Cruise Performance Chart/RPM Setting/Press Alt Ft

8 Answers

1. Best Answer
   
   

   
   

   Kent Shook on Oct 30, 2010

   1) Most CFI’s and renters will generally use 75% power – They don’t own the airplanes and don’t have to pay directly for the fuel and maintenance! But 75% is an OK number to use or they wouldn’t put it in the manuals. 
   A couple of reasons to select a lower RPM would be to make the trip more efficient or increase your range. Slowing down by just a few knots can cause a big improvement in fuel consumption – On a long trip that might require three fuel stops at 75% power, you could instead fly at 65% power and that might increase the range sufficiently that you would only need two fuel stops, effectively making the trip faster despite the slightly slower airspeed.
   2) Whether or not you really need to recalculate in flight depends on the conditions. In general, a reduction in power and airspeed will increase your range enough that you’ll still land with sufficient fuel. The exception would be if you have a very strong headwind, as slowing the airplane down will mean you will be exposed to that headwind for a longer time, making the per-mile fuel consumption higher. The important thing here is DON’T RUN OUT OF FUEL. Keep track of your fuel consumption per *hour* and if you think partway through the flight that you may not make your destination without tapping into your reserves, make an extra fuel stop. Pre-flight calculations are just your starting point to give you an idea of how much fuel you should expect when you land. Know how close you are to needing reserves and know how any changes in the plan (winds aloft, power settings/airspeeds, etc) will affect you, and use that information to determine how to adjust your plan – Be it recalculating, or maybe making an unplanned stop for fuel.
   Generally, training flights are well within the range of our airplanes, even to make an out & back trip, but when you get into real cross-country flying later, these are the kinds of decisions you’ll need to make. I have a personal minimum fuel reserve of at least 1 hour – And I might add to that if I’m flying in an area where there is limited fuel availability, few airports, bad terrain, or bad weather. Fuel = Options.
   3) 2250 is a pretty low power setting in some airplanes – For example, an Archer with a max RPM of 2700. 2250 may be a 60% or 55% power setting at 2000 feet, but at 4000 feet it might not be in the performance charts because it’s only 45% or 50% power. It’s OK to fly at that power setting (unless there’s a limitation elsewhere in the POH) but it’s simply such a low power setting that the manufacturer decided people weren’t going to use it in real life so they didn’t need to develop, test and publish that data.
   4) Pretty much. Pressure altitude is what your altimeter reads at 29.92 inches of mercury. If you’re experiencing abnormally high or low pressure, it can be significant, but generally not. Since each .01 inch is equivalent to only about 10 feet of altitude, it generally doesn’t affect things much. If you had an altimeter setting of 29.42 or 30.42, that would make your cruise altitude 500 feet off from pressure altitude (which is uncommon), 28.92 or 30.92 would make it 1000 feet off which is rare. The highest altimeter setting I’ve ever seen was 30.84; the lowest was just last week at 28.82. But in most weather conditions, there won’t be a significant difference in pressure altitude as it relates to power settings.
   5) Yes, do include all of the conversions. The purpose of going through these exercises as a new pilot is to understand how to use the charts properly and to learn what factors affect aircraft performance and how. Simply skipping steps is a bad way to learn.
   That’s actually a good point with all of these things. Yes, we do a lot of things “the hard way” in training. The purpose of that is not that we NEED to do them to fly safely in training (training flights rarely push the envelope), but so that we understand when we need to do them, and how to do them, in real-world flying. It’s been a long time since I’ve opened up the POH of an airplane just to fly to another airport less than an hour away for a $100 hamburger. However, when doing something like flying 3 people out of an 1800 foot obstructed grass strip, it’s important to know that the charts do need to come out and how to use them.

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   Aviatrix on Oct 30, 2010

   Thanks, Kent!  My thoughts exactly on the short field/obstace situation! 
   I appreciate all the time you have devoted to answering my confusing questions.

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   Brian on Nov 02, 2010

   “4.) And concerning the cruise performance chart press alt ft column, my instructor says just select the cruise altitude you intend to fly and use those numbers. Is this correct? ”

   If you wish to be more accurate you can quickly grab a density altitude calculation. Since density altitude is the pressure altitude you can expect your aircraft to perform as if it were at, density altitude will be most accurate. As Kent explained, the differences are often small enough that we can forgo the calculations.

   Telling the examiner “well it usually doesn’t matter” typically doesn’t work to well in my experience however. That said, you should know how to calculate density altitude and understand why you’re using it. If you don’t understand why, say so and I’ll attempt to elaborate.

   “5.) AND FINALLY on the time, fuel and distance to climb chart”

   As this got me in trouble as a private pilot I’d like to pass along some information: The time to get to the altitude will remain constant. The fuel burn, being a function of the time, will also remain constant. The DISTANCE TRAVELED changes based on the winds, so you can ignore this column and instead calculate the distance given the charted time, charted speed, and winds for the day.

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   Aviatrix on Nov 03, 2010

   Brian, this may be wrong, but I usually listen to AWOS and they give me the density altitude.  I know density altitude is “pressure altitude corrected for non-standard temperature.”  I can calculate it by knowing the pressure altitude and the outside air temperature.  But TO calculate it, I’d either have to look at density altitude charts..or (thank goodness!)…just use my electric flight computer.  Luckily, I’m near sea level (elevation 178 ft.)…so it’s never THAT much.  I guess technically before making calculations, I’d have to go out to my plane…set my altimeter to 29.92…and then come back inside and make my “time, fuel and distance to climb” based on the pressure altitude.
   Also, the chart is based on “at 2450 lbs”…which is the max takeoff weight.  I just thought of this.  I wouldn’t know HOW to calculate this if my plane was at 2,000lbs if the chart is based on 2450.
   However, my question regarding the “press alt” column was actually regarding the cruise performance chart.  I KNOW I get my pressure altitude by setting the altimeter to 29.92…but beyond that…I wouldn’t know how exactly how to calculate pressure altitude at the altitude that I intend to fly.
   For instance, if I want to fly at an altitude of 6,000 feet, I would look at the 6,000ft. press alt column on the cruise performance chart and get those numbers (at my desired RPM setting).  I wouldn’t know HOW to calculate my pressure altitude AT 6,000 feet.
   Once I get my pilot’s license (knock on wood!), how “real life” are these charts?  For instance, do VFR pilots actually sit down and get these calculations whenever they intend to fly?  Or, can you just use a TAS estimate of 120 (most of my ktas colums are between 98 – 120)…and a fuel burn at 10 gph for fudge factor (the max of my gph columns is 9.2).
   Also, if I pick my desired RPM setting…I SHOULD technically STAY at the RPM setting during the entire duration of the flight, right?  If not, my calculations would become inaccurate.  THAT WOULD TAKE ME FOR-EVER mid-flight, and I’d have to have my nose buried in the charts and calculator.
    

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   Aviatrix on Nov 03, 2010

   And forgive me, I meant to say fuel RATE…CONSUMPTION RATE…rate (or GPH) is based on RPM and press altitude. 
   So, my previous posting should have read:  “Or, can you just use a TAS estimate of 120 (most of my ktas colums are between 98 – 120)…and a fuel CONSUMPTION RATE at 10 gph for fudge factor (the max of my gph columns is 9.2).”
   Lol, confused now?
    
    

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   Aviatrix on Nov 03, 2010

   Also, if the reported density altitude at my airport is 1,200 ft…My plane behaves as if it was at 1,200 ft. once I takeoff.  This is not an actual altitude, but it merely used for peformance.
   But once I climb TO 1,200 ft…does the density altitude also increase (two-fold) as well?  I.E. – If the density altitude is high, does it increase more and more as you ascend? 
   I’m not sure…b/c the higher you go, the colder it gets and the pressure decreases.  (that is unless you have a temperature inversion, etc)

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   Kent Shook on Nov 03, 2010

   In most training airplanes like the C172, weight has only a negligible effect on cruise performance, and the worst performance is at gross weight so that’s what the performance charts use. On the takeoff and climb performance, though, weight has a much larger effect, so those charts should include some different weights.
   Pressure Altitude can be estimated based on the actual altimeter setting – Every .01 change in barometric pressure changes the pressure altitude by about 10 feet, as I explained in my first answer to 4) above. For example, if it’s a high-pressure day and your altimeter setting is 30.20 (0.28 above standard), your pressure altitude would be about 280 feet lower than indicated (whether at field elevation or cruise altitude), not a whole lot of difference.
   Density altitude, however, can vary significantly, and the plane performs based on the density altitude. If you were to fly from LA to WI and back in January, for example, there would be a large temperature difference, the density altitude would probably vary by about a few thousand feet, and you would see a noticeable difference in takeoff distance and climb performance between the two. As an extreme example, I have taken off in the same airplane from a field with a DA of less than 1000 feet, and another field with a DA over 12,000 feet. The airplane used *three times* the runway at 12,000 feet DA than it did at under 1,000 feet DA.
   I would not say that VFR pilots pull out performance charts for every single flight. They’re a starting point for you to be able to plan flights, but as you continue to fly a particular airplane you’ll learn how it actually performs. Using these charts gives you a baseline to learn from, and a way to estimate performance.
   You do NOT want to “have your nose buried in the charts and calculator.” Always, always, ALWAYS fly the airplane, first and foremost. You should have a general idea what changing RPM will do (higher RPM = higher fuel consumption) and how that affects your particular flight – If you increase RPM, will the higher fuel burn cause you to get low on fuel?
   “Fuel burn” is what most pilots say instead of “fuel consumption rate,” so I don’t think you confused anyone. 🙂
   Finally, as to density altitude: If the density altitude at the surface is 1,200 feet, you should expect it to go up from there as you climb. Here’s a quote from the Pilot’s Handbook of Aeronautical Knowledge:
   “In the atmosphere, both temperature and pressure decrease with altitude, and have conflicting effects upon density. However, the fairly rapid drop in pressure as altitude is increased usually has the dominating effect. Hence, pilots can expect the density to decrease with altitude.”
   For the Density Altitude to not climb at roughly the same rate as the airplane, there would need to be a higher-than-normal temperature lapse rate, and to see the same DA at any altitude as you do at the surface would require an EXTREME lapse rate. So, if the DA on the ground at your airport is 1200 feet, when you’re flying at 3000 AGL you should expect the DA to be roughly 4200 feet.

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   Brian on Nov 06, 2010

   “does the density altitude also increase (two-fold) as well?”

   That depends on the lapse rate. Standard is 1″ mercury per 1,000 feet. One inch of mercury is how we represent pressure change. So, in your example of a 1,200 foot density altitude on the ground let us assumes a standard lapse rate. With this assumption we can also assume that at an actual altitude of 1,000 AGL you’re density altitude will have gone up 1,000 feet as well, to 2,200.

   I personally just assume standard lapse rate for these calculations. Realistically, you could recalculate density altitude for each 3,000 foot increment using the forecasted (which has it’s own inaccuracies being a forecast) winds aloft chart. However, I find this to be an unnecessary step for a basic VFR flight. When I start doing transatlantic flights in my Cessna then I’ll make sure I have this information. 🙂

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