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Density Altitude

“Safety: reflect on it or it may reflect on you.”
-Transport Canada Aviation Safety Letter-
Issue 3/2002

Here we are looking squarely at summer: June, July and August. It’s a beautiful day. What say we get a few friends together and go for a burn? An excellent idea as long as we remember that beautiful, warm days can hide some potentially dangerous situations if we are not informed and prepared.

Looking through accident reports from the past several years for a particular time of year can reveal some interesting and instructive information. We hear lots of stories about the dangers of ice and snow and freezing in winter, but what are some of the hazards of this time of year that have led pilots in Canada to grief? One of the items that appears in too many accident reports as a contributing factor is Density Altitude. Let’s take a look.

On June 8, 1997, a Piper J4A in the initial climb after take-off from Ponsonby, Ontario, was observed by witnesses to bank steeply to the right, pitch nose-down, and strike the ground. The pilot was fatally injured; the passenger later died of his injuries. The aircraft was destroyed by fire. Findings: 1) The high gross weight of the aircraft resulted in reduced aircraft climb performance. 2) The aircraft stalled, for undetermined reasons, at too low an altitude to be recovered.

On August 3, 1997, a Cessna 337 on fire patrol out of Nelson, BC, flew low up a valley, commenced a steep, left turn and crashed into the mountainside. The aircraft was destroyed at impact; the two occupants were fatally injured. Findings: 4) The combination of high air temperature and altitude reduced aircraft climb performance. 5) When the pilot entered a steep turn to avoid the rising terrain, the stall speed increased; as a result, the aircraft stalled.

On June 13, 2000, a Cessna 180E, about five miles north-west of Campbell River, BC, over McIvor Lake, entered a left-hand, steeply banked climbing turn. During the turn, the aircraft’s nose dropped abruptly and the aircraft descended to strike the shoreline in a near-vertical, slightly left-wing-low attitude. The three occupants died on impact. The aircraft sustained substantial damage. Findings: 1) The pilot, while manoeuvring the aircraft, induced an aerodynamic stall. 2) The heavy weight of the aircraft increased the risk of a stall. 3) The initiation of a low-speed climbing turn increased the risk of a stall.

On July 1, 2000, an Aeronca 65-CA attempted to avoid trees at the end of the runway at Fort Steele, BC. As it approached the trees, its bank angle appeared steep, and the aircraft pitched nose down, descended rapidly and struck a house. The pilot and passenger were seriously injured. The aircraft was substantially damaged. Findings: 1) The aircraft was close to its maximum gross take-off weight and had degraded performance because of the relatively high density altitude. As a result, the angle of climb was too shallow to clear the trees at the end of the airstrip. 2) The pilot’s attempt to manoeuvre to avoid the trees resulted in a stall at an altitude that was too low for the pilot to recover.

Pulling the threads of these accidents together leads us to some important safety considerations for flying in the warm, summer months. In each of these accidents the pilots were qualified for the intended flight and there appeared to be no mechanical problems with any of the aircraft prior to their contact with the ground. All of the aircraft were at or approaching their gross weight and temperatures were above standard, in the 20 to 30 degree Celsius range. In the case of the 180E, no information is given regarding temperature, but the accident occurred on 13 June and the weather included light rain and mist.

In ground school, we generally discuss, in one form or another, the concept of the “4 H’s”: high, heavy, hot and humid, also known as altitude, aircraft weight, outside air temperature and humidity level. Each of these is a factor affecting aircraft performance. Manoeuvring an aircraft increases G loading, which increases wing loading and stall speed, just to put icing on the cake. Our “ideal” scenario for light aircraft operation would be low altitude, a light load and a cold, dry day. In each of the accidents described above, at least three of the four H’s were on the wrong side of pretty, and the pilots were manoeuvring just prior to the stall and subsequent crash.

One of the key windows we have for predicting reduction in aircraft performance is to understand the concept of density altitude. We all learned how to calculate density altitude back in ground school and probably retained that information right through the written exam. Understanding how the concept relates to aircraft performance and making the needed allowances for the effect it has might just save your life.

In simple terms, density altitude is pressure altitude corrected for temperature. It gives us what I like to think of as the aircraft’s experiential altitude: the altitude at which the aircraft “thinks” it’s flying. Let’s work through an example.

Pressure altitude is easy enough. We just set our altimeter to 29.92, standard pressure, and read pressure altitude off the dial. Or, if we know the altimeter setting from listening to an ATIS broadcast, we can take our station pressure—the altimeter setting we get off the ATIS broadcast—and subtract that from 29.92. We know that 1” Hg is equivalent to 1000’ in altitude; a higher than standard station pressure will indicate a lower pressure altitude and a lower than standard station pressure will indicate a higher pressure altitude. In terms of performance, “High station pressure: happy. Low station pressure: lookout.”

Let’s say the station pressure, or our altimeter reading, is 29.02” Hg. We take standard pressure, 29.92, and subtract our station pressure, 29.02, getting a difference of +0.90 or 900’. Remember that 1” Hg is equivalent to 1000’ of altitude (+0.90 x 1000 = +900). The pressure drops as we increase altitude so we know that our pressure altitude will be 900’ higher than our station altitude. If our aerodrome altitude is, for example, 1800’, our pressure altitude is 2700’ (1800’ + 900’).

With this information and the outside air temperature (OAT), we can go directly to a handy Koch Chart. You can find a copy of the chart in your Canadian Flight Supplement, Section C, Planning. The Koch Chart gives us a quick, generic reference for determining performance degradation in relation to pressure altitude and temperature, which is another way of saying density altitude. It basically does the density altitude calculations for us.

In our example, we determined our station’s pressure altitude is 2700’ and we’ll give ourselves an OTA of 30 degrees C, just to make things interesting. If we draw a line on the Koch Chart connecting those two points—2700’ and 30 degrees—we see we can expect a decrease in rate of climb of approximately 50% and an increase in ground roll of approximately 80%.

If a C-172 at gross weight, for example, will require an 865’ ground roll at standard temperature and pressure at sea level, we can expect, under the conditions we have chosen for our example, a ground roll of 1557’. If our normal SL rate of climb is 680’/min, in our example scenario we can expect a rate of climb of only 340’/min. These are not numbers that would encourage a prudent pilot to launch off a short runway with trees at the end unless he or she did some careful calculations to ensure a safe take-off was certain.

If we want to be a bit more specific, we can determine the density altitude ourselves and work from there. Using our E-6B Flight Computer, we look in the window for True Airspeed & Density Altitude, set the air temperature over the pressure altitude and read the density altitude.

In this example we read a density altitude of approximately 5000’. Getting back to our example, this means that although our aerodrome has an altitude of 1800’ ASL, our aircraft “thinks” and, more importantly, behaves as if it were operating at 5000’ ASL.

If we want to find more specific numbers for our particular aircraft rather than use the generic data from the Koch Chart, we can go to the POH and work out the numbers for rate of climb and take-off roll. Comparing sea level figures to our example scenario, my quick calculations show, for a Cherokee Warrior, PA-28-151, the rate of climb will be reduced by 34% and the take-off roll will be increased by 79%.

Aircraft weight can significantly affect aircraft performance. The basic calculation for determining the loading factor is (present weight/max certified weight)2 (1). If we go back to our example C-172 aircraft with a gross weight 2300 lbs., which had a 865’ ground roll at gross weight at sea level, and reduce its weight by, say, 20% to 1840 lbs., we determine that its take-off roll will be reduced to 553' [(1840/2300)2 x 865 = 553]. Take a look in your own aircraft’s POH, if it has the numbers for various take-off weights, and see what a difference a few pounds can make.

We mentioned humidity earlier and I don’t want to just let it get by without a wee bit more detail. While humidity is not nearly as significant a factor as temperature and pressure or weight, it does have adverse effects on aircraft performance. Given two masses of air with the same temperature, the moist air mass will always be less dense.

Water vapour in the air affects both engine power output, with piston engines, and the amount of lift an airfoil can produce. High levels of humidity, in effect, increase the density altitude by decreasing the ambient air density. In extreme conditions with high humidity levels and high temperatures, a piston engine aircraft may experience as much as a 12% reduction in power output (2). Your 250-hp engine is now putting out 220 hp.

What all these calculations demonstrate for us, without getting lost in the particular numbers, is that heat, altitude, weight and humidity can seriously degrade aircraft performance. In high density altitude conditions, our take-off roll will be extended, our rate of climb will be reduced and our aircraft’s ability to manoeuvre will be impaired. As pilots responsible for ensuring safe flight, what we need to know and understand about these factors is the specific ways they will affect the aircraft we are intending to fly.

A hot day does not cause an accident. Neither does operating an aircraft at gross weight or in a light drizzle or taking-off from an aerodrome with a high density altitude. However, coupling these factors with a serious underestimation of their effects on performance and a pilot who thinks he or she can manoeuvre an aircraft in the same manner and expect the same performance as at sea level on a normal day can set the stage for a tragic outcome.

Just as we know visibility, turbulence and icing are factors that must be considered before commencing flight, so too are heat, altitude, weight and humidity. Knowing what to expect from an aircraft under any particular flight conditions allows us to make the decisions that will result in a safe, enjoyable flight.

1. Kershner, William K., The Advanced Pilot’s Flight Manual, Sixth Edition, Iowa State University Press, 1994, pg. 76
2. ibid., pg. 76