[{"data":1,"prerenderedAt":344},["ShallowReactive",2],{"learn-\u002Flearn\u002Fdensity-altitude-and-aircraft-performance":3,"learn-nav-\u002Flearn\u002Fdensity-altitude-and-aircraft-performance":313},{"id":4,"title":5,"body":6,"date":255,"description":256,"draft":257,"extension":258,"faqs":259,"howTo":269,"keyTakeaways":269,"meta":270,"navigation":271,"path":272,"quiz":273,"seo":299,"series":269,"seriesOrder":269,"sources":300,"stem":310,"topic":311,"__hash__":312},"learn\u002Flearn\u002Fdensity-altitude-and-aircraft-performance.md","Density altitude and aircraft performance",{"type":7,"value":8,"toc":245},"minimark",[9,20,26,31,52,59,63,90,94,111,131,136,145,149,163,191,194,198,224,228],[10,11,12,19],"p",{},[13,14,18],"a",{"href":15,"className":16},"\u002Flearn\u002Fglossary#gt-density-altitude",[17],"glossary-link","Density altitude"," is the single number that explains why an aircraft that leaps off the runway on a cold morning at sea level struggles on a hot afternoon at a mountain strip.",[21,22,23],"blockquote",{},[10,24,25],{},"This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.",[27,28,30],"h2",{"id":29},"what-density-altitude-means","What density altitude means",[10,32,33,34,38,39,44,45,51],{},"An aircraft does not care about its height above the sea; it cares about how dense the air is. Density altitude is the way pilots talk about air density in familiar units. Formally, it is ",[35,36,37],"strong",{},"pressure altitude corrected for non-standard temperature",": the altitude in the International Standard Atmosphere (",[13,40,43],{"href":41,"className":42},"\u002Flearn\u002Fglossary#gt-isa",[17],"ISA",") at which the air would have the density you currently have. The ",[13,46,50],{"href":47,"rel":48},"https:\u002F\u002Fwww.faa.gov\u002Fregulations_policies\u002Fhandbooks_manuals\u002Faviation\u002Fphak",[49],"nofollow","FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25)"," and the FAA pamphlet \"Density Altitude\" (FAA-P-8740-02) both define it this way.",[10,53,54,55,58],{},"Air thins out when it is ",[35,56,57],{},"hot, high, or humid",". On such a day the density altitude is higher than your real altitude, and the aircraft behaves as though it had been lifted to that greater height, even while it sits on the ground.",[27,60,62],{"id":61},"the-standard-atmosphere-it-is-measured-against","The standard atmosphere it is measured against",[10,64,65,66,69,70,73,74,77,78,83,84,89],{},"The reference is the ISA, the agreed model of a standard day. At mean sea level the ISA defines a temperature of ",[35,67,68],{},"15 degrees C"," and a pressure of ",[35,71,72],{},"1013.25 hPa"," (29.92 inHg), with temperature falling at about ",[35,75,76],{},"2 degrees C per 1000 ft"," (more precisely 1.98 degrees C, or 6.5 degrees C per kilometre) through the lower atmosphere. These values come from the ",[13,79,82],{"href":80,"rel":81},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fmanual-of-the-icao-standard-atmosphere-extended-to-80-kilometres-262500-feet-doc-7488",[49],"ICAO Standard Atmosphere (ICAO Doc 7488)"," and are reproduced in the ",[13,85,88],{"href":86,"rel":87},"https:\u002F\u002Fwww1.grc.nasa.gov\u002Fbeginners-guide-to-aeronautics\u002Fearth-atmosphere-equation-metric\u002F",[49],"NASA Glenn Earth atmosphere model",". Because the ISA is an international standard, the same baseline underlies both FAA and EASA performance data, so density altitude itself does not differ by authority. What differs from aircraft to aircraft is the performance penalty, and that lives in the approved flight manual.",[27,91,93],{"id":92},"a-worked-estimate","A worked estimate",[10,95,96,97,100,101,106,107,110],{},"You will not find a density-altitude needle in the cockpit, so pilots estimate it. Start with ",[35,98,99],{},"pressure altitude",": set the standard pressure 1013 hPa (29.92 inHg) and read the altimeter, or take your field elevation when the ",[13,102,105],{"href":103,"className":104},"\u002Flearn\u002Fglossary#gt-qnh",[17],"QNH"," is close to standard. Then correct for temperature. A widely used rule of thumb adds about ",[35,108,109],{},"120 ft to the pressure altitude for every degree C the air is warmer than the ISA temperature"," for that pressure altitude.",[10,112,113,114,118,119,122,123,126,127,130],{},"Take a strip at 5000 ft elevation on a day when the QNH is near standard, so pressure altitude is roughly ",[115,116,117],"code",{},"5000 ft",". The ISA temperature there is ",[115,120,121],{},"15 - (2 x 5) = 5 degrees C",". If the actual outside air temperature is ",[115,124,125],{},"30 degrees C",", the air is ",[115,128,129],{},"25 degrees C"," warmer than standard. Applying the rule of thumb:",[10,132,133],{},[115,134,135],{},"5000 + (120 x 25) = 5000 + 3000 = 8000 ft",[10,137,138,139,144],{},"Your aircraft now performs as if it were at 8000 ft, not 5000 ft, before you even account for humidity. For real planning, use the performance charts in the aircraft flight manual or the FAA's density-altitude chart rather than the rule of thumb; the ",[13,140,143],{"href":141,"rel":142},"https:\u002F\u002Fwww.faa.gov\u002Fsites\u002Ffaa.gov\u002Ffiles\u002F2022-11\u002FDensity_Altitude_lg.pdf",[49],"FAA density-altitude chart"," (the Koch chart) shows the take-off-distance and climb penalties directly.",[27,146,148],{"id":147},"why-performance-falls-away","Why performance falls away",[10,150,151,152,156,157,162],{},"Less dense air hurts an aircraft in three ways at once, as the ",[13,153,155],{"href":47,"rel":154},[49],"FAA"," and the US ",[13,158,161],{"href":159,"rel":160},"https:\u002F\u002Fwww.weather.gov\u002Fzse\u002FHighDensityAltitude",[49],"National Weather Service"," both explain:",[164,165,166,173,179],"ul",{},[167,168,169,172],"li",{},[35,170,171],{},"The engine makes less power."," A normally aspirated piston engine breathes the ambient air, so thinner air means less oxygen and less power. (A turbocharged or jet engine resists this up to its critical altitude.)",[167,174,175,178],{},[35,176,177],{},"The propeller produces less thrust,"," because it too is working in thinner air.",[167,180,181,184,185,190],{},[35,182,183],{},"The wing produces less lift at a given indicated airspeed,"," so the aircraft must reach a higher ",[13,186,189],{"href":187,"className":188},"\u002Flearn\u002Fglossary#gt-true-airspeed",[17],"true airspeed"," to fly, lengthening the take-off run and reducing the climb gradient.",[10,192,193],{},"Humidity makes it worse still. Because water vapour is lighter than the dry air it displaces, humid air is less dense than dry air at the same temperature and pressure, lowering power and lift further. The basic calculation usually ignores it, so a hot and humid day is worse than the number alone implies.",[27,195,197],{"id":196},"common-pitfalls","Common pitfalls",[164,199,200,206,212,218],{},[167,201,202,205],{},[35,203,204],{},"There is no density-altitude gauge."," You have to work it out from pressure altitude and temperature, so do it deliberately before a hot or high departure.",[167,207,208,211],{},[35,209,210],{},"Forgetting humidity."," On a muggy day the real penalty exceeds the dry-air estimate.",[167,213,214,217],{},[35,215,216],{},"High terrain plus a hot day."," Mountain strips on summer afternoons combine high elevation, high temperature, and often rising terrain ahead, a classic and well-documented performance trap.",[167,219,220,223],{},[35,221,222],{},"Trusting sea-level instincts."," Take-off and climb numbers from a cool day at low elevation do not transfer to a hot day at altitude.",[27,225,227],{"id":226},"in-pilot-efb","In Pilot EFB",[10,229,230,231,234,235,240,241,244],{},"Pilot EFB shows you the ",[35,232,233],{},"decoded"," weather, including the temperature and the pressure setting from the ",[13,236,239],{"href":237,"className":238},"\u002Flearn\u002Fglossary#gt-metar",[17],"METAR",", so the raw inputs to a density-altitude estimate are in front of you. It does ",[35,242,243],{},"not"," compute density altitude or aircraft performance for you, and it is not a certified Electronic Flight Bag. It is a study and planning aid: work your take-off and climb numbers from the approved aircraft flight manual and the manufacturer's performance charts, and treat those as the source of truth. Saved weather stays readable offline; pulling a fresh observation needs a connection.",{"title":246,"searchDepth":247,"depth":247,"links":248},"",2,[249,250,251,252,253,254],{"id":29,"depth":247,"text":30},{"id":61,"depth":247,"text":62},{"id":92,"depth":247,"text":93},{"id":147,"depth":247,"text":148},{"id":196,"depth":247,"text":197},{"id":226,"depth":247,"text":227},"2026-06-10","What density altitude is, how to estimate it from pressure altitude and temperature, and why high, hot and humid conditions quietly rob an aircraft of performance.",false,"md",[260,263,266],{"q":261,"a":262},"What is density altitude?","Density altitude is pressure altitude corrected for non-standard temperature: the altitude in the International Standard Atmosphere at which the air density equals the density you actually have. When the air is hotter, higher or more humid than standard it is less dense, so the density altitude is higher than your true altitude and the aircraft performs as if it were higher up.",{"q":264,"a":265},"Why does high density altitude reduce aircraft performance?","Less dense air means the engine draws in less oxygen and makes less power, the propeller has less air to bite, and the wing produces less lift at a given speed. The result is a longer take-off run, a shallower climb and a higher true airspeed for the same indicated airspeed. The FAA Pilot's Handbook of Aeronautical Knowledge sets out these effects.",{"q":267,"a":268},"Does humidity affect density altitude?","Yes. Water vapour is less dense than the dry air it displaces, so humid air is lighter than dry air at the same temperature and pressure, which raises the effective density altitude and reduces power further. Humidity is often left out of the basic calculation, so treat a hot, humid day as worse than the figure alone suggests.",null,{},true,"\u002Flearn\u002Fdensity-altitude-and-aircraft-performance",[274,283,291],{"q":275,"options":276,"answer":281,"explanation":282},"Density altitude is best described as which of the following?",[277,278,279,280],"Your true height above the sea","Pressure altitude corrected for non-standard temperature","The altitude shown directly on a cockpit gauge","The reading on the altimeter set to local QNH",1,"Density altitude is pressure altitude corrected for non-standard temperature: the altitude in the ISA at which the air would have the density you currently have.",{"q":284,"options":285,"answer":281,"explanation":290},"Using the common rule of thumb, how much do you add to pressure altitude for each degree C the air is warmer than the ISA temperature?",[286,287,288,289],"About 60 ft per degree C","About 120 ft per degree C","About 1000 ft per degree C","About 2 ft per degree C","A widely used rule of thumb adds about 120 ft to the pressure altitude for every degree C the air is warmer than the ISA temperature for that pressure altitude.",{"q":292,"options":293,"answer":247,"explanation":298},"Why does humid air make density altitude worse at the same temperature and pressure?",[294,295,296,297],"Water vapour is denser than the dry air it displaces","Humidity has no effect on air density","Water vapour is lighter than the dry air it displaces, so humid air is less dense","Humid air cools the engine and adds power","Water vapour is lighter than the dry air it displaces, so humid air is less dense than dry air at the same temperature and pressure, lowering power and lift further.",{"title":5,"description":256},[301,303,304,306,308],{"label":302,"url":80},"ICAO Doc 7488: Manual of the ICAO Standard Atmosphere",{"label":50,"url":47},{"label":305,"url":141},"FAA density altitude chart (Koch chart)",{"label":307,"url":159},"US National Weather Service: high density altitude",{"label":309,"url":86},"NASA Glenn: Earth atmosphere model (metric)","learn\u002Fdensity-altitude-and-aircraft-performance","Weather","mk-HEo5_8V90nRDkBoG06xGgO77xZlVcdXg7761bNNI",{"related":314,"newer":333,"older":339,"series":269},[315,321,325],{"path":316,"title":317,"description":318,"date":319,"topic":311,"draft":257,"minutes":320,"series":269,"seriesOrder":269},"\u002Flearn\u002Freading-a-surface-analysis-chart","How to read a surface analysis chart","Decode a surface analysis chart: isobars and the pressure gradient, highs and lows, warm, cold and occluded fronts, and what the big picture tells you before you read the METAR.","2026-06-20",3,{"path":322,"title":323,"description":324,"date":319,"topic":311,"draft":257,"minutes":320,"series":269,"seriesOrder":269},"\u002Flearn\u002Fthe-international-standard-atmosphere","The International Standard Atmosphere (ISA)","What the ICAO International Standard Atmosphere is, its sea-level values and lapse rate, and how ISA deviation underpins altimetry, performance and density altitude.",{"path":326,"title":327,"description":328,"date":329,"topic":311,"draft":257,"minutes":330,"series":331,"seriesOrder":332},"\u002Flearn\u002Fhow-to-read-a-sigmet-and-airmet","How to read a SIGMET and an AIRMET","What SIGMETs and AIRMETs warn of, the phenomena codes, how long each stays valid, and a worked SIGMET decoded field by field, with the ICAO baseline and the US differences.","2026-06-18",5,"decode-the-weather",4,{"path":334,"title":335,"description":336,"date":337,"topic":338,"draft":257,"minutes":332,"series":269,"seriesOrder":269},"\u002Flearn\u002Foffline-first-preflight-briefing","The offline-first preflight briefing","What the rules require you to check before flight, how to build a self-brief, and the honest difference between offline-first and working fully offline.","2026-06-11","Briefing",{"path":340,"title":341,"description":342,"date":343,"topic":311,"draft":257,"minutes":330,"series":269,"seriesOrder":269},"\u002Flearn\u002Faircraft-icing-explained","Aircraft icing explained","How airframe ice forms, the difference between rime, clear and mixed ice, what makes supercooled large droplets so dangerous, and where to find the icing forecast.","2026-06-09",1781989191336]