Thermometer at 100 degrees

Density altitude. We cannot see, smell, or taste it. However, it is something that must not be ignored. There was an incident in which four people died because they failed to account for density altitude: three Marine Corps helicopter pilots went up to a high altitude airport to pick up a passenger with their baggage, and, on a hot day, took off and tragically never got out of ground effect.

Density altitude is the measure of air density relative to a standard day. A standard day is barometric pressure of 29.92 inches of mercury (in Hg) and sea level air temperature of 15°C (59°F). Density altitude is pressure altitude corrected for non-standard temperature, so pressure and density altitude are the same in standard day conditions. As the temperature rises above standard and altitude increases, the density of the air decreases, resulting in an increase in density altitude. The important part about pressure altitude is that the aircraft performance data under non‑standard conditions is obtained using pressure altitude.

Density altitude is how the airplane thinks it is performing and is not meaningful when considering height above any given point.

Thermometer at 100 degrees

Your “book numbers” are going to be wrong when it’s this hot.

The standard datum plane, which by international agreement is considered to be representative of the atmosphere for pressure altimeter calibrations and other purposes, is a level where the atmospheric weight is 29.92 in Hg on the barometer. As atmospheric changes occur, the datum plane changes to either above or below sea level. Pressure altitude is obtained by setting the altimeter to 29.92 in Hg. Then the altimeter will indicate altitude in feet above the standard datum plane or the height the aircraft is above the actual pressure of 29.92 in Hg. This, however, would not necessarily be the distance above MSL. It would be the height above the imaginary plane, which would be displayed as either above, at, or below mean sea level. Pressure altitude can also be seen as the height the aircraft would be if the actual atmospheric pressure at sea level were 29.92 in Hg.

Another way of determining pressure altitude is to subtract the altimeter setting from standard barometric pressure, 29.92 in Hg. Take the difference and, referring to Table 1 below, convert altitude to feet. Add this to the field elevation or altitude at which you are flying, and this will result in pressure altitude. As a refresher, barometric scale on the altimeter is calibrated from 28.00″ to 31.00″ and is read in inches Hg. Each increment on the barometric scale represents ten feet.

Along with the different types of densities come different types of altitudes and temperatures. The chief components, in the order that they affect density altitude, are pressure, temperature, and dew point. These variables are directly affected by the weather. When one or more of these variables varies, the density altitude varies. To take this one step further, density altitude can occur at any location regardless of the altitude, time, or weather conditions. If the barometric pressure decreases, and the temperature and dew point increase, so does the density altitude.

In this configuration, the density altitude could possibly be, for example 5000 feet, while the airport field elevation is actually at sea level. Now imagine being at an airport where the field elevation is 5000 feet, then add another 5000 feet for density altitude. The aircraft would be performing at 10,000 feet. I have personally experienced density altitude at sea level which is illustrated by bad aircraft performance. As I broke ground at Catalina Island one day, the stall warning horn went off.

Density altitude directly affects the aircraft’s performance. Using standard barometric altimeter of 29.92 in Hg as a datum, a high barometric setting, and include a high or low air temperature and dew point, then the density altitude is less. On the other side of the datum, with a low barometric pressure, high or low air temperature and dew point, the density altitude is higher. A high barometric setting means low density altitude; a low barometric setting means high density altitude. A combination of increasing the physical altitude and temperatures at the same time dramatically increases the density altitude.

The engine and airfoils of the aircraft rely on air density for performance. Warm air molecules expand, resulting in fewer molecules of air, which makes the air thin.

Flying forces the thin air over the airfoils, resulting in either a minimal to a significant effect in reducing the aircraft performance. There is a reduction in lift and drag when the air is thin with an increase in density altitude. The takeoff, rate of climb, distance to climb, fuel, time, service ceilings, and landing performance are reduced by the effects of density altitude.

The most vulnerable segment of flying affected by density altitude is the takeoff and climb phase. Density altitude should also be taken into consideration in the landing phase. Use the normal indicated airspeed on the approach, because of higher true airspeed. Anticipate a higher ground speed and longer landing roll after touchdown.

As you fly into warmer air, the engine has less air to burn so there is less power. Engines consume air by weight and at lower density, the same volume of air will have less weight. With less mass (weight) of air available due to lower density, the engine produces less power. In ideal conditions, combustion of the ratio of fuel-to-air mixture is constant, so when the mass of air available for burning in the engine drops, so does the fuel flow. Engine horsepower decreases because the engine takes in less air to support combustion. The propeller efficiency (thrust) is reduced because it exerts less force at higher density altitudes versus lower density altitudes. In other words, the air is thin and there is less air for the propeller to grip. Turbochargers only help the engine think it is at sea level, which increases the amount of air entering the engine to generate more horsepower. However, turbochargers do nothing for the propeller or airfoils.

What does all this mean in the real world, and when should density altitude be taken into consideration? First, let’s perform a comparative analysis using the Cessna 172 POH, and different altitudes like sea level and 8000 feet using percentages in performance change. Remember, this is according to the book. By looking at the results, density altitude should always be taken into consideration when flying. Here is a comparative analysis from a 1979 Cessna 172N POH for data on performance:

Performance

Category

Altitude
Sea Level, 15°C 8000 Ft, 15°C
Takeoff distances

(Max Wt Short Field)

No obstacles

56KIAS / 805ft N/C KIAS / 1740ft
Landing distances

(Max Wt Short Field)

No obstacles

60KIAS / 520ft N/C KIAS / 700ft
Max. Rate of Climb 73KIAS / 770 FPM 69KIAS / 349 FPM
Max Rate of Climb for:
            Time (min) 0 15 (-1°C)
            Fuel (gal) 0 2.7 (-1°C)
            Distance (nm) 0 19 (-1°C)
Cruise Performance 116KTAS / 8.4GPH 122KTAS / 8.4GPH
Range (40 Gal Tanks) 472 nm / 114 KTAS 485nm / 122KTAS
Endurance (40 Gal Tanks) 4.1 hrs 4.1 hrs

To summarize the comparative analysis example above: takeoff distance increases by 54% and landing distance increases by 26% when altitude and temperature change. Maximum rate of climb has a decrease in airspeed of 5%, and feet per min climb a decrease of 55%. Naturally there is a decrease in maximum rate of climb for time, fuel, and distance because sea level is sea level and climbing to 8000 feet naturally takes more effort to reach altitude. Cruise performance increases 5% in airspeed at the same fuel consumption. Range performance increases 5% in distance and 7% in airspeed. There is no change in endurance.

So what does this really buy you? One: Be extremely careful on takeoffs and landings, and be exceedingly patient climbing to altitude. Two: There is a gain of six KTAS at the same fuel consumption in cruise performance, or 13nm and seven KTAS in range performance. For example, the cost of a $100 per hour aircraft remains constant but the hours decrease with altitude at the same fuel consumption. In terms of dollars, in order to save $10 in range, $30 will be used in order to reach altitude.

To move to our next phase, let’s solve for density altitude. Our iPads, glass cockpits, and weather reports from ASOS give us density altitude. Nonetheless, if you find yourself one day on some remote landing field without any of this information, don’t panic. There is always the good old fashioned way of solving for density altitude. We do not need a calculator, computer, slide ruler, or an abacus. All we need is today’s modern miracle, a Post-It, and a workable number two pencil. Put them together with some pre-Civil War mathematical knowledge. Here is an example.

Known:

  • Station Pressure = 28.80 in Hg
  • Field elevation = 8000 feet MSL
  • Surface temperature = 30°C (86°F)

Calculation:

  • Pressure altitude at field elevation = ((29.92 – 28.80) x 1000) + 8000 ft = 9120 ft
  • The standard temperature for 9120 feet from Table 2 = -2.831°C
  • Variation of temperature = 30°C – (-2.831°C) = 32.831°C
  • Density altitude = 9120 + (120 x 32.831) = 13059.72 feet or 13,100 feet

Let me add some details. Variation of temperature is the variation of the actual temperature from standard temperature at the pressure altitude. The 120 is the approximate change in density altitude per 1°C, variation from standard temperature. Or, you can solve the whole problem by referring to Table 3 after solving for pressure altitude and variation of temperature, and obtain density altitude.

As friendly reminder during those hot summers days, check the aircraft POH performance charts before venturing out. Performance charts can be in either density altitude or pressure altitude. Also, take into consideration the following: age of the aircraft, condition of the runway (whether it is a grass strip, sod, wet or soft, rough field, dirt, gravel), and distance relative to the mountains.

If you run into an atmospheric condition under which the aircraft cannot fully perform, then instead of taking the risk, do not fly or stay at a lower altitude where the aircraft can reasonably perform. Go flying early or late in the day. There always plan B: go swimming, drink a nice cold Frappuccino from Starbucks, both, or whatever it may be, but it is better to be safe than sorry.

Norm Ellis
Latest posts by Norm Ellis (see all)
27 replies
  1. Tom Slavonk
    Tom Slavonk says:

    Great article Norm; thanks for sharing all that information. With the hot summer months just ahead it’s good to keep DA top of mind, especially in the high country of Colorado. Thanks for the great refresher!

    Reply
  2. Karrpilot
    Karrpilot says:

    I do my annual vacation to see mother in September. She’s in Colorado Springs, Colorado. I am based in Illinois. Density altitude is really not much of an issue for me in Illinois, but it certainly is in Colorado. As i go up in altitude closer to Colorado, i notice both the manifold pressure dropping and the airspeed decreasing on the gages. Not much, but it did concern me. Trying to make any adjustments to correct that condition was in vain, as i was in the hottest part of the day. Luckily i fly a 182 RG lightly loaded. And solo. But i can surely realize what would happen if i was fully loaded with passengers and baggage. Nothing to fool around with or discount.

    Reply
  3. Mac MCCLELLAN
    Mac MCCLELLAN says:

    Density Altitude Number is Useless
    The concept of density altitude is crucial to safety. The actual density altitude calculation is useless to nearly of all of us. When many ASOS announce that the density altitude is xxx, what does the careful pilot do with that information? Nothing I know of, other than to be alerted to check the useful data.
    No standard pilot’s operating handbook (POH) or airplane flight manual (AFM) that I know of has density altitude performance information. Instead there are performance charts where you enter the actual air temperature and the airport elevation and see required runway for takeoff, climb rate and other performance data you need for safe operation.
    The charts don’t show density altitude. You can’t enter density altitude. And you can’t read density altitude. But you can see all the actual performance data you need by knowing the air temp and elevation.
    When you hear the density altitude is high, or know that it probably is, forget that number and check the charts so you have the real and useful data you need for safe operation. Pilots of transport airplanes do it before every takeoff so they know the safe airspeeds for takeoff, the required runway length, and that the airplane meets minimum climb gradient requirements.
    Mac Mc

    Reply
    • Ron Blum
      Ron Blum says:

      Not only are your comments rude and derogatory towards the author, they are also incorrect. All AFM, POH, Owner’s Manuals, etc. use PRESSURE ALTITUDE as one of the lookup parameters on the performance charts. Airport elevations don’t change with the weather.

      Reply
      • Mac MCCLELLAN
        Mac MCCLELLAN says:

        Hi Ron,
        The reason I’m concerned about the misleading and possibly confusing density altitude number is that a very experienced CFI that I know believes that’s the number you use to enter the takeoff performance chart for airport elevation, or pressure altitude, if you prefer. And I doubt he is alone in that incorrect belief.
        The density altitude is a very useful attention grabber, but not at all useful in calculating actual performance for most aircraft.
        Mac Mc

        Reply
        • OngoingFreedom
          OngoingFreedom says:

          Mac, thanks for your OP. You beat me to it: calculating DA is as useful as quantity of tube and fabric repair taught to me during my A&P training back in 1990-91. Both used to be very important but now mostly reserved for a few, very old aircraft.

          We really need to drop the “Check density altitude” from our ATIS’ (and magazine articles) in favor of showing the importance of using the tables your airplane came with. Which use DA in their calculations. Turn to Ch5 in any modern POH/AFM, find pressure altitude, find temp and viola, you just accounted for DA, you just didn’t have to find it for yourself.

          High, hot humid and heavy. When these go up, performance goes down.

          Reply
        • Ron Blum
          Ron Blum says:

          Mac, respectfully, I will again disagree with your words. I would rather fly with your experienced CFI. friend. Elevation is the wrong word and should not be associated with aircraft performance … ever. Although the experienced CFI should go into performance tables with PRESSURE altitude, going there with density altitude will be slightly more conservative (it will account for the non-standard (higher) temperature twice).

          Going into and out of Leadville, CO, the airport elevation is always 9,934 feet. Using the 10,000′ line of the performance tables on a high, hot, humid summer day will often result in the airplane being asked to takeoff and climb where it cannot. PRESSURE altitude is the label on the top of that column.

          PS. I have had a long career and now own a business that makes aircraft performance charts.

          Reply
  4. Ron Horton
    Ron Horton says:

    As a DPE each of my Private and Commercial Pilot Practical Tests include a scenario of a airport with a short(er) airport and a given set of (warm) Wes the conditions. The candidate must tell me if we can land and then takeoff from the airport. Since most candidates now show up in aircraft built in the 60s and 70s, their POH takeoff and landing data is not intuitive with regard to DA. This is an area where a lot of them are unprepared to answer the question. I urge all CFIs to make sure your students are given real world scenarios even if you fly from non-mountainous states. A popular airport near us is BQ1 (BBQ restaurant on the field) and you can’t take an older 172 in and out of there at gross weight on an afternoon in July or August.

    Reply
  5. Clive Wilton
    Clive Wilton says:

    Thanks for an interesting article, however, it would be even more interesting if you included the units which everyone outside the USA uses for pressure. Most of the world doesn’t recognise inches of mercury but prefer to speak in terms of mB of hPa.
    A shame that the article seems to be only for US based pilots….

    Reply
  6. Norm Ellis
    Norm Ellis says:

    Thank you Clive. I wasn’t thinking about this. It would be nice if we all had one system of measurement, it would be less confusing.

    Reply
  7. Bruce Knight
    Bruce Knight says:

    Great article. DA was one of the most difficult aspects to safe flying that I faced when learning- mostly, as the author states at the beginning, because it is abstract. I never had any issue with PA, but DA was another matter. Fast forward many years and problem solved, good thing since I am in CO. Someone above suggest using mB for pressure. It would be useful to also know how the factor “120” is expressed in Fahrenheit, since ATIS usually uses that unit (if it’s spoken it’s F, if it’s written it’s C). Regarding the comment on perf tables- in single pilot flight, if you have your head down using tables, other than possibly a check ride, your head is in the wrong place- if DA is above 10,000 in a C172, you’d better be ready. Thanks for the well written and organized information.

    Reply
  8. Dale Hill
    Dale Hill says:

    Norm, Great article! I had a run-in with DA when flying the T-38. Sitting in the squadron one day, I got a call from the front desk. They needed a couple of hours put on an airplane so it could go into phase. I was asked if I would join another IP to take a quick Out & Back sortie to get some instrument time. As an IP, we had to let students do the flying, only grabbing ‘stick time’ when we demoed something. Flying with another IP, we could each get a leg of flight time with some instrument approaches for currency. We both jumped at the opportunity and started flight planning. We were stationed at Vance AFB in NW Oklahoma, it was a summer day, but we had 8,000 feet of runway and a twin engine, afterburner equipped ‘Hot Rod’ to fly! We decided to make a single hop to Colorado Springs and make our turn at Peterson AFB. I was going to ride in the front seat on the way up, acting as the IP and safety observer; I would get my instrument time in the back seat on the way home. We took off and everything was going smoothly until my fellow IP made his first approach at ‘Pete Field’. He flew a nice ILS, transitioned to a back seat touch and go and, as he pushed the throttles up, he said over the intercom, “Something’s wrong!” I asked him what was going on and he said, “The airplane is sluggish!” As the IP onboard, I took control and immediately felt the sluggishness. By this time, we were well down the 13,000 foot runway and I did not want to take the barrier. So, I shoved both engines into afterburner and felt them light off, which quickly got us to flying speed. As soon as I was safely airborne, I raised the gear and then the flaps. I told tower we were going to proceed to downwind and enter on initial for an overhead full stop. In the T-38, a normal overhead pattern is flown at 1500 feet. On an overhead pattern in the T-38, the final is flown at 175 KIAS, final approach at 155 KIAS and touch down is at 130 KIAS. However, you add one knot per every 100 pounds of JP-4 remaining above 1000 (we had about 1500 pounds on board, so my final turn was flown at 180 KIAS, final approach at 160 KIAS and touchdown was 135 KIAS). I touched down in the first 200 feet, aerobraked and then applied the wheel brakes. We slowed in plenty of time to pull off before the departure end. As we switched to ground and got clearance to taxi to the ramp, the other IP told ground control he was switching to the ‘metro’ frequency. I listened as he got the temperature and pressure altitude. He then got real quiet as he looked at the charts in our checklist where we computed our takeoff data. He then told me, “We were in a ‘Category 3’ situation.!” That meant that, on initial takeoff, the distance to accelerate to Single-Engine Takeoff Speed plus 10 knots required by regulation (SETO +10 as it was termed) and then abort the takeoff was longer than the runway available. Thus we were not allowed to make a takeoff. After parking, we called back to the Supervisor of Flying (SOF) and were promptly read the proverbial riot act. After he calmed down, he told us, “Be there tomorrow morning at sunrise and get that airplane off the ground while it is still Cat 1!” We did exactly that and I have been VERY conscious of DA ever since!

    Reply
    • Norm Ellis
      Norm Ellis says:

      Dale I cannot know how to say thank you so much for sharing this. OMG! This is for real stuff not to mess with. I know exactly what you are talking about. I flew out of El Toro Marine Air Station for 10 years with club aircrafts. Our company was anything from C5’s to T-34C’s. Wake turbulence was more of issue. However, going into Prescott one time in a M20J we were indicating 80kias with a groundspeed of well over 100kias on landing. The controller said nice good on landing….

      Reply
  9. Tom Lisec
    Tom Lisec says:

    The Jeppesen textbook states that “At standard temperature, pressure and density altitude are the same” . And their table shows that relationship. (Guided Flight Discovery, Private Pilot, page 8-7.) And logic would seem to support that. Your table #3 does not show that. Why does table #3 show a difference?

    Reply
  10. Jack Morris
    Jack Morris says:

    The POH for my airplane has eight performance charts (graphs), to calculate important things such as takeoff and landing distances, rate of climb, true airspeed, engine power etc. Each one of these charts, except for the last one, is expressed in terms (as a function of) Density Altitude and requires that value in conjunction with other parameters such as RPM, Indicated Altitude and Temperature to arrive at the correct result (e.g. Takeoff distance). The last chart is the exception and is used to compute Density Altitude as a function of Pressure Altitude and Temperature. So we can see from this that DA varies not only with altitude but also with temperature. We can also see that the aircraft and engine designers base their performance calculations for our aircraft on Density Altitude, not Pressure Altitude. Otherwise, they would just produce charts as a function of pressure altitude.

    The significance of Density Altitude is that it incorporates both pressure and real temperature. Pressure Altitude is based on pressure and an idealized temperature based the standard lapse rate with can vary significant from the real value. Thus Pressure Altitude will be less accurate (except on a standard day) than Density Altitude and consequently any results derived purely from Pressure Altitude will be less accurate. This difference can be significant when you consider how the sun can produce large temperature gradients near the earth’s surface on a summer day. Temperature has a great effect on airplane performance.

    Reply

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