Things aren’t always what they seem. One of the pleasures of this job is hearing from readers. Some write to correct my errors or to note my omissions. Some are hostile, though the population recommending my immediate consignment to the infernal fires has for some reason dwindled in recent years. Maybe they went first. Some ask questions. Some of the questions are standard ones, but some are ones that would never have occurred to me and get me thinking about stuff in new ways.

LAST WINTER, FOR EXAMPLE, cold spells in the middle Atlantic states produced reports of negative density altitudes in places where they aren’t usually seen. A reader wrote to inquire what a negative density altitude meant and what effect it would have on his flying.

A negative density altitude is no different from a positive one, but the question made me reflect upon confusions that can arise from our use of proxy scales for measuring things – in this case, using an altitude as a way of describing a physical property. A negative density altitude is quite a different thing from a negative density. It’s hard to imagine what a negative density would be like – similar to antigravity, I suppose – but a negative density altitude is an ordinary enough thing. At the Dead Sea in Israel and Jordan, where the water surface is 1,363 feet below sea level, it must take a very hot day just to get the density altitude above zero. At any near-sea-level location, a high barometric pressure or a chilly temperature will bring out the minus signs.

Density altitude is a proxy scale – one kind of measurement used in place of another because it is more convenient or more familiar, or because it can be anchored at some readily comprehensible value like zero or one. A scientist might express the density of air in slugs per cubic foot or kilograms per cubic metre. (In case you’ve never quite gotten slugs, air weighs around .08 pounds per cubic foot at sea level. The air in a beach ball a yard in diameter would weigh, if you could weigh it, about a pound. Water is almost 800 times heavier.)

   ‘negative density altitude is no different from a positive one’

Air becomes less dense when it gets warmer, and is also less dense at higher levels in the atmosphere (even though it’s colder there); so warming and ascending produce similar effects. We employ the proxy scale because it is handier for pilots to know that conditions are roughly – a lot of aviation is about roughly – what they would be on a warmish day in Denver than to base a go/no go decision on the information that the air density at the airport today is .063 pounds per cubic foot.

It is possible to have a negative density altitude.

The operational implications of negative density altitude are minor. A reciprocating engine will deliver a little more than its rated power, because denser air contains more oxygen and so the engine can burn fuel at a greater rate; but maximum power is used only briefly and engines have built-in margins of safety. Indicated airspeed will be greater than true airspeed. But with all that power the airplane will quickly leave those creepy negative density altitudes behind anyway.

Another proxy scale that we don’t think much about is pressure altitude. Below the so-called transition altitude, 18,000 feet in the U.S., pressure altitude doesn’t have much practical importance, but in the flight levels everybody sets the altimeter to 29.92 (1013mb) regardless of the local altimeter setting. When you’re at FL240 the altimeter reads 24,000 feet, but you’re really wherever the pressure is 11.59 in Hg. In other words, a flight level is really not an altitude at all, it’s a pressure.

The most important and potentially the most dangerous proxy we use is speed. Another reader’s question got me thinking about the tangled role of airspeed in flying and how it is related to stall-spin accidents in the traffic pattern.

Here is the question:

A friend of mine lowers his flaps while turning base and again while turning final. He does so to lessen the pitch sensation that accompanies the flap deployment. Instead of the plane changing pitch only in the vertical plane some of the force is directed into the turn. Is this safe? I don’t know aerodynamically what is happening with the wing. Although I don’t think extending the flaps would affect the bank angle (save an asymmetric deployment), would it in some way tighten the turn radius and raise the stall speed, or would the flap extension offset that effect? Could this practice lead to an uncoordinated turn?

Like doctors of different nationalities, pilots sometimes talk about familiar phenomena in terms that other pilots do not understand. It took me several readings to untangle the threads here.

      ‘add flap during wings-level portions of the approach’

First, as a matter of basic flying technique, if I were a flight instructor, I would teach students to add flap during wings-level portions of the approach, not during the turns, for the sake of clarity and simplicity. The pitch change with flap deployment just complicates the management of the turn; why pile one complication on another? A skilled pilot wouldn’t have any trouble with it, but that is not a good reason for making it standard practice. Besides, different aeroplanes pitch differently with flap deployment, some nose-up, some nose-down, and an unfamiliar airplane could surprise you at an awkward time.

The explanation that adding flap during turns “lessens the pitch sensation” may be subjectively true, but I think it is misleading. For the humans inside the airplane, and their glasses of water, the sense of down and up remains the same even in a turn (unless it is an uncoordinated turn); the pitching effect of lowering flap is in the same axis, relative to the aeroplane, as it would be if the wings were level. If the Cirrus pilot perceives the turn as somehow lessening the pitching sensation, I think it is only because the changing G forces in the turn mask those due to the flap.

Adding flap would not affect bank angle, but it would probably reduce the turning radius because the airspeed would drop, and turn radius is a function of bank angle and true airspeed and nothing else. Adding flap never raises the stall speed; it always lowers it. But the stall speed in a turn, while mathematically quite predictable, is always a bit of a mystery to pilots. Fiddling with the flaps during a turn just deepens the mystery.

As to whether this practice could lead to an uncoordinated turn, the answer is no. Flap deflection has nothing to do with turn coordination, which is simply a matter of keeping the fuselage lined up with the direction of flight and is the job of the rudder.

The key to all these puzzles is angle of attack. Angle of attack is arguably the single most important piece of information a pilot can have when handling an airplane at low speed, for instance during the landing approach, and yet it is one that remains hidden behind the proxy of airspeed.

For as long as I’ve been writing for aviation magazines we have been harping on the importance of angle of attack and decrying the lack of emphasis on it in flight training and cockpit instrumentation. The FAA has finally awakened to the voices – not just ours – crying in the wilderness and has produced a rule that will make it easier for AoA instruments to gain approval. It will be interesting to see whether a pilot population conditioned to rely on the airspeed indicator will adopt an unfamiliar, but superior, instrument. Will pilots stick with the surrogate, or go for the real thing?

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