Teacherjh wrote:

It's what I was taught. I think it's also in the PTS for instrument flight
(though I don't have one handy). I haven't found it in a printed reference.

PTS standards for straight and level flight (FAA-S-8081-4C) states
applicant maintains heading within 10 degrees, altitude within 100 feet
and airspeed within 10 knots.

Which really leads me to another question.

If I am assigned an altitude of 5000, does this allow me from 4900 to
5100 or does it mean that I stay within 4950 to 5050? I always took it
as no higher or lower then 50 feet of my assigned altitude.

Allen
(working on my instrument rating)