drift down speed?

[number]

Can anyone give me the definition of Drift Down Speed?
What is it for? Any practical example of its meaning?
thank you

[Mukka]

In the unlikely event that an aircraft has engine failure, the aircraft will slow down to min drag speed for the remaining engines.

Consequently, the lift generated will decrease, and the aircraft will "drift down" to a lower altitude where the increase in air density gives sufficient lift.
(Remember lift is proportional to air density and speed^2.)

It is useful to calculate the drift down altitude for an aircraft weight so you know in the event of engine failure, if terrain clearance will be a problem.

Hope this helps.

[411A]

Drift down speed is used when, for a variety of reasons, either a powerplant is shut down for precautionary reasons or fails enroute, necessitating continued flight at a lower altitude.
The speed selected is usually that to obtain the most nautical air miles, depending on terrain enroute.
Normally engine thrust is increased on the remaining engine(s) to compensate for the failure. In addition, a lower cruising speed is required.
But not always....for example, Howard Hughes operated a Lockheed Constellation in cruise flight (at a lower altitude than normal) using only three engines at cruise power, and obtained a slightly faster cruise speed than with four operating engines.
Oddly enough, this was not possible with Douglas airliners of the time (DC6 for example).
The Constellation was a very clean design, especially the 1649A.

[quid]

In practical application, the driftdown speed will give you the slowest rate of descent if you lose an engine at altitude. In swept wing jets, it's min drag speed. (...and of course this varies with weight.) It will keep you higher longer while you formulate a plan.

I disagree that it will give you the most air miles. Min drag will give you the most "time" before flameout, but best optimum range (specific range) speed will yield the most NAM per unit of fuel. That speed is significantly faster than min drag.

[411A]

The Lockheed TriStar with positively the most advanced flight management system (for its time) offered two distinct drift-down speeds in the event of an engine shutdown.
A slower speed with high thrust set on the remaning engines to minimise height loss where terrain was important, and a faster speed with two-engine cruise thrust to obtain more favorable nautical air miles per unit of fuel used.

Either selected at the touch of three buttons.

Oddly enough, both the TriStar and the B707 offered very similar engine out speeds/increased fuel consumption...approximately 430 KTAS and an increase of 500kg/hr.

Not too shaby for old iron.;)

[m&v]

drift down very useful in the engine out over terrain scenario.
On the a320,IF one loses an engine over the mtns ,select Max Continuous on the good engine (disconnect the Auto Thrust to retain MCT).Slow to drifdown speed,and select the alt (about FL230).It will take you about an hour to reach that level,at a Driftdown rate of about 300'per min-at a TAS of about 420.
This gives one lots o'time to consider the options(Diversion)
Cheers;)

[ITCZ]

So what are we aiming for?

(a) Minimum/flattest descent angle/best glide range? Sounds like an AoA or IAS at given weight for best Lift/Drag ratio.

Or

(b) Minimum descent RATE? at a slower IAS and higher AoA giving a steeper descent angle and reduced glide range, but perhaps more time at a higher altitude that may give higher TAS and lower specific fuel consumption.

????

[Longhauler]

The FMC on the 744 provides two sets of driftdown speeds and altitudes - one based on min drag and the other based on long range cruise (all based on 2 or 3 engines operating as the case may be). I always note the E/O Max Alt as it will not only tell me how far we need to drift down but also what the rate of deceleration is likely to be at the current weight and altitude.

LH

[quid]

ITCZ-

I guess it would depend on the situation. If you're over the ocean and a long way from a suitable airport, miles would be what you're shooting for. If on the other hand, you're over the Alps or crossing Tibet, you'd want to minimize your rate of descent until you've got a plan.

[Alex Whittingham]

Generally the target speed in the driftdown is VMD which gives the minimum descent angle, important for terrain clearance. If you need to minimise your rate of descent to get the checklists complete before you hit the sea use VMP, a lower speed.

[ITCZ]

Hmm, not sure that everyone here is talking the same talk. I have had a bit of a look at the Aussie regs and situation and think I have an answer....

Drift downs are not an all engines inop situation. They are a terrain clearance procedure in the event of one inop in the enroute section of a flight

Drift downs are called for by the Australian regs in CAO 20.7.1b, which is the performance criteria for heavy metal -- takeoff, enroute, missed approach and go around performance in the all engines and one or two inop situations.

Basically, if portion of a flight will be conducted in an area where the LSALT/MORA/MSA is HIGHER than the one inoperative ceiling of your aircraft, you can still fly that route segment provided that should you lose a donk at the most inconvenient point, your DESCENT ANGLE will be such that you will be able to continue/double back/fly an escape track to an area where the LSALT/MORA/MSA is AT or BELOW the one engine inoperative ceiling.

This suggests to me that we fly a higher speed than a minimum descent RATE speed - how about a minimum descent ANGLE speed, like a best L/D speed?

A minimum RATE would perhaps be applicable if (1) we had stuffed it up and recklessly flown over an area where the LSALT was above one inop ceiling (2) we were not concerned about terrain clearance and wanted to maximise time before impact, such as preparing a cabin for emergency landing or ditching. That would be an emergency rather than an abnormal situation, perhaps an all engines inop situation (fuel starvation/exhaustion) instead of just one or more inop

Agree? Disagree? What did he say;)

[GlueBall]

Had a few jet engine shutdowns in my lifetime. Even during cruise at high gross weights, the airplane had enough MCT power on the remaining engines to maintain at least FL230. In my case, unless I'm in the immediate vicinity of Mt. Everest, there is no worry about "terrain clearance."

[quid]

GlueBall-

I guess you've been fortunate to have flown a/c with that kind of performance. We have several models in our fleet that any warmer than ISA +10, our altitudes would be around 14-15,000. Here in the US, that's a problem west of Denver.

ITCZ-

We plan both Terrain Avoidance (on filed route) and Driftdown (with escape routes). Many times we'll be too heavy to dispatch TA, so we have to dispatch DD. The US regs say no person may "take off" an airplane at such a weight, etc. Now, if you're in flight, that FAR was satisfied at takeoff. The PIC is now free to fly any route he chooses. He may choose to avoid a line of thunderstorms, or pick one with smoother air, etc. That wouldn't be reckless. Also, you may find that flight conditions (ISA, fuel burn, winds, etc.) may be different than planned. In those cases, minimum rate may be what you're looking for.

[4dogs]

Folks,

Driftdown can be a couple of things.

411A is correct in regard to maximising range for two cases: first, for calculating ETOPS threshold distance (always much further than steady state OEI CRZ suggests) and, second, for fuel efficiency.

The others are correct for the third case of minimum descent angle for terrain clearance.

ITCZ, Oz requires driftdown planning for 3/4 engine aircraft where 2 engines may be inoperative - often more critical than the OEI case for a twin.

Stay Alive