Originally Posted by
yanrair
For start, I hope nobody disagrees that you can fly a perfect circuit using GPS only with ALL pressure instruments including IAS and altimeters not working. ?
I'm not sure how you define "perfect", but for me it would include following the manufacturer's recommendations on airspeed - especially during takeoff and landing. Say you're in a C172 or PA28 with a 20kt headwind straight down the runway. If you just assume that airspeed = ground speed, then you're going to be taking-off 20kts faster (airspeed) than recommended - except that you won't be able to because the plane will insist on flying before that point. Then you turn downwind, and instead of the normal 80kts-ish downwind you're doing more like 60kts - which is not a stall, but is not much fun either. More challenging is landing; if you come in at 20kts above recommended approach speed, it's going to happily fly straight off the other end of the runway. That is, if you haven't already torn the flaps off and crashed by lowering full flap at 20kts above the maximum flap speed...
Of course, if you know your power settings then you can fly a perfectly acceptable circuit without any air or ground speed indication, and in either of those planes you can feel your airspeed just in the control response. Or if you have accurate wind readings and a bit of time to do the maths then you can convert ground speed into airspeed. However, assuming that the pilot knows the power settings, can feel the response, or has accurate wind readings and spare time may be a mistake.