuel consumption to add a little bit of extra safety margin. When I did my checkride I had the correct calculation and the round up on my plan and explained to the DPE that it was an added safety measure.
Click to expand...
That is the correct answer for A checkride. It actually matters not that much what numbers you use. Exact or with a safety factor as long as you can explain the process and concepts of how and why you used those number.S Calculate exact if you like and then add a safety factor. Or add some safety factors based on fuel burn, TAS and then still add a bit of a safety factor.

Checkride are more just to make sure you understand the concepts. You may not use them the same normally but understanding why you have to turn 15 degrees off course for a crosswind could be important, and of course figuring out how much headwind/tailwind affects your flight is useful for planning fuel stops.

Brian
 
Never a doubt roll heavy if you don’t know the plane well. And once you do you have to assess your risk level- me it’s top off every chance I can...

Had I not planned pessimistically I would have been a glider pilot earlier this year. I flew a new to him old 150 home for a friend. Well the plane drank 2-2.5 gallons an hour more than book! First fuel stop really opened my eyes... my reserve had kept me airborn...
 
sarangan said:
There are two extreme answers to this. One is to never carry a drop of extra fuel than needed because you are wasting fuel by carrying extra fuel, and it is ok to land with less than the legal reserve because legal reserves only applies for takeoff. The other extreme is to always takeoff with full tanks, and the only time you have too much fuel is when you are on fire. Where you fall in between this is going to depend on your personal judgement.
Click to expand...

You forgot the 3rd and forth schools of thought: If fuel is really cheap, fill it to the brim, tip tanks included. If fuel is really expensive, fill it just enough to get to an airport that has really cheap fuel......
 
I always rounded off in the most unfavorable direction: 7.5 gph becomes 8.0 for planning purposes; a forecast tailwind is ignored. I fall back on Murphey's Law when flight planning.

Bob
 
Plan exactly, then add a reserve at the end. There's good reason to do this. You want a reserve, not impecise (even if they are on the conservative side). As you fly this you want to compare your actuals with the planned values. If you expect to fly for 1:10 on a leg and burn 13 gallons, you don't want to just "accept" 1:25 and 15 gallons as "normal" or "expected" and blunder forward without taking the difference into consideration.

Our type society each year runs a proficiency "event" where prior to takeoff you estimate your exact time and fuel burn. You get a point for each second (fast or slow) you come in and for each tenth of a fuel (either way) that you are off. Low score obviously wins. I came in tied for first one year with 30 points. Usually, the winners are down in the single digits.
 
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