View Full Version : Speed flown after Drifting down?

D driver
12th Sep 2015, 12:17
So,In a turbojet aircraft, after an engine failure in cruise, we hold the altitude till we get to the Vmd and then maintain the Vmd till the thrust from the remaining engine is enough to balance the drag. This gives us the minimum angle of descent. what does it mean by enough to balance the drag? is it that we want to maintain Vmd for cruise at the lower altitude also? or is it descended to a lower altitude where we can maintain LRC? If LRC then, Since after an engine failure, the most important aspect is range(i think), then why LRC? it does give 1 percent reduction in range...what speed is actually to be flown after an engine failure and why?

12th Sep 2015, 17:37
If there is no obstacle problem you descend at .78/300kts.(or whatever for the aircraft type) at fixed thrust at MCT to OEI LRC altitude and then cruise at OEI LRC cruise speed. LRC gives you the range. If you have obstacles below you then reduce speed to best L/D ratio and descend at that speed till obstacles are cleared then descend to OEI LRC cruise altitude and follow the same procedure.

12th Sep 2015, 22:42
Ddriver, good work on asking when you are not sure.
This subject is sometimes poorly understood even by people flying airliners.
A common mistake is to think that the speeds used to determine the 'EDTO area of operations' is the speed that you should fly at. This is not the case at all. In the 737's that I fly the OEI speed that is used to calculate the maximum diversion distance is M0.79/310kts and that gives an area of operations of 825nm for 120min EDTO. Quite a few pilots I fly with think that they should fly these speeds if they lose an engine in the cruise. That is not the case. There is nothing to stop you flying those speeds if they are what you deem appropriate in the circumstances as the EDTO diversion fuel is calculated using those speeds, but it is up to you and your Captain or First Officer ( ultimately the Captain) to decide on what speeds are most appropriate at the time.
Is it most appropriate to descend at Vmd? If you are trying to maintain altitude due terrain or traffic then probably yes.
Is it most appropriate to descend at M0.79/310kts? If you need to get on the ground ASAP due fire or critically injured pax etc probably yes, but take into account the new altitude you will have to be at. Will it put you in icing or a front?
Is it most appropriate to descend at an intermediate speed and transition into LRC? If you want a margin ( to stall and over speed)due to turbulence and to arrive overhead with more fuel in the tanks then probably yes.
You can see from the above scenarios that each situation is different and requires thought. It is a good idea to think prior to the situation developing. For example, if you are making an ocean crossing there is a good chance you can rule out descending at Vmd and just use your current Mach number into the IAS for LRC if no traffic is about.
we hold the altitude till we get to the Vmd
That is nice in theory but I would have a think about what your priorities are before deciding that is the highest priority. For example, there have been incidents ( and I can think of one accident) where trying to achieve this has resulted in loss of control. Keeping your aircraft flying safely is your number one priority and if you are very high, beginning a descent may be your highest priority and you may only have 4 or 5 seconds to initiate it before being 'behind the drag curve' and things starting to unravel.
Know what your actions will be when you see an engine roll back.
Know roughly ( within ten knots) what your OEI LRC speed will be and exactly what your Vmd is.
Be careful to do your own research rather than relying solely on what one Captain or Check Captain tells you as the advice given is not always of the standard it could be. Ask several trusted people.
I think your question is a very good first post.

D driver
17th Sep 2015, 15:19
thanks for the inputs vilas and framer but the question remains why LRC and not Vmd. I understand different scenarios demand different speeds altitudes etc. But my main concern is all the checklists I hv recntly referred to mention selcting LRC for engine out cruise. I wish to know why LRC and not Vmd(MRC)which is the ideal speed for maximum range...does it have something to do with the fact that LRC gives 4 % higher speed over MRC and thereby reducing the time penalty?? which also brings to mind how EDTO states time limits from landing area and not range?? what is the role of time in all this??

19th Sep 2015, 16:31
M LRC is slightly faster than MMR and gives 99% specific range of MMR but 1% loss in range is almost recovered by the higher speed. Non ETOPS aircrafts are restricted to 60Mts. operation with OEI. That is where LRC speed helps.

20th Sep 2015, 06:55
Is the threshold time 30 Mts or 60 Mts?

20th Sep 2015, 08:41
My mistake. I meant 60 mts.

20th Sep 2015, 10:20
At MRC you are speed-unstable, which is why it is preferable to fly LRC. Nice explanation here: MRC And LRC Tech Ops Forum | Airliners.net (http://www.airliners.net/aviation-forums/tech_ops/read.main/318022)

FE Hoppy
20th Sep 2015, 11:45
Some confusion on this thread as to what MRC is on a Jet.

Max range is aprox 1.32 VMD and you certainly are not at the bottom of the drag (power required) curve.

For any drift down the first priority is not hitting the ground. The best way to achieve that is to fly at the speed which gives you the lowest vertical speed.

Once you are sure the ground is not an issue then you can think about changing strategy to arrive at your new destination either as quickly as possible or with as much remaining fuel as possible.

The speed to achieve either will be decided by your current and future altitude and mass.

Piltdown Man
20th Sep 2015, 12:07
...what does it mean by enough to balance the drag?

This refers to your level off altitude, after drift-down. At high altitude, you don't have enough thrust operating at single engine to maintain your level. The thrust generated by the remaining engine operating at maximum continuous thrust will increase as the aircraft descends and eventually it will equal the airframe drag. This process is applicable when you wish to maintain altitude in order to miss high ground.

Once you have levelled off, you have to decide what you want to do next. If terrain clearance allows, you now have to consider where you wish to go. If it you choose somewhere close by, you might even be too high! But if flying a long distance, an appropriate speed has to be chosen and if fuel is a problem, the speed to pick will the one that gives the best ground distance per kilo. Almost certainly that speed will entail flying at an airspeed higher airspeed than Vmd meaning that your aircraft will again descend as you accelerate to this speed. Again, the descent will be arrested as the thrust equals the drag resulting from flying at this new, higher airspeed. Assuming you have compliant ATC, it might be worth trying to "block off" some sky ahead of you, say from 1,000' beneath to 3-4,000' above you level and allow the aircraft to slowly climb as it gets lighter. It will also give a margin should temperature fluctuations and turbulence change your performance.


21st Sep 2015, 01:01
The best way to achieve that is to fly at the speed which gives you the lowest vertical speed.
Personally I think that is too broad a statement. Often times ( in my experience most times) the best way to avoid hitting the ground is to ensure control of the aircraft is maintained. The best way to achieve that is to descend at current speed initially. The purpose of that is to avoid the situation where the aircraft is decelerating towards the back side of the drag curve with bells and whistles blaring during the 'startle factor' phase. I realise there will be many and varied opinions on the above and that it is probably a case of 'each to their own'.
The above assumes no traffic on TCAS.

22nd Sep 2015, 22:43
Max range is aprox 1.32 VMD and you certainly are not at the bottom of the drag (power required) curve.

FE Hoppy, can you give me some clarification around the 1.32 VMD you quoted above? I went flying this morning and noted that our VMD was 236kts. I got this speed from the single engine drift down page in the FMC, (I tried verifying it using AOA from the HGS but after doing some reading I realise that my understanding of the AOA/ VMD relationship is woeful.)
TAS was 440kts Mach was 0.72
Using the 1.32 you gave gives us an approximate MRC of either
311 kts IAS or Mach 0.95 which are obviously incorrect.
I am not particularly knowledgable regarding jet aerodynamics so I would like to know if I have missed something or if your statement is incorrect. Can you help me there?
Thank you.

FE Hoppy
22nd Sep 2015, 23:59
maximum range speed (jet) [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-453278.html)

What was your level off altitude? What was LRC at your level off altitude?

And why do you think drift down speed is Vmd?

At high alt wave drag starts to negate the 1.32 rule in modern jets but not so much at lower alts. You need to look at the single eng holding speed at bottom of descent and MRC at the same alt. Less likely to be above Mcrit and below MDD.

so to add a little flesh.

on a small jet I know well.

Drift down speed at 46 T = 209kt (best L/D)
Level off alt at 209Kt = 21800kg gross Level off weight @ 44T
Max alt single eng LRC 44T = 15000
LRC 44T at FL150 = 254
Holding speed at FL150 44T = 192

1.32* 192 = 253.44

23rd Sep 2015, 03:48
Thanks for the reply FE Hoppy.

What was your level off altitude? What was LRC at your level off altitude?

Level off would have been approximately FL210 and LRC would have been within a few knots of 270 IAS.

And why do you think drift down speed is Vmd

The FMC guide and the FCOM both have statements to the effect " For engine out (ENG OUT) conditions, the FMCS displays the maximum continuous thrust N1, maxi-mum altitude capability and optimum airspeed based on maximum continuous thrust, minimum drag speed, and current gross weight" so I am pretty sure that 236kts was within a knot or two of VMD and applying 1.32 is nowhere near MRC .
To try and tie in some figures with the ones you provided below I'l provide figures from the 737-800 manuals we use.
At FL200 and 65T s/e hold speed is 222kts
At FL200 and 65T the 2 eng hold speed is also 222 kts
So using the graph you provided above, LRC should be 222x1.37= 304kts and MRC should be
222x 1.32 = 293kts.
Both of those figures are a long way off. From the Boeing charts LRC under the same conditions is 267kts at FL200.
So what I am thinking is that the chart you reproduced above can be relied upon for some aircraft types and not others, or for some altitudes and not others.
Either that or I have a fundamental misunderstanding of what the graph is saying.
More of your thoughts would be appreciated ( I just might learn something )
Thanks again, 73qanda

FE Hoppy
23rd Sep 2015, 11:46
Did you also read the thread I linked?

The chart is generic and typical of those used for ATPL theory.

23rd Sep 2015, 13:23

I'm quite intrigued by this thread. Although not current, I am multi piston rated. I think what interests me here is the science behind what's going on. As a physicict, I like it when the real world works the way science class tells you it should - drifting down on one engine until equilibrium is reached, but I never thought about the airplane climbing as it gets lighter (I knew that's what Comet did back in the day of inefficient turbojets). As I read the practical suggestiong of asking for an altitude block, I was perplexed because I was focused on "drift down," and why would you need blocked altitude at equilibrium? A mental speed bumb later I realized it was for the possible climb.

[One of the things I like about PPRUNE is the combination of technical AND practical information.]

Big jets are way outside of my experience. The biggest thing I've flown from start up to shutdown is a Piper Navajo.

I know fuel consumption is dependent on altitude and setting. What happens to total fuel consumption and range when flying on one engine at full power and lower altitude when compared to two engines at cruise power at cruise altitude? I know drag decreases with slower speed, but what is the net result?



24th Sep 2015, 14:38
FE Hoppy, I have just read the thread you linked now and that answered my question. Old Smokey has it sorted.Thus, whilst the 1.32 Vmd 'rule' is a good approximation at lower levels, it becomes redundant at typical cruise levels for jet aircraft.

Thanks for your time.

D driver
25th Sep 2015, 15:06
well thnx @piltdown tht ws helpful...@vilas that is precisely my query why not Vmd which gives the "maximum" range. Why is default speed LRC not MRC? as range is the worst affected flight parameter after an engine failure...any other deviation called for by the peculiarities of specific contingencies could then have been accounted for from thereon( i.e.from MRC)...also why is max distance from adequate aerodrome in terms of time not distance? whts the role of time here?

25th Sep 2015, 15:18
also why is max distance from adequate aerodrome in terms of time not distance? whts the role of time here?
If you were a passenger in a cabin that was filled with smoke, would you want the pilots to go to the closest airport or the airport that they can arrive at earliest?

25th Sep 2015, 15:21
vilas that is precisely my query why not Vmd which gives the "maximum" range. Why is default speed LR
VMD doesn't give max range though.

D driver
26th Sep 2015, 15:12
my bad @73qanda i meant VMRC...and what u said makes sense given it is a "smoke" event...but like i said not withstanding peculiarities of specific contingencies which can be many requiring different courses of action...am only talking about a simple engine failure event with no other problems accompanying.

D driver
12th Oct 2015, 12:23
anybody got anything new??

12th Oct 2015, 23:03
It's pretty difficult for me to understand what you are asking D driver.
Can you simplify your question?

D driver
14th Oct 2015, 11:40
1. Why the speed to be flown after engine failure and subsequent drift down is LRC(several checklists mention that) and not MRC since range is the worst affected flight parameter?
2. Does time has any significance in this(as we know LRC is a faster than MRC and would take a lil less time), since ETOPS also imposes time limits at OEI cruise speed? If so how does it play its part?

15th Oct 2015, 07:34
1/ It is a nice balance between fuel efficiency and time spent aloft single engine.
( ie 1% more fuel but 5% less time)
2/ Time is a factor as per the statement above.
, since ETOPS also imposes time limits at OEI cruise speed? If so how does it play its part?
The role that time plays for OEI cruise is mainly at the planning stage. Your company would have had approval for a maximum time and then used a relatively fast speed in order to get a descent range ring of operations, that doesn't mean you have to fly that speed. The speed you do fly is up to the crew at the time ( Captain ultimately). Some companies place a maximum to aid in decsion making airborne.
For example, if you are unlucky enough to have a failure at the most critical point and then flew the speed that the company based it's EDTO application on you would arrive at your chosen field with much less fuel than if you flew s/e LRC.
Each situation will be different. If you have a lot of fuel you might want to go faster to minimise the time aloft. If fuel is critical you might accept being airborn a bit longer in order to arrive with more fuel.
Does that help?

sleeve of wizard
15th Oct 2015, 07:58
What speed to fly is like asking how long a piece of string is? Assuming you are in a twin engine aircraft. Many factors come into play,
If you have a short distance to go do you want to be flying at LRC then having to hold because you still haven't completed all of your approach preparations.

As for ETOPS, your company through the regulatory authority will have approved you to a certain time limit, using this time limit a maximum diversion distance will be calculated (under EASA, isa, nil wind etc etc). Assuming the worst happens and an engine failure occurs at the ETP (which coincides to be at the maximum diversion distance) you may exceed (legally) your ETOPS time limit due to actual winds, not flying the diversion speed as defined in your operations manual etc.

D driver
16th Oct 2015, 14:58
always helpful @framer.....bt some irritating doubts still linger....is LRC chosen jst b'cuz it feels like a "nice speed"?or does it have some solid reason cuz like i said range wud be worst affected in oei thus somehow making MRC sound a better speed to me...we can thence always adjust our speed according to the requirements of a particular scenerio for eg LRC or even a higher speed etc..