OEI Driftdown Computations
How are enroute driftdown critical points computed? Huge question, I know. Let me see if I can narrow this down a bit?
First, we have an obvious driftdown situation. Second, We're not using a terrain analysis method but rather we're using the IFR altitudes MEA, MOCA, MORA. We have a route segment with a minimum IFR altitude (2,000 ft mountainous terrain clearance) that is above the OEI positive net enroute flight path (at the MEA - 1,000 ft) and we want to apply driftdown to find the critical point along this route that will allow us to dispatch over this route segment past the critical point to lower terrain. Such an example might be departing Denver, CO westbound.
As I understand driftdown, we pick a point and altitude on that route that will allow us to drift down over the route segment that is limiting in both directions. If that is not possible, we need a driftdown alternate(s) that allows driftdown between the departure critical point and the destination critical point.
How is the critical point computed and how is driftdown net climb gradient (negative in this case) computed? Would you compute the average OEI net enroute climb gradient between cruise altitude and the route segment altitude and determine the distance required to driftdown to the route limiting altitude and then designate the point past the route segment starting point as the departure critical point?
I am trying to determine a method by which an operator can determine driftdown points without having to perform a terrain route analysis. This would be for US 14 CFR 135 operators who do not have access to the airline-style terrain databases.
Thanks in advance.
Rich Boll
Wichita Kansas