Calculating Drift down after Engine failure
Thread Starter
Join Date: Aug 2005
Location: Not Ardua enough
Posts: 266
Likes: 0
Received 0 Likes
on
0 Posts
Calculating Drift down after Engine failure
As an acedemic exercise I was trying to discover how drift downs are calculated after engine failures, particularly B744, B777-200 and A330, A340-300.
Do the manufacturers produce tables for a given AUW, or in the event of multi engine failure do crews just descend until they can maintain height and speed ?
Any info welcome.
Do the manufacturers produce tables for a given AUW, or in the event of multi engine failure do crews just descend until they can maintain height and speed ?
Any info welcome.
As mentioned, its in the AFM.
Usually (my experience ) in a graph form allowing for differing conditions. I end up memorizing the worst case scenario altitude as my bottom number. Most jet's I'm aware of can maintain suffecient altitude to clear the highest obstacle in the continental US. That said an engine failure at altitude isn't the biggest concern. Its losing one shortly after T/O from a mountainous airport. There companies use a few different methods for driftdown. If you lose an engine from A to B, divurt to ABC, from C to D go to DEF.
Usually (my experience ) in a graph form allowing for differing conditions. I end up memorizing the worst case scenario altitude as my bottom number. Most jet's I'm aware of can maintain suffecient altitude to clear the highest obstacle in the continental US. That said an engine failure at altitude isn't the biggest concern. Its losing one shortly after T/O from a mountainous airport. There companies use a few different methods for driftdown. If you lose an engine from A to B, divurt to ABC, from C to D go to DEF.
ARINC,
Company escape procedures, pre planned course of action that will be in a manual somewhere saying what to do. Escape procedure are based upon AFM specified aircraft driftdown technique (eg MCT M0.82)
Not made up by the crew in the heat of the moment.
Company escape procedures, pre planned course of action that will be in a manual somewhere saying what to do. Escape procedure are based upon AFM specified aircraft driftdown technique (eg MCT M0.82)
Not made up by the crew in the heat of the moment.
Just another number
Join Date: Jun 2001
Location: UK
Age: 76
Posts: 1,077
Likes: 0
Received 0 Likes
on
0 Posts
ARINC
The actual driftdown altitudes can be found in the QRH for given weights and temperatures. However, for the B744, a good 'rule of thumb' is to subtract the aircraft weight from 630,000 (3 engine) or 500,000 (2 engine). Hence at a weight of 300,000kgs the driftdown altitude on 3 is 33,000ft and on 2 is 20,000ft. (knock the 000 off if you work in tonnes and FL's)
Airclues
The actual driftdown altitudes can be found in the QRH for given weights and temperatures. However, for the B744, a good 'rule of thumb' is to subtract the aircraft weight from 630,000 (3 engine) or 500,000 (2 engine). Hence at a weight of 300,000kgs the driftdown altitude on 3 is 33,000ft and on 2 is 20,000ft. (knock the 000 off if you work in tonnes and FL's)
Airclues
Thread Starter
Join Date: Aug 2005
Location: Not Ardua enough
Posts: 266
Likes: 0
Received 0 Likes
on
0 Posts
Thanks Capt Airclues (Read a few of those in my time)
So I assume this is the sort of calculation the BA crew did when they lost an engine on departure from KLAX. 3 engines across the atlantic at FL330 ?
So I assume this is the sort of calculation the BA crew did when they lost an engine on departure from KLAX. 3 engines across the atlantic at FL330 ?
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
I can assure you they would have been far more precise than applying instant 'rule of thumb' for an operation that was planned in good time over the continental US! They would have gone through the graphs for a second failure too, and compared to en route terrain.
Join Date: Jun 2001
Location: UTC +8
Posts: 2,626
Likes: 0
Received 0 Likes
on
0 Posts
This myth about losing an engine over the Himalyas and being in imminent danger is journo stuff. If you're in a 74 over Mt. Everest and an engine goes on vacation, it's not going to result in a crash dive to a lower altitude.... Furthermore, it would take no more than 15 minutes flying time to "clear" the extreme terrain heading south, in a gradual MCT descent, towards northen India.
Besides, by the time that you're cruising over the Himalayas, from wherever you're coming from, you'll most likely be already light enough to maintain FL 300 on three motors.
Besides, by the time that you're cruising over the Himalayas, from wherever you're coming from, you'll most likely be already light enough to maintain FL 300 on three motors.
Join Date: Jul 2004
Location: Sydney NSW
Posts: 513
Likes: 0
Received 0 Likes
on
0 Posts
no crash and burn
No crash and burn because of ample thrust reserves. Captain Airclues provides some good hints and Rainboe dashes in to remind us (if ever we thought so) that rules of thumb are tests of reasonableness, written in putty, that have not hardened into tablets of stone. [Are there tablets of stone?]
Yes it is all in the AFM but drawing on Captain Airclues’ suggestion you can gain an insight into the broad brush picture of how figures get into the AFM in the first place. He has helped shed light into the dark cavities between my ears.
What Airclues did not say is that 300,000kgf is the weight at which the 747-400 can cruise at FL370 ISA conditions M0.85 and a lift coefficient of 0.5 where it is fat dumb happy and economical. But if you could manage to keep trimmed and driftdown with exactly the same L/D ratio it is a relatively simple task to draw up a graph and Airclues’ simple examples work beautifully.
CL is fairly easy to calculate. CD is more difficult but all you need to do in a paper exercise is to take a reasonable stab. You don’t have to be correct to test a theory just to be reasonable.
My value of CD is based on two visits to Seattle, Book No 5 in the Cambridge series on aeronautics and something to do besides drinking in hotel bars when away.
If an aircraft drifts down with constant L/D ratio each of the coefficients CL and CD maintain constant proportionality but density and speed vary. (Actually compressibility drag probably varies with the square of CL and Mach No – see Book No 5).
For constant weight and ISA conditions all the way down @ FL370 M0.85, FL330 M0.77, FL200 M0.58 the maths produces the same answers: CL = 0.5, CD = 0.0286 and L/D = 17.483 using my crib below.
I use http://www.desktopaero.com/stdatm.html as my tame atmosphere computer and take CD = 0.015 + ((CL)^2)/20.73 + CDcc [which latter value I just stuck down 0.0015 for the sake of argument]. Just determine a rational basis for CD and stick to it to obtain a guesstimate of L/D and therefore the thrust the engines are required to supply.
Using a spreadsheet and the graph function you can then pick your own height and speed. Bingo they much resemble the AFM.
Now, bailing out? I don’t think so. Whatever you might think of Mr Boeing’s products I haven’t come remotely close but there was a lot of fuss a while back about driftdown on the B777 with Airbus alleging it would have to use route L888 to avoid the jaggy bits. That was when the GE "85" (bitch, bitch) couldn't pull the skin off a rice pudding and captains steered around suspect patches that might have been discarded bubblegum.
Rgds
E & KO Sally on tour, 346-9 as we speak!
Yes it is all in the AFM but drawing on Captain Airclues’ suggestion you can gain an insight into the broad brush picture of how figures get into the AFM in the first place. He has helped shed light into the dark cavities between my ears.
What Airclues did not say is that 300,000kgf is the weight at which the 747-400 can cruise at FL370 ISA conditions M0.85 and a lift coefficient of 0.5 where it is fat dumb happy and economical. But if you could manage to keep trimmed and driftdown with exactly the same L/D ratio it is a relatively simple task to draw up a graph and Airclues’ simple examples work beautifully.
CL is fairly easy to calculate. CD is more difficult but all you need to do in a paper exercise is to take a reasonable stab. You don’t have to be correct to test a theory just to be reasonable.
My value of CD is based on two visits to Seattle, Book No 5 in the Cambridge series on aeronautics and something to do besides drinking in hotel bars when away.
If an aircraft drifts down with constant L/D ratio each of the coefficients CL and CD maintain constant proportionality but density and speed vary. (Actually compressibility drag probably varies with the square of CL and Mach No – see Book No 5).
For constant weight and ISA conditions all the way down @ FL370 M0.85, FL330 M0.77, FL200 M0.58 the maths produces the same answers: CL = 0.5, CD = 0.0286 and L/D = 17.483 using my crib below.
I use http://www.desktopaero.com/stdatm.html as my tame atmosphere computer and take CD = 0.015 + ((CL)^2)/20.73 + CDcc [which latter value I just stuck down 0.0015 for the sake of argument]. Just determine a rational basis for CD and stick to it to obtain a guesstimate of L/D and therefore the thrust the engines are required to supply.
Using a spreadsheet and the graph function you can then pick your own height and speed. Bingo they much resemble the AFM.
Now, bailing out? I don’t think so. Whatever you might think of Mr Boeing’s products I haven’t come remotely close but there was a lot of fuss a while back about driftdown on the B777 with Airbus alleging it would have to use route L888 to avoid the jaggy bits. That was when the GE "85" (bitch, bitch) couldn't pull the skin off a rice pudding and captains steered around suspect patches that might have been discarded bubblegum.
Rgds
E & KO Sally on tour, 346-9 as we speak!
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
Contained in AFM data?.............Not always, but the information required to make the computation of the Drift-Down profile is contained in the AFM.
ARINC, As you posed that it was an acedemic question in discovering how drift downs are calculated after engine failures, here's the procedure that I use for calculating the Time / Distance / Fuel / Altitude profile for "less civilised" aircraft that do not provide the resultant profile in the AFM -
The AFM should provide -
(1) The One Engine Inoperative (OEI) speed schedule to be flown following engine failure, and
(2) the OEI Gross/Net climb gradients for the complete range of Weights, Pressure Heights, and Temperatures, and
(3) the Referred Fuel Flow for the remaining engine/s at MCT.
For a given Start weight, En-Route Pressure Height, and Temperature deviation from ISA, extract the OEI gradient, TAS, and Fuel Flow.
.1. For a very small increment of fuel (I do it 1 Kg at a time), calculate the time taken to consume the fuel increment at the MCT Fuel Flow,
.2. Apply the gradient to the TAS to compute the horizontal distance flown, and height lost (or gained) for the incremental time to consume the fuel,
.3. For the new height, and new gross weight after consuming the incremental fuel, return to step .1. keeping a running total of Time, Distance, and Fuel, until the gradient has reached zero. Drift-Down has ended when the gradient is zero, but the process may be continued to ascertain continued En-Route Climb as fuel continues to burn off.
Such a procedure as described above will obviously involve literally thousands of 'cycles' between step 1 and step 3, and is only really suitable for a computer programme unless you have a lot of time on your hands. From several computer runs at a series of Starting Weights, Temperatures and Flight Levels, a series of Drift-Down tables may be produced for the AOM.
With respect to the journalistic 'Himalayan death plunge' alluded to in earlier posts, it is quite typical, at normal cruise levels, weights, and temperatures to see a Drift-Down which may take up to one and a half hours to complete. Even, immediately following engine failure at cruise speed and setting MCT, it can take up to 5 or 6 minutes of level flight before the speed has reduced to Drift-Down speed, and Drift-Down commences. Sorry journos, you'd better go looking for a 'death plunge' somewhere else.
PS - The D.D. table is coming Rich, it's coming, work is the curse of the Pruning classes.
Regards,
Old Smokey
ARINC, As you posed that it was an acedemic question in discovering how drift downs are calculated after engine failures, here's the procedure that I use for calculating the Time / Distance / Fuel / Altitude profile for "less civilised" aircraft that do not provide the resultant profile in the AFM -
The AFM should provide -
(1) The One Engine Inoperative (OEI) speed schedule to be flown following engine failure, and
(2) the OEI Gross/Net climb gradients for the complete range of Weights, Pressure Heights, and Temperatures, and
(3) the Referred Fuel Flow for the remaining engine/s at MCT.
For a given Start weight, En-Route Pressure Height, and Temperature deviation from ISA, extract the OEI gradient, TAS, and Fuel Flow.
.1. For a very small increment of fuel (I do it 1 Kg at a time), calculate the time taken to consume the fuel increment at the MCT Fuel Flow,
.2. Apply the gradient to the TAS to compute the horizontal distance flown, and height lost (or gained) for the incremental time to consume the fuel,
.3. For the new height, and new gross weight after consuming the incremental fuel, return to step .1. keeping a running total of Time, Distance, and Fuel, until the gradient has reached zero. Drift-Down has ended when the gradient is zero, but the process may be continued to ascertain continued En-Route Climb as fuel continues to burn off.
Such a procedure as described above will obviously involve literally thousands of 'cycles' between step 1 and step 3, and is only really suitable for a computer programme unless you have a lot of time on your hands. From several computer runs at a series of Starting Weights, Temperatures and Flight Levels, a series of Drift-Down tables may be produced for the AOM.
With respect to the journalistic 'Himalayan death plunge' alluded to in earlier posts, it is quite typical, at normal cruise levels, weights, and temperatures to see a Drift-Down which may take up to one and a half hours to complete. Even, immediately following engine failure at cruise speed and setting MCT, it can take up to 5 or 6 minutes of level flight before the speed has reduced to Drift-Down speed, and Drift-Down commences. Sorry journos, you'd better go looking for a 'death plunge' somewhere else.
PS - The D.D. table is coming Rich, it's coming, work is the curse of the Pruning classes.
Regards,
Old Smokey
Last edited by Old Smokey; 10th Sep 2005 at 16:40.
Join Date: Dec 2001
Location: down-route
Posts: 413
Likes: 0
Received 0 Likes
on
0 Posts
Interesting to see Captain Airclues' rule of thumb for B747-400 drift-down.
The BAe146(RJ100) rule of thumb is to subtract aircraft weight from 63,000 (as opposed to 630,000 for the B747-400). For example a 40 ton aircraft will drift down to 23,000 ft.
The BAe146(RJ100) rule of thumb is to subtract aircraft weight from 63,000 (as opposed to 630,000 for the B747-400). For example a 40 ton aircraft will drift down to 23,000 ft.
