The Operating Enviroment and Aircraft Performance

Do not Be Dense about Density Altitude   

          It is a beautiful summer day in Leadville, Colorado. The National Weather Center is forecasting that the weather will be sunny skies with a high temperature around 85°F and humidity at 30 percent. The winds will be calm and pressures at 28.75 hPa during the day. You decide that today is a perfect time to go flying, so you start gathering the necessary items to start your pre-flight and flight plan. After you checked the Airplane Flight Manual (AFM) and completed a thorough walk around the aircraft, you are now ready to start the engine. You notice during the start that there was an abnormal slow acceleration for the engine core speed (N2) to build up before introducing fuel and an even slower time for the engines to stabilize to idle. This often happens during the summertime since air density is low, so your situational awareness did not trigger any red flags. Once the engine is at idle, you then complete the before takeoff checklist and receive clearance from the airport tower to start taxiing onto the active runway for a rolling takeoff. After the aircraft is aligned with the runway, you advance the throttles smoothly and continuously to takeoff power until the propeller speed (N1) reaches 50 percent. 

          During the takeoff roll you start to realize the engine is not stabilizing above 50 percent N1 while you are advancing the throttle to the required maximum continuous thrust. You look at the airspeed indicator and its reading 60 KIAS while approaching two-thirds of the runway. You start to pull back on the control wheel to initiate rotation for lift off and climb. The aircraft starts to lift off the ground while you rotate the nose to increase the angle-of-attack (AOA) and rate-of-climb. At this time tower authorizes for you to approach 1500 feet for the departure leg; however, the aircraft will not climb any higher than 700 feet even when you advance the throttle at a higher percentage. At this point, you believe there might be a malfunction with the engine or a performance issue, so you turn into a downwind leg and request a final approach with the tower. Once safely landed on the ground you are taxied off the runway. You start to investigate what the probable cause could have been and found that the National Weather Center reported a possible high-density altitude for today. You then realized that you never calculated the density altitude in your performance since the Pilot’s Operating Handbook (POH) are generally based on standard atmospheric conditions at sea level which is 59 °F and 29.92 inches of mercury (FAA Safety, 2008). Luckily for you, it did not cost your life. This is an important lesson on understanding the significant influence density altitude has on aircrafts and engine performances, especially on a hot and humid day.  

          So, what is density altitude and its environmental factor? Well, density altitude is the pressure altitude corrected for nonstandard temperature variations (PHAK, 2021). In other words, it is the altitude the aircraft "thinks" or "feels" it is flying at, which is much higher than what is read from the altimeter. The importance of knowing density altitude is to be able to determine the aircraft performance. The density of air decreases with altitude. The higher the density altitude, the more the aircraft's performance will decrease. Regardless of the actual altitude at which the aircraft is flying, it will perform as though it is operating at the existing density altitude, which can be thousands of feet higher than the altitude indicated on the altimeter. Therefore, if an airport like Colorado’s Leadville-Lake County whose field elevation is 9,934 Mean Sea Level (MSL) and has a reported density altitude of 15,000 feet, an aircraft will operate as if the airport elevation were 15,000 feet. This causes a huge environmental factor to the aircraft’s performances, or worst, a deadly consequence before a pilot has a chance to sense its presence.

          Some effects density altitude has on aircraft performance includes a slower acceleration on takeoff that requires a longer takeoff roll, a reduced engine thrust or propeller efficiency, and a reduced lift during climb since there is a reduction in power (Reynolds, 2012). To minimize the impact, every pilot should consider these three important environmental factors that contribute to high density altitude. They include height, temperature and humidity. The higher the altitude, the less dense the air is. The warmer the air gets, the less dense the water molecules are in the atmosphere, since water vapor is lighter than air. Pilots must be sure to determine the reported density altitude and check the appropriate aircraft performance charts carefully during preflight preparation. A pilot's first reference for aircraft performance information should be the operational data section of the aircraft owner's manual or the POH. (FAA Safety, 2008). Therefore, if density altitude is reported, a pilot can properly calculate the following data by increasing the takeoff distance, reducing the rate-of-climb, and increasing the landing roll distance. If the AFM or POH is not available, use the Koch Chart to calculate the approximate temperature and altitude adjustments for aircraft takeoff distance and rate of climb. Failure to plan for these adjustments can result in a mishap, exceeding a limitation or an accident.

Reference:

Aircraft Performance. (2016). Pilot’s Handbook of Aeronautical Knowledge.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/13_phak_ch11.pdf

Density Altitude. (2008). Federal Aviation Administration Safety. 

https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56396/FAA%20P-8740-02%20DensityAltitude%5Bhi-res%5D%20branded.pdf

Jim Reynolds. (2012). The Front. NOAA’s National Weather Service.

https://www.weather.gov/media/publications/front/12jul-front.pdf