Takeoff and landing comprise about three percent of flight time, but account for roughly half of all accidents — one significant contributor is density altitude. As you probably know, density altitude is pressure altitude (what you see when you set 29.92 in your altimeter’s Kollsman window) corrected for non-standard temperature. For non-turbocharged aircraft, density altitude can have calamitous effects on takeoff, climb performance and landing distance in less time than it takes to explain it. Although flatlanders are particularly susceptible to underestimating the effects of density altitude, this doesn’t only happen in the hills.
Hot summer days near sea level can increase density altitudes by as much as 3000 feet.
Inside Information: The only regulations mentioning the knowledge of density altitude in Title 14 CFR are given in reference to recreational and private pilots (and this includes both Parts 61 and 141). The subject isn’t mentioned in commercial, ATP, or even CFI aeronautical knowledge requirements!
CAUSE…
Getting ready for any anticipated high-density altitude departure actually begins after your previous arrival.
- For each Fahrenheit degree increase, density altitude increases by roughly 60 feet, or about 1000 feet for each 15 degrees more than standard.
- Humidity further decreases performance as water displaces air molecules in the combustion chambers of your engine or over your wings and propeller (mostly the former).
Important: Always check POH figures against current conditions, runway lengths, and runway conditions. Most have performance corrections for certain reference combinations of pressure altitude and temperature. Remember though, performance data in your POH was obtained by a test pilot using a new airplane. Also, because accelerate-stop distances from rotation speed aren’t given for single-engine airplanes, it might make sense to use the ‘50 foot obstruction‘ distance as a minimum runway length for any ‘high altitude‘ takeoff, or take the POH’s minimum runway length and add another 50%. That way, if you need to abort, you get a second chance where before, maybe there was none.
…AND EFFECT
Density altitude works against you in the following ways:
- Lower horsepower (indicated by reduced rpm in fixed-pitch props, and about one inch less of manifold pressure per 1000 feet in constant-speed props) and reduced climb capability.
- True air speed can be significantly greater than indicated air speed for the same required lift. For example, at 5000 feet density altitude, your TAS is 108 knots vs. 100 at sea level; at 10,000 feet, that’s 117 knots! So, on takeoff roll, it takes longer (seemingly forever) to attain the needed TAS for departure.
- For every 1000-foot increase in density altitude, takeoff roll increases by at least 10% — that’s a very rough rule. Landing roll is less affected.
A normally aspirated engine loses about half of its horsepower at 10,000 feet.
WHAT THIS MEANS TO YOU
On The Ground: Pilots tend to react to the lower acceleration, the increased vibration at a higher ground speed, the increased sensation of speed itself (and the oncoming trees) by performing premature rotation, which only adds drag and further extends the takeoff roll. In fact, one might not be able to climb out of ground effect and accelerate further.
Danger: The most common takeoff error is using apparent ground speed to initiate rotation.
Defense: Don’t climb until the ASI says you can! If you must leave the ground (rough surface, angry spouse, etc.) let the airspeed build in ground effect. (And remember to make sure that your tires are properly inflated.)
In The Air: The higher true airspeed combined with a reduced climb capability results in a flatter (less steep) climb. The most critical part of a high density altitude departure is often clearing obstructions after getting off the ground. One must accept the flatter climb, because just as premature rotation will only extend the takeoff roll, climbing too steeply can more easily raise the induced drag to the point where it exceeds the now-reduced thrust. Learn about the surrounding terrain, and know escape routes through the lowest terrain.
Danger: Gradually rising terrain can still appear to be level! Knowing your rate of climb can be critical.
Defense: Dividing your climb rate in feet per minute by your speed in nautical miles per minute (which is just your speed in knots divided by 60) will tell you your climb rate in feet per nautical mile, which makes it easy to determine whether you’d squeak by — or not. Obviously this is the kind of drill you’ll prefer to include in your preflight planning.
At 5000 feet, climb rate is about two-thirds that at sea level (and this doesn’t account for any downdrafts).
THREE DONTS OF HIGH DENSITY ALTITUDE
- Failure to abort: If you haven’t reached three-quarters of rotation speed within the first half of the runway, abort the takeoff! Prompt decisions are even more critical on soft grass strips or when water and snow accumulation further increase needed distance.
- Pressure (from others): Don’t allow your passengers to influence your judgment. You may need to shuttle back and forth with reduced load (including fuel) at a cooler time of the day. Follow your POH if you want to nail Vy for a given weight. (If you have no accurate data, reduce Vy by about one percent for every 1000 feet of density altitude.)
- An over-rich mixture: If the carburetor heat drop during your mag check is even larger than normal, and not followed by a slow increase in rpm, this is an indicator that additional leaning is needed. (The mixture would already be too rich, and adding carb heat would make it more so.) At power settings under 75% or density altitudes over 5000 feet, normally aspirated engines must be leaned for maximum takeoff power in hot temperatures. Lean the mixture either for maximum rpm (fixed pitch) or best power (for constant speed props).
For each percent weight increase, you double the percentage increase of runway you’ll use up.
COMPLICATIONS
Adding to the density altitude problems are other factors such as unfavorable winds and loss of control or over-control during crosswinds, runway slope, improper takeoff configuration, and exceeding appropriate weight limits…
- A headwind of 10% of takeoff speed decreases takeoff/landing distances by 19%. A tailwind of that same 10% increases the distance by 21% (about the same). When headwinds or tailwinds reach 20% of takeoff speed, distances decrease by 36% or increase by 44%. At 30% winds, those percentages become 51% less and 69% more.
- Runway slope in the US is usually not a major consideration; they seldom exceed 2% (a bit over one degree). Tailwinds add a much greater penalty than an up-sloping runway. According to AC 91-3 of the Civil Aviation Authority (of New Zealand, that is) change in takeoff distance for each 0.1% of slope is about 0.5%. (So if your 3000-foot runway has a rise of 30 feet, that’s 5% more runway.) If terrain allows, it is usually better to take off uphill with a headwind than downhill with a tailwind (with exceptions like Aspen, Colorado).
- Humidity isn’t a factor for aerodynamic efficiency, but super-steamy jungle air can reduce reciprocating engine power by 10%.
FAILSAFE
If your airplane doesn’t have a POH, or the one you do have looks like it belongs with a small appliance and you’ve misplaced your E-6B, here’s an interesting visualization aid: the Koch chart. It’s easier than negotiating the diagonals of a density altitude conversion chart, and you can understand in one glance how much density altitude can affect takeoff distance and rate-of-climb. They used to be on the back ‘data section‘ side of sectionals. You might want to print this out and keep a laminated copy in your flight bag. It will give you a quick appreciation for how quickly things can happen when it’s hot and high:
