Richjb,
The information that you’re looking for is quite straight-forward to develop from AFM data. What you’ll need to develop this data is –
(1) The En-Route One Engine Inoperative Climb Gradient Table,
(2) The MCT ‘Referred Fuel Flow’ data (Such that the Fuel Flow may be calculated for any Pressure Height / Temperature combination), and
(3) The OEI Climb / Driftdown speed Schedule.
Before determining the Drift-Down Time / Distance / Fuel profile, it would pay to determine if a Drift-Down was necessary at all in the first place. If the OEI ceiling is above the MEA, then there should be no further requirement to do the considerable extra work required to develop the profile. This can be gained from the En-Route One Engine Inoperative Climb Gradient Table alone (I am familiar with both of the types that you mention, and this data is available from the AFM).
The calculation of OEI Net and Gross Ceiling, and OEI Drift-Down are much better achieved by writing a computer programme for them, due to the very tedious work involved. It only took me about 2 days to write the programmes for both of these scenarios, so not a Herculian task.
OEI Net and Gross Ceiling
The En-Route One Engine Inoperative Climb Gradient Table provides Net climb gradient for a complete range of Weight, Pressure Height, and Static Air Temperature. The annoying part is that as SAT is provided, temperature information is applicable to the corresponding Pressure Height only, therefore, go through the table and convert the temperatures to ISA deviation (using lapse rate of 1.98°C per 1000 feet in the Troposphere, and a Tropopause temperature of -56.5°C at 36089.2 ft is much better than the rounded off figures commonly used). Interpolate between the ‘oddball’ ISA deviation temperatures to create a new table with ISA deviation columns instead of SAT columns. Then, to find the Net ceiling, run your finger up the column for the ISA deviation, and your ceiling is where the Gradient is Zero.
This is the Net ceiling, being Gross Ceiling degraded by 1.1% for 2 engined aircraft, so, to find the Gross Ceiling, run your finger up the column again to find where the Gradient is minus 1.1%, and you’ve found the Gross Ceiling. You can repeat the procedure again for Engine Anti-Ice ON etc. to find ceiling data applicable to other configurations.
Don’t forget that the ‘artificial’ Net ceiling is used to determine the legally required MEA clearance, and, having satisfied this, use Gross Data for Flight Planning (at the much higher level).
OEI Drift-Down
This is definitely computer programme stuff, achievable by hand only if you have a week or 2 to spare. Here’s the routine –
Have at hand the AFM En-Route Gradient Data converted to ISA deviation as earlier mentioned. For a given start weight at maximum all-engines altitude, calculate the Drift-Down in small increments of Gradient, Fuel Flow, and TAS found from the Climb / Drift-Down speed schedule. I do it in increments of 1 Kg of fuel used.
.1. For a given ISA deviation, at the start Pressure Height, for the Instantaneous Gross Weight and temperature, calculate the Gradient, TAS, and Fuel Flow. (In a computer programme you’ll need an aerodynamic module for TAS calculation from the Mach/IAS Climb / Drift-Down speed schedule).
.2. For the weight increment chosen (1 Kg for me), calculate the number of seconds that, at the Fuel Flow, it takes to consume your incremental unit of fuel.
.3. For the Gradient, TAS, and time to consume the incremental unit of fuel, find the Horizontal and Vertical distance covered.
.4. From this, find the new Altitude and Instantaneous Gross Weight, keeping a running tally of Distance covered, Time, and Fuel used from the start of Drift-Down.
.5. ‘Capture’ selected Flight Levels / Altitudes as you pass them, particularly noting the Gross Weight on passing, then you have a Drift-Down profile from that level for that weight.
.6. With the new IGW and altitude, go back to step .1. until you run out of fuel. You will now have a table providing Time, Distance and Fuel between incremental Flight Levels.
You will note that, if you start ABOVE the OEI ceiling, the data will indicate a profile bottoming out at the ceiling, and then commence a cruise-climb. If you start BELOW the ceiling, the data series will show a climb (almost) to the OEI ceiling, but entering a cruise-climb, as the OEI ceiling is never reached because it keeps getting higher as fuel burns off.
That’s the long winded reply, lengthy but necessary. Now, if you want to save doing a lot of that work I may be able to help you Richjb, , but I’ll need to know your e-mail address for the required attachments. PM me if you’re interested.
Regards,
Old Smokey