Net performance
 

I was looking at the performance tables the other day and it gives a gross level off altitude and a net level off altitude in the event of losing one engine at altitude and drifting down. I understand that gross performance is reduced to net, but the differences are huge - in some cases gross and net altitudes differ by 9,000ft!

Could anybody help me understand why this large difference exists? Is is simply just that the numbers are factored to reduce gross performance to net or is there another additional factor at work here that I don't understand??
:confused:

Hufty.

9,000 ft sound excessive. The standard factor applied is 1.1% for twins and 1,6% for quads. There are then additional corrections to be made for anti-ice and ISA deviations.

A 9000 foot difference between Gross and Net level-off altitudes following OEI Drift-Down is not at all out of the ordinary.

The 1.1% Gradient ARBITRARY penalty is a quite large performance degradation factor, it's only 0.8% for a 2 engined aircraft in what I consider the most critical phase of OEI flight, the Second Segment Climb.

Viewed another way, an aircraft at 250 KIAS at 25,000 feet Net level-off altitude at ISA SHOULD, all things being equal, be capable of a 1.1% Climb Gradient, which translates to a Rate of Climb of 405 Feet per Minute. That's a respectable reserve of performance at the Net Ceiling.:ok:

Use Net ceilings to ensure En-Route terrain clearance, and, having satisfied that requirement, use Gross data for onwards Flight Planning in the OEI condition at the Gross Ceiling:ok:

(Of course, you may operate below the Gross Ceiling to achieve LRC etc., but the intent of Net performance, as in all other phases of flight, is to ensure terrain clearance).

Regards,

Old Smokey