Originally Posted by
TonyWilliams
I think Alaska did that in the 1990's with their MadDog fleet. They let it wander plus or minus 200 feet in cruise, knowing that ATC computers won't detect the altitude change until it hits over 250 feet.
Not sure how smart that is in RVSM.
You are talking about the MD80 PMS, "Performance Management System", a.k.a. "Perf." I believe we still have the option of using it today at AA. It is kind of like a poor man's VNAV. Perf actually works quite well but you have to input a lot of data. If you take the time to put everything in, it will climb, cruise and descend quite smoothly, bringing throttles back to flight idle in descent instead of "low lim."
I never used it - didn't like the +/- 175 ft it used to maintain speed in cruise. However, I believe it is still approved for use in RVSM (haven't flown it in 3 years, so not 100% sure.)