Accident Analysis

Aviation's Seasonal Affective Disorder

Ask any group of new pilots to list the most engaging subjects in private pilot training, and there is an excellent chance that not one of them will mention density altitude. The reverse may be true, and the topic may appear prominently on a list of subjects that made them glaze over. Fortunately, they may tell you, they didn't spend much time on the subject before moving on to something more interesting.

Most pilots agree that it was worth learning that air density decreases as altitude, air temperature, and humidity increase, and that pilots must be aware that this combination can turn a usually friendly flying environment into a dangerous realm. But unless you did your training during the summer in a place where density altitude is a major consideration, chances are that the closest thing you got to an illustration of what density altitude can do to aircraft performance was a look at the takeoff-distance performance chart in the pilot's operating handbook. Maybe you made a mental note to answer the inevitable knowledge-test question by responding that density altitude "reduces an aircraft's climb performance." And then your training may have turned to concerns that seemed more... well...relevant. You did not make a field trip to an accident site. Nor did you load an aircraft to its limits, find a high, short field on a sticky, hot day, and seek out peril where it lives. That would be crazy. You were forewarned and presumably forearmed.

Presumably.

The effects of density altitude - an obtuse term for atmospheric conditions that affect an engine's ability to make power at any altitude - must be experienced to be truly understood. No chalk talk ever demonstrated the contrast between the way a low-powered airplane leaps from a runway on a cold winter day but merely drags itself into the air from the same strip on a sultry afternoon. A pilot who trained at sea level and never ventured high into the hills will give little thought to the technique of best-power leaning, and he will take what he gets with a full-rich mixture on most takeoffs. But to the mountain or high-desert flier, it is a perfectly routine element of preparation for flight to adjust for max power, because every little bit of performance counts.

On August 2, 2000, density altitude became extremely relevant to a pilot in Taberg, New York, and the lesson was presented with none of the banality of ground school. The unlikely chain of events leading to the accident began, as it so often does, with an unlatched door, on a PA28-180 that was heavily loaded. The sequence of events, according to the National Transportation Safety Board's report, followed this course: "During a cross-country flight with three passengers, the airplane's upper door latch opened and the pilot intended to perform a precautionary landing at a nearby airport to secure the door. The pilot landed on a 1,500-foot-long private grass strip, secured the door, and attempted to depart the grass strip to the east. The pilot aborted his first attempt to take off, because the airplane was unable to take off within a 'reasonable' distance of the end of the runway. During the second attempt, the airplane became airborne about 500 feet before the end of the runway, and the pilot lowered the airplane's nose 'to accelerate while remaining in ground effect.' Near the end of the runway, the pilot pitched the airplane up; however, the airplane began to descend toward a cornfield and impacted terrain."

Density altitude was estimated to be 2,188 feet at the time of the accident. An ironic side note is that, unlike so many others, this pilot did not lose control on noticing the open-door problem, but he still was unable to avoid trouble after his precautionary landing placed him in a situation that made takeoff a risky affair. After a sudden change of plans, it is always prudent to study your new circumstances carefully before making the next go/no-go decision.

Operating a low-powered airplane in high density altitude conditions with all seats occupied should be enough to put any pilot on the alert. In Putnam, Illinois, on an August day in 1989 with the temperature at 95 degrees, only one other factor was needed to thwart a Cessna 150 pilot's bid to outclimb the terrain after takeoff. According to the NTSB, "The pilot reported that when he taxied to take off, he used carburetor heat...because of the high humidity. While taking off he noted that the aircraft would not gain airspeed after liftoff or climb sufficiently to clear obstacles. He turned to avoid a hangar, but the aircraft subsequently hit trees and crashed." A post-accident examination of the aircraft revealed that the carburetor heat was in the full On position. Why was this worth special comment by investigators? Recall that the way to verify that carb heat is working during a pretakeoff runup is to place it in the On position for a moment and observe a drop in engine rpm. If the carb heat control is functioning, turning it on will bypass the carb-air filter, sending air warmed by the exhaust manifold into the engine. Warm air is less dense than cooler air - hence the drop in engine rpm and associated reduction in engine power.

It's worth noting that in both of the preceding examples, the high density altitude conditions became an accident cause only after other problems - an open door in one instance and the failure to remove carb heat before takeoff in the other - had compromised the planned flight. In both cases, a pilot attempting to combat one problem fell victim to another.

All pilots are taught the three Hs of density altitude - high, hot, and humid. Most also learn to beware the combination, because each compounds the effects of the other two. Unfortunately, this is a lesson that not every pilot takes to heart. On August 6, 1989, at Tom's Place, California, "a PA28-140 entered a high-elevation basin located above 10,000 feet msl in the mountains," an NTSB accident report noted. "The rim of the basin was determined to be above 12,500 feet. The density altitude at 10,500 feet msl was about 12,500 feet. The aircraft's best rate of climb was less than 400 feet per mile. The floor of the basin rose in elevation about 400 feet per mile in the area of the accident site. The aircraft collided with the terrain." This accident resulted in two fatalities.

It is a different message from the previous reports in that it reminds us to set aside the common notion that once a successful takeoff has been performed, we can forget about density altitude for the rest of the flight. This was a case where the terrain literally climbed at a higher rate than the aircraft. To put it another way, the conditions at 10,500 feet msl that day left the 150-hp airplane capable of climbing at a rate to be expected under standard conditions at 12,500 feet. That was not good enough.

The good news about density altitude is that it rarely sneaks up on a pilot. The atmospheric conditions that rob a normally aspirated airplane of its ability to make power are observable, or at least predictable, before the flight ever leaves the ground. What should raise a warning flag for any pilot is the prospect of taking off under those conditions in a low-powered or non-turbocharged airplane, with a heavy load, from a short runway or over a course traversing sharply rising terrain.

Your airplane's operating handbook will tell you what kind of performance you can expect. Remember that this assumes that the aircraft is in tip-top shape and the pilot's technique is equal to the task. But how to factor the high-hot-humid-heavy variables into handling an emergency in flight is something else to ponder. The standard classroom discussion has little to offer here. As the Piper with the open door and the Cessna 150 with the carb heat on full during takeoff illustrate, the important thing to remember here is to be sure that the actions we take to remedy one problem do not simply create another. Every decision must be made in a broader context that takes into account having to operate in high density altitude conditions that leave us aerodynamically weakened and in need of an extra margin of safety.