TW,

Not so. FL500 is the generally accepted altitude where pressurized suits are required and by FL620 your blood at body temperature boils.

I think you are mixing up pressurized breathing with pressurized suits which is needed above FL400. I've flown to FL280 with an Alps mask on a MH system and had no difficulties. I know pilots who pushed this limit to FL320 with a MH system and an unpressurized system but this is certainly not recommended. The A14 will allow you to pressure breath at FL400 and above and apparently even a short duration trip to FL500 is possible. I personally would not go above FL450.

If you recall the recent passing of Bob Harris you will note that he flew to FL490 with a redundant A14 system. Probably the best lesson learned from his flight was the importance of having a well secured mask. Apparently he had to limit his climb because he couldn't keep his mask on as the pressure increased! He did not have a pressure suit.

Jim
VVII