A few comments regarding reader feedback (“Unicom”) in your September issue: Jim Pipers explanation of O2 partial pressure changing “only slightly” with density altitude is a little misleading. The partial pressure of oxygen is exactly 21 percent of the density altitude air pressure. For example, the absolute air pressure at 10,000-feet density altitude is 21.145 in. Hg; the partial pressure of oxygen is 4.44 in., or 74 percent of PPO2 at sea level. A 26 percent decrease in available oxygen has my attention. 288 Morris Holmes comments are more troubling. In another life, when I regularly flew in a MOA, we were not looking for LBFs (little bitty, etc.). We were working hard learning new combat maneuvers or practicing old ones, at high Gs and often at high Mach numbers. If you were to fly a PC-12 through a hot MOA like that every day at 12,000 to 18,000 feet, you would be dead within a year. You would have had the right to be there, but you would still be dead. The guys getting their undies in a bunch about the F-16 rendezvous should relax. I would trust that Air Force pilot 10 feet off my wing more than a GA pilot (including me) at a quarter mile. His message was, get out of my airspace so I can do my job. Finally, the horizontal lift component and increased stall speed at a 30-degree angle of bank (p. 24) continues to be a widely-believed myth. Angle of bank has zero to do with stall speed-its only application is steep turns during a BFR. In an example of how the myth works, a writer in another magazine discussed how Cory Lidles Cirrus SR20 might have avoided hitting that apartment building by rolling to 60 degrees, taking care not to stall because of the angle of bank. If I am about to hit a building, I am going to roll to 90 degrees, firewall the throttle, and pull enough Gs to get a light buffet. The nose will fall through enough to provide flying speed, or I will roll to 120 degrees and pull the nose through. According to your chart, I would fall out of the sky at 90 degrees angle of bank. But I dont, because I am using the airplanes energy (including full power) to change its momentum away from the building, not to maintain level flight. Airplanes stall at a given angle of attack, but all else being equal, the G-load is the pilots indicator, not angle of bank. Great magazine-I read it cover to cover each month. Dave Oberholtzer We agree, right up to the point about steep turns. The missing element? The chart you referenced presumes level flight. If one trades altitude for airspeed, the chart doesnt apply. Overgross After reading Octobers Accident Probe, “Overgross,” about operating an aircraft at more than its certified gross weight, I wonder if were being encouraged to regard 288 the gross weight limitation as sometimes being an unnecessary intrusion into a pilots option to choose which limitations to observe, and which to ignore. The fatal aircraft accident cited in the article shows that no performance data exists for overgross operations, and four people died while someone operated the aircraft more than 100 pounds above its gross weight. The fact the aircraft was “only” about four percent overweight seems to be given in a manner intended to somehow mitigate the seriousness of violating the basic weight limitation, but lets take a deeper look. Examining the steep upward slope of the drag curve in the slow speed regime shows an aircraft “only” four percent overweight will experience degraded climb performance much greater than four percent, and probably far more than the drag caused by a “low tire.” How much more drag? We dont know, but we do know that this airplane crashed and people died! Did a mere 100 pounds-only four percent-make the difference? The article also made no mention of the Part 91 regulatory requirement that “no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual….” Each Flight Manual gives a limiting maximum gross weight (and also the necessary balance information). Every pilot is responsible for complying with these limitations. The article headline, “Flying your aircraft above its maximum gross weight can be done, but not in all circumstances,” seems especially casual in light of the serious fatal accident and the lack of performance data which is then discussed in the article. Joe G. Cabuk, Jr. We dont advocate operating above max gross. But, this being the real world, we know it happens. As we noted, our editors airplane is regularly operated 7.5 percent above its original gross weight and without revised performance charts, thanks to an appropriate STC. The issue isnt whether to operate above max gross-its done all the time, whether by mistake or by design. Instead, its more a matter of how to do it, whether deliberately or not, and why this crash happened. Etymology 288 I really enjoyed Phil Blanks “Batteries Not Required” (October 2008) as I am a firm believer that fully understanding navigation basics is a prerequisite for using todays electronic magic. However, as a minor quibble, I am dismayed to see the incorrect spelling and derivation of the “dead reckoning” term once again foisted upon us. I am worried that a new generation of 
San Diego, Calif.
Via e-mail
The term “ded reckoning” is a product of amateur etymologists in the last 50 years who basically made up a story to fit their conjecture. For a useful discussion of real etymological research, please see tinyurl.com/3nzhlj.
John Hunter
Hillsborough, N.C.
We had an internal discussion about this prior to publication. We concluded the “dead” version made no useful sense in explaining how to navigate while “ded” did.
We also note your reference, in addition to an exhaustive discussion, finds, “In an FAA pamphlet titled Dead Reckoning Navigation, aviation writer Barry J. Schiff agrees, saying, According to popular definition, dead reckoning is short for deduced reckoning or, as the old-timers used to say, youre dead if you dont reckon right. In truth, however, the term originated with maritime navigation and refers to reckoning or reasoning (ones position) relative to something stationary or dead in the water.” Who are we to argue with Barry Schiff?
However, that same reference notes, “In practice, the term is usually abbreviated DR, which is probably safest if you dont want to get into an argument.” Which is our new policy.
