Impostors in the Cockpit

Sunday, June 15, 2014 at 05:51 PM ET

Verified

** Even in the sun-drenched Maldives, low
pressure will produce negative density
altitudes — but not negative densities.**

Key Takeaways:

The article highlights aviation's common use of "proxy scales" (e.g., density altitude, pressure altitude) to represent complex physical properties in more convenient but sometimes confusing ways.
It clarifies that a negative density altitude is a normal condition indicating very dense air, not a "negative" physical property, and its operational implications are minor.
Speed is identified as a critical but potentially misleading proxy for understanding an aircraft's state, especially concerning stalls and safe low-speed flight.
Angle of Attack (AoA) is presented as the single most important, yet often overlooked, parameter for safe flight, with the FAA now facilitating the approval of AoA instruments.

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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 mid-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.