Descending Deceleration 737

[Olendirk]

Hey Guys

we´re talking about a classic 737. For example: 40Nm out,20 knots tailwind,300kts on the tachometer. I would say I should be at around 8000 ft to make it smooth. When I want to do a descending deceleration, do I put the power out at 20NM from the airport with a shallow descend rate?

How do you handle that? I dont like the 330kt level off with deceleration.

Thanks so much guys


OD

[framer]

If your tacho is doing 300kts it's a safe bet the a/c is as well, with that in mind I reckon your example of a 14nm slowdown is fine. Take the power off at top of descent.

[BOAC]

I think you forgot "250kts below 10,000ft"?

[MikeAlphaBravo]

Maybe he was given "no speed restrictions":)

[Olendirk]

free speed yes. I tried it one time. You fly good in profile. Reaching the decel point on the map, you spin in the vs out of the descend page in the fms and reduce the speed .so now, it worked fine, but nobody could tell me whether the rate of descend in the fms works out with the deleceration segment of the map mode. any hints there?

as i said, it worked fine

how do you handle that?

Cheers

[BOAC]

MAB - this has been discussed at length before - that means "no ATC speed restictions" and does not negate the regulatory limit.

[Olendirk]

Well, could somebody maybe answer my question;-)

[porch monkey]

Well, I'm new to the 737, (NG), and we were asked the other day, passing 9300 ft whether we could get in if we had 23 miles to run. We were at 310 kts at the time. It actually wasn't as hard as I thought. We were 250kts by 5000agl.

[Olendirk]

Ok i try to make the question more precise: what do I need for deceleration , how many nm, with light tailwind, normal weight and a sinkrate of 1000ftper min?

any expereience?

[Sciolistes]

About 10nm from 3xx kts to near clean speed with a V/S of 1000fpm.

[MikeAlphaBravo]

Ok BOAC, Hows about "High speed approved"

[BOAC]

Same! ATC approval, not regulatory.

[MikeAlphaBravo]

"maintain 300kts or greater"???

[Rocket Launcher]

300kts + 20kts tail - 180kts = 140kts to lose to intercept glideslope at 180kts flap 5. 140kts @ 10kts per nm = 14nm for deceleration.

40nm - 14 = 26nm for descent.

26nm x 3 = 7800' agl for a smooth descent on the safe side. anything faster or higher you may need to bring gear down early on the g/s.

regards

[BOAC]

MAB - I think we need an ATC input here - it has been a grey area for ages. I believe it to be an ICAO rule in certain airspace.

[mbcxharm]

Re: 250 below 10k. As usual, it depends...

From a UK perspective, as published in the AIP (ENR 1.4), as we would expect it depends on the class of airspace.

Class A: "As published in procedures or instructed by ATC"
Class B: "As published in procedures or instructed by ATC" (but there ain't any!)
Class C: "As published in procedures or instructed by ATC" (IFR) and "Below FL 100: 250 kt IAS; OR lower when published in procedures or instructed by ATC" (VFR)
Classes D/E/F/G: "Below FL 100: 250 kt IAS; OR lower when published in procedures or instructed by ATC." (IFR and VFR)

Appendix 4 to Annex 11 has the ICAO version...basically the same, except it doesn't mention the 'procedures or instructed by ATC bits'.

[BOAC]

Thanks mbcx - there guys - see? I told you it was straightforward.:) Kind of depends where you are.

For me, at 300kts 'tacho', I would be happy at 10,000' at 40nm. You need about 3 miles/1000' with 300 and -20, so at 30 miles you would be around 7000' if you maintained 300kts. Target (still air) is ballpark 30nm at 10,000 ft/250kts for a throttle closed to 210kts at 10 miles. Maintaining 300 to level-off (3000'?) needs another 12 miles, leaving you at 18nm from touchdown at 3000'/300kts - easy! You never know, you might also please Rainboe with a perfect CDA! One would also expect the -20 to reduce on the way down too if you are landing straight-in or it is time to whinge about a runway change:)

Of course I'm sure Olen is also aware of descent rate restrictions relating to MSA which may also affect his speed?

[Denti]

It really depends on country. In germany there is a note on each chart that the 250 below 10.000 does not apply to class C airspace, which is placed around any somewhat bigger airport. So you actually have to check the AIP of every country you operate in and be aware of the differences or just (its much easier) use 250 below 10.000 anyway. By the way, even in the UK i was told by ATC to keep 300 kts minimum well below 10.000, and personally i do not like that at all since the birdstrike risk goes up a lot below that altitude, not to mention that the classic windows were only tested up to 280kts.

[PantLoad]

On the 737, you can figure a descent profile at 250 KIAS of about 3:1. That is, it will take three miles to descend 1000 feet. (This assumes clean configuration, idle thrust.)

If you have a bit of tailwind, figure the 3:1, but add a mile or two for the tailwind.. For example, you're at 10,000 feet and 250 KIAS, maybe 25 knot tailwind throughout the descent, and you're cleared to descend to say, for example, 4000 feet. That's 6000 feet of descent...at a 3:1 profile...that's 18 NM. Adding maybe three or four miles for the tailwind, gives you 23 or 24 NM to do this level change from 10,000 feet to 4000 feet.

Note: If you're above 10,000 feet (What I'm saying is, you're not restricted to the 250 KIAS speed limit.)...descending at say, maybe, 300 KIAS...figure a 2.5:1 profile. So, descending from FL 350 to 10,000 feet is a level change of 25,000 feet. Idle descent, no speedbrake...no wind...figure 2.5 X 25 = 62.5 NM. Really, just round it up to 65 NM...it'll be more than OK. Now, add 50 kts of tailwind in this descent example, and I'd add at least 10 more miles...try, say, 75 miles for this example.

The old rule of allowing one mile for each ten knots of deceleration is pretty good. And, I really don't think it makes THAT much difference if you drive down to 10,000 feet at 300 KIAS, do an abrupt pull-up to level at 10,000 to slow to 250, then continue the descent...OR....do a gradual deceleration while you descend...say, for example, start easing the nose up at 12,000 feet, timing it so you're at 250 KIAS as you descend through 10,000....I really don't think either way makes much difference in your descent profile. (I vote for the smooth method...but whatever works for you, works for me....)

The old rule of being at 10,000 feet AGL, 250 KIAS, 30 NM from the airport is also a good one. I used to figure...if I have to go past the airport to turn around and land the opposite direction....10,000 feet, 250 KIAS, 25 NM out is good. OR, the other extreme...landing straight in...be at 10,000 feet, 250 KIAS at 35 miles out. If you'll be entering a base leg to land...then, the 30 out at ten and 250 is great. Again, add or subtract a mile or two for wind.

If you're off in your descent planning, you won't be off much. Too low, add a small bit of thrust. A bit too high, speed brake for a few seconds will make the needed correction. One common error is for pilots to use flaps/slats as a speed brake. I suggest you use the flaps/slats at the scheduled speeds that your company's SOPs dictate. If you're REAL high, speed brake and/or gear will most times give you the drag you need to get back on the desired profile.

While your company's SOPs are more important than any other publication, most operators use the 1000 feet AGL to be stabilized in IMC conditions or 500 feet AGL in VMC conditions. The idea is to plan your descent to acheive these restrictions. The closer you come to doing an idle descent from cruise altitude...to easing up the thrust to be stable at 1000 feet....well, that's quieter, more fuel efficient, easier on the plane, easier on the passengers, forces you to be more aware of a descent profile that gives you adequate terrain clearance, etc.....

If you really want to get fancy...plan on the last 1000 feet of descent at 500 feet per minute....allowing that extra distance to do this. AT 250 KIAS, figure 270 Kts ground speed....that's 4.5 miles a minute...500 FPM descent requires two minutes to descend 1000 feet....so add 9 miles to do this. I'd round it up to an even ten miles extra in your descent planning to accomplish this....you're good to go.

So, you're descending from 9000 feet to 5000 feet (250KIAS, idle, clean, no wind)....from 9000 feet to 6000 feet is a level change of 3000 feet. Figuring the 3:1 profile gives you 9 miles. (I'd round it off to ten miles...just for ease of figuring.) Then, for the last 1000 feet of descent (from 6000 feet to 5000 feet)...at 500 FPM...add another ten miles. So, if you're able, allow 20 miles for that 4000 feet of descent.

Anyway, have fun with the 737. It's a great airplane. It simply requires a little more descent planning than other planes. But, it's a great glider!


Fly safe,

PantLoad

[Capt. Inop]

Anyway, have fun with the 737. It's a great airplane. It simply requires a little more descent planning than other planes. But, it's a great glider!

Second that :ok:

[DC-ATE]

PantLoad has pretty well got it figured out.

Another thing you can try is descending at 350 to 12,500. Reduce to 1000FPM. At 10T you'll be right at 250. Simple. Continue down at 1500FPM. Slow to 200 at 3000AG. Select Flaps 1, 2, 5, 10, 15 as the speed gets to the appropriate spot.

All this at Idle, and assuming ATC is no problem.

As you cross the Marker, throw the gear out and select Flaps 25. Slide down the G/S and at 1000AG go to F30. At 500 AG, F40. If it wasn't for the "stabilized approach" procedure now used by everyone (I think) you could have landed without ever having to have touched the throttles (excuse me.....thrust levers!) from 35,000 feet!

Unfortunately, I guess you'll never get to try this all the way to touchdown these days. But, the -37 is a great little machine. Spent nine years on it in both seats.

BTW, this only works on the -200. The -300 takes a little more fiddling. I'm sure all the later models would be the same as the -300. Only spent a short time on the -300.

[Denti]

Another thing you can try is descending at 350 to 12,500.

That could lead very fast to a tea without biscuits at the headquarter. Do it 20 kts slower and it might be safe. At least on -300 and higher models.

[Yon Garde]

Stating the obvious I know, but why not just do it in VNAV. Works smoothly and efficiently (almost) every time unless of course ATC or other traffic is being unhelpful. Intervening with numerous pitch modes usually just increases workload with no real benefit. Just my 2 cent's worth :)

[framer]

I agree.I normally only go into all this stuff if VNAV isn't playing the game due traffic.

[BOAC]

Works smoothly and efficiently (almost) every time - well, a little 'off thread' now, but I would disgree. I am constantly amazed at the poor pitch control of most 737s 3-7) in a/p VNAV. Whether it be maintenance or design faults I know not.

[framer]

Maybe it's maintenance? I find that if there are accurate winds, temp and qnh in the descent forecast pg then it does a good job on the 300.

[Sciolistes]

There are differences in our fleet, some OK some not so good. But there better ones are still course and clumsy in this regard.
Stating the obvious I know, but why not just do it in VNAV. Works smoothly and efficiently (almost) every time
On the Classic, inputing a descent forecast still results in the ubiquitous DRAG REQUIRED nearly every time. It seems one needs to fake additional tailwind to ensure a smooth VNAV PTH descent.
Intervening with numerous pitch modes usually just increases workload
with no real benefit. Just my 2 cent's worth
Agreed. If it isn't working out in VNAV then just extend the rwy centre line, LVL CHG and work it out from there.

[framer]

one needs to fake additional tailwind to ensure a smooth VNAV PTH descent.

Ha ha good point, I have to admit that I round up tail winds and round down head winds....maybe thats the key with the ole 737's.