I really liked the pressurization article (Under Pressure, June/July 2020). We use the [supplemental oxygen] masks quite a bit - or used to. On a long-range shuttle we regularly fly, we take three pilots. That means we rotate pilots every 90 min. or so, and when there is only one pilot at the controls, the other must wear a mask anytime we are above FL 250.Then came COVID-19. Decontamination procedures are paramount, and nobody wants to put their face in someone else's mask. Evidently the FAA agreed and moved the altitude for mandatory wearing from FL 250 to FL 410.I have to comment on "Under Pressure" (June/July 2020). After five years as a T-38 instructor, then 30 years as a Delta pilot, I flew Lear 35s for five years in my retirement. During refresher training at FlightSafety International, the instructors beat into us about the green valve for the oxygen bottle located in the Lear's nose on the right side. When the valve is open - which we checked during pre-flight - it could appear to read "Off. It seems that the green-painted valve control, when it was new and in the "Open" position, displayed an arrow on its stem pointing clockwise to the off position. But in every one of the five Lear 35s I flew, the arrows had been worn away from repeated use so that all the viewer saw was "Off".
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