Driftdown and Letdown difference???

[SOB737]

Hi all,

What is the letdown? difference with driftdown?
I know that the driftdown is procedure when you loose one engine, you decrease your speed on your live engine to get the Vmin drag, as consequence a decreasing of lift and a drift down in altitude but what is the let down?

Thanks to all for your lights ;-)

[bfisk]

I believe you are incorrect; driftdown is a procedure where max continuous power and best glide speed will result in a negative climb gradient, ie engine failure above single engine service cieling. Making a drift down calculation will in some cases enable you to overfly terrain which is close to or above you s.e.s.c.

A letdown is sometimes used to describe an instrument approach procedure and does not nescessarily refer to an emergency procedure, and does not have anything to do with a driftdown.

[SOB737]

Hi bfisk

For driftdown we are talking about the same thing but with different words. When you loose one engine, you set the max continuous thrust and best glide speed (min drag) on the live engine, as consequence a negatif climb gradien (speed decreasing, lift as well as per formula, so driftdown). Ok we are on same level...

Now I don't have any idea about "letdown".
I have to answer to the question: "what's difference between driftdown and letdown?"

Anybody else for this question?
thank you

[waren9]

Whats wrong with bfisks 2nd paragraph? Its correct.

[wiggy]

OK, I'll be another "Anybody else": as far as I understand your question bfisk has already given you the correct answer .

"Driftdown" is a specific term and is used in connection with an engine failure on a multi engine type, as in "what is the maximum driftdown altititude for a single engine failure at this weight".

"Letdown" is a less specific term, sometimes applied to instrument approaches, as in "given the weather, what's a good letdown to 27 left".

[Old Fella]

The answers given by bfisk and wiggy are correct in relation to aviation operations. As for the personal application, Driftdown is what happens to one's waistline with age and Letdown is when the pick-up line doesn't work anymore. Sorry, thread drift I know.

[SOB737]

Thanks to all for your answers.

What do you mean by "the pick-up line doesn't work anymore"
Can someone explain me the letdown, I don't understand...

many thanks

[Wizofoz]

Let Down is a somewhat antiquated term for :Instrument approach", meaning a procedure to descend when visual reference (due to cloud or fog) is not available.

In earlier times, these consisted of prodedures to ensure the aircraft was over safe terain, then "Letting Down" to a safe altitude, hopefully below the cloud.

Precision or non-precision approach is the more usually used terminology these days.


HOWEVER,

As you are obviously a keen student, I hope you won't mind me pointing out an easily made error in your statement. You do not lose (NOTE not "LOOSE") height because of "Decreased lift due to decreased speed as per lift formula."

You increase AofA and thus Cl so that, in a steady descent, Lift still equals Weight. If this were not so, the aircraft would continuosly ACCELRATE vertically downward, which it does not.

It descends because THRUST is no longer sufficient to balance DRAG in level flight. As the aircraft descends, the available Thrust increases, until it is sufficient for level flight.

Old Fella was trying to be funny. Probably doesn't translate well to either your generation or your native language.

[SOB737]

Thanks Wizofoz, thanks Old Fellam, Just wake up about the joke...sorry but I have the head in my books and try to prepare my next interviews...if I can get one...

I discussed the letdown with a friend also who said that the "let down" was used in the old ATPL, and we don't find it anymore in the JAR one.
He remembered something about the obstacle clearance of 2000ft and 1000ft for one of them...but he didn't remember exactly...

Thanks guys for your help, so I suppose the difference is the term used in the JAR ATPL or old one.

cu :ugh:

[roljoe]

hI,

according to my old memories..atpl theory.long time ago..

There were 2 ways to consider the situation in montaneous area, when experiencing a seious failure..for ex one engine out..

Operationnaly, we have to consider:

1) are we able to maintain , after an initial descent (due to the eng. failure) a minimum of 1000 ft above all obstacles while levelling..
This is the let down rule..

2) No we will not be able to maintain this obstacle margin, but we can select a routing enabling us to avoid all obstacles along the descent wit a min margin of 2000 ft. This is the drift down rule..

The final status is the same..maintain at least 1500 ft agl above selected field..and able to give a positiv gradient..
That's what I remember..

[SOB737]

That's exactly what would like to know, thank you roljoe :D

[FE Hoppy]

AMC OPS 1.500
En-Route – One Engine Inoperative
See JAR-OPS 1.500
1 The high terrain or obstacle analysis required for showing compliance with JAR-OPS 1.500 may be carried out in one of two ways, as explained in the following three paragraphs.
2 A detailed analysis of the route should be made using contour maps of the high terrain and plotting the highest points within the prescribed corridor’s width along the route. The next step is to determine whether it is possible to maintain level flight with one engine inoperative 1000 ft above the highest point of the crossing. If this is not possible, or if the associated weight penalties are unacceptable, a driftdown procedure should be worked out, based on engine failure at the most critical point and clearing critical
obstacles during the driftdown by at least 2000 ft. The minimum cruise altitude is determined by theintersection of the two driftdown paths, taking into account allowances for decision making (see Figure 1).
This method is time consuming and requires the availability of detailed terrain maps.
3 Alternatively, the published minimum flight altitudes (Minimum En route Altitude, MEA, or Minimum Off Route Altitude, MORA) may be used for determining whether one engine inoperative level flight is
feasible at the minimum flight altitude or if it is necessary to use the published minimum flight altitudes as the basis for the driftdown construction (see Figure 1). This procedure avoids a detailed high terrain contour analysis but may be more penalising than taking the actual terrain profile into account as in paragraph 2.
4 In order to comply with JAR-OPS 1.500(c), one means of compliance is the use of MORA and, with JAR-OPS 1.500(d), MEA provided that the aeroplane meets the navigational equipment standard assumed in the definition of MEA.


Driftdown is driftdown following thrust loss. Both the 1000' and 2000' clearances apply.

Letdown is an archaic name for a procedural approach.

In most cases GOOGLE is your friend.

[Conan The Barber]

roljoe is correct in as much as in some parts of the world the term 'letdown', is used to describe a more or less normal descent after an engine failure when no terrain considerations apply.

[FE Hoppy]

is it an official term or just a habbit picked up that has gone into general use?


looks like they still use let down to describe instrument approaches in SA
http://www.caa.co.za/resource%20center/AIP's/2007/S059-07NOV07-Letdown%20Procedure.pdf

and I think QGH was the old q code for a DF "talk down" in about 1066