The scenario exists in many iterations; the outcome boils down to only a few: Mix high ambient temperatures and high humidity and this morning’s more-than-ample- piece-o-cake runway morphs into this afternoon’s high-risk proposition.

Dave Higdon  |  01st August 2010
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Dave Higdon
Dave Higdon

Dave Higdon is a highly respected, NBAA Gold Wing award-winning aviation journalist who has covered all...

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Hot & High
Density Altitude can make even Sea Level too high.

The scenario exists in many iterations; the outcome boils down to only a few: Mix high ambient temperatures and high humidity and this morning’s more-than-ample- piece-o-cake runway morphs into this afternoon’s high-risk proposition.

If you fail to address and adjust to density altitude (DA in aviation speak)- your trip may - best case - only make its way into NASA’s monthly Call Back as a harrowing near-miss tale told by an anonymous reporter feeling relieved to have survived. As a worst case though- failure to deal with high DA will make page-leading headlines as only deadly- unnecessary accidents can.

At its basic most-scientific level- density altitude is atmospheric density expressed in altitude terms corresponding to the corrected altitude of the standard-atmosphere conditions.

More simply put: DA is a number describing how high you would have to be on a standard day to experience the air density adjusted for the temperature and humidity. Pilots start hearing about and learning about DA in ground school- and pretty much the instant they start flight training. They learn to calculate DA and adjust their flying in accordance with DA conditions.

To offer some perspective- consider Augusta Municipal Airport (3AU) which sits at 1-328 feet above mean sea level (msl); a full 4-200 feet long- Augusta’s runway is plenty for most piston singles- twin- propjets and light jets… at least- on a mild day. Standard Day conditions of 29.92- 56 degrees and nominal humidity and the air would feel to the airplane like it is at the posted elevation – 1-328 msl. Should the outside air temperature read 90 degrees Fahrenheit- relative humidity 60 percent and the altimeter be at 29.80- however- and the runway length can become an issue for many an airplane.

Under these conditions the plane is flying in the same air density you’d encounter on a standard day – but at about 4-200 msl. In other words- the engine- prop and airfoils would all perform- on the ground- as if they were already at 4-200 msl. Kick the temperature up to 100 degrees and the DA approaches 5-000 msl; at which point the plane feels like it is on the ground at Centennial Airport outside Denver – while sitting on the ground in southeast Kansas.

DA impacts all aircraft – not only those little propeller aircraft typically associated with these types of discussions. Doubt this fact at your peril. It matters not a jot that the aircraft employs two or three high-thrust turbofans- a turboshaft with a shaft horsepower rating in the low four figures- or a high-performance piston engine.

Beyond the engine-power losses turboprops and piston engines suffer- those sideways airfoils in their propellers also lose efficiency – meaning the prop blades don’t work as effectively. As for airframe airfoils- DA is again an equal-opportunity threat- impacting wings- tail feathers and control surfaces.

High DA robs airfoils of their ability to work at the maximum level of efficiency. That especially goes for turbine engines- which suffer from both the lower ambient atmospheric pressure of a high DA day- as well as from the efficiency losses of all those miniature airfoils used in the fan- compressor core and power turbines. So pretty much every part of the airplane- airframe and powerplant- suffer to some extent in high DA conditions- all losing their ability to work at their maximum capabilities – a fact even when we wisely calculate and adjust to the conditions.

More DA- less performance; high DA- more runway needed; up the DA- lower the climb rate: You can consider these all as givens. High DA robs the wings of their ability to lift- requiring higher speed to make up for the loss.

High DA reduces the stall margin for a wing too- meaning that even at your fastest you may have trouble climbing out of ground effect. Couple the higher speed need with a reduced ability to produce power and you come to a point where you need far more runway to safely negotiate take-off or landing.

Check the performance charts of any jet or propjet of your choosing; the numbers tell the tale. For example- the TBM 850 with its 700shp Pratt & Whitney Canada PT6A-42A needs only 2-840 feet of runway at sea level on a standard day. Put the same airplane at 5-000 msl- kick the temperature to 77°F and it needs 4-282 feet of pavement – flat- level- with clear climb and approach zones at each end.

The difference is 1-442 feet of runway. It might have been more had Daher Socata not opted for a flat-rated engine that guarantees 500shp up to ISA+55 Celsius – or 131 degrees Fahrenheit.

How about a Cessna Aircraft Citation Mustang? On an ISA day the Mustang and its PW615F turbofans need 3-110 feet of runway; yet if you kick DA up to 5-000 msl- the runway number more than doubles- to 6-600 feet.

One more example - this time a Gulfstream G500: At the ISA standard day and elevation- the G500 needs 5-150 feet of runway. Change the conditions to our 5-000 msl/77°F conditions and 7-680 feet of runway becomes the minimum.

As noted- performance suffers from high DA- across the board: take-off and landing distances increase; rotation and approach speeds go up; stall speed increases; climb rate drops. Try to take off from 5-500 feet of runway when the trusty E6B pilot calculator or Electronic Flight Bag program says you really should have 7-500 and what’s likely to happen? You- your aircraft and crew will likely experience an opportunity to examine the overrun area of real estate beyond the runway’s end.

The luckiest never clear the runway and never have to come back down; the unluckiest transition to a struggling- weak climb that doesn’t take the airplane out of ground effect – and then the airplane hits something and the flight ends- usually badly…very badly.

Flip this scenario to an arrival and one of two things are likely: the airplane stalls short of the runway as the crew attempts to slow for a normal-feeling touchdown speed; or the plane- holding extra speed across the threshold- touches down long and runs off the end of the runway – again- ending very badly for all on board and the aircraft they occupy.

As noted earlier- this high DA phenomenon is not limited to high-elevation parts of the world. A temperature of 110°Farenheit on a sea-level runway jacks the DA up above 3-200 msl – more if the humidity is particularly high or the barometric pressure particularly low. This can especially be the case- between March and October- and DA should be a daily checkpoint for planning flights- payloads- airport stops and flight distances.

So what solutions come to mind when the passengers want to “Go- go- go!” but the DA numbers say- “No- no- no!”? Actually a number of adjustments can help bring aircraft performance up to a point where the crew can work within the DA.

The easiest and simplest solution is to wait for temperatures to drop; as temperatures drop- so goes DA- back toward sea level. This solution may involve staying overnight or starting out before temperatures peak. At this time of year- many a flight instructor and air-transport pilot suggests getting in or out – or in and out – before 10am. Depending on the field elevation- DA may not return to useable levels until late at night or the following morning.

Another alternative is to use a different airport- if that’s practical. Picking a field with a runway long enough even for the ISA+25/5-000 msl conditions largely eliminates the take-off and landing concerns – though climb performance remains an issue to consider if flying in mountainous areas.

The crew can fuel for a shorter leg - less fuel means less weight to lift. Consider opting to keep the kerosene load to only what’s needed to get to another airport with lower elevation- lower DA- longer runways or all of the above. Similarly- if the aircraft is carrying materials – or people – who can be safely- conveniently off-loaded- consider lightening the payload that way. Either weight-reduction step reduces runway needs and will help climb rates- whether used collectively or individually.

Pressure to fly when DA conditions clearly work against your chances of success should be off limits. The simple truth here: the laws of physics in play pay no heed to schedule; they care not for position; and they are unimpressed with status or any other man-made concern. DA is what DA is and the experts to heed are the ones the company trusts to manage the airplane and its occupants in a safe- sane manner.

Maybe the solution is an unhappy one: say- shipping a box back to the office rather than hauling it in the company plane. Or maybe the unhappy solution involves arriving home late because the flight had to be broken into two legs to manage DA- available runway and take-off capabilities. At the worst case: you and the crew bed down for the night in a comfy hostelry near the airport – and then get out before sunrise- at the coolest part of the day. But all lead to the happier outcome that the airplane and its occupants have a far greater chance of arriving safely back at the home-base - and viewed with perspective- that’s an outcome worth a few minor inconveniences.

We close out this month’s ‘Safety Matters’ piece with a graphic (see eMag version) offering examples of how DA changes with temperature.


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