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13th Jul 2007, 22:26
Hi Folks,

i know this topic has been discussed several times, but I'm still confused about some things... 737-300/500:

1. Common rule seems to be: ALT x3 = Distance needed for Altitude Loss. So far so good, but does this calculation include deceleration from descent speed to 250kts and to 180kts at GS capture? According to my operators books it does, but it doesn't really work out well if i do my crosscheck at say 8000ft and have 24NM to go. I always end up using speedbrake. Or does the Formula Alt x3 = Distance required (without considering speed reduction) only apply to TOP OF DESCENT calculation, considering that you make a pretty steep descent gradient initially and then have spare mileage to cover the speed reductions from descent speed to 250kts and from 250kts to 180kts?

2. What about a heavy aircraft? Do you add some miles? Any rules of thumb?

3. What is your mental maths during a high speed descent? The aircraft achieves a higher descent gradient, but on the other hand i also need more track miles to slow it down...so, do I overall need MORE or LESS mileage? Any rules of thumb?

I just started on the 737 and would really appreciate if someone could shed some light on this for me... :confused:

best regards

blackmail
13th Jul 2007, 22:55
top of descent = alt x3 in nm.
+10nm for deceleration from 300kias to 250kias to 210kias
for b738ng :per ton above/below 60ton(maxlanding wt = 65.3ton) : +/- 1 nm
for average wind correction :per 10 kts : +/- 2 nm tailwind/headwind.
during descent : groundspeed divided by 2 = rate of descent in ft/min for a 3°/5% glide path.
kind regards, sorry, can't get rid of the 2 smileys,
bm

FCS Explorer
13th Jul 2007, 23:03
don't know about 300/500 but NG has very nice VNAV descend function, just enter the winds on the desc forecast page for levels 300/200/100 if significant, usually does the trick. if there will be tail wind during descend, put in about 20% more wind speed.
after all its usually 21 or 22 minutes from T/D to DEST. if you get above path and have to perform a high speed descend go below the virtual vnav-path, 300 feet for every 10 knots above the econ-desc-speed that the path is based on, so if for example 300 is econ desc and you dive with 330, go 900 below the path, reduce to 300 to get back to econ profile.
if you are lets say 15-25 NM final and still slightly high you may also go for flaps 1/2/5 keeping the speed at around 210/220. that creates way more drag and you are already configured and slow.

A37575
14th Jul 2007, 03:15
and still slightly high you may also go for flaps 1/2/5 keeping the speed at around 210/220. that creates way more drag and you are already configured and slow.

From the 737 FCTM:

"Normally, descend with idle thrust and in clean configuration (no speedbrakes)...the speed brake may be used to correct the descent profile if arriving too high or too fast...plan the descent to arrive at traffic pattern altitude at flaps up manaoevering speed approximately 12 miles from the runway...losing airspeed can be difficult and may require a level flight segment. The flaps are not normally used for increasing descent rate. Normal descents are made in the clean configuration to pattern or instrument approach altitude. When thrust requirements for anti-icing result in less than normal descent rates with speed brakes extended, or if higher than normal descent rates are required by ATC clearance, the landing gear can be used to increase the rate of descent."

Using the flaps for more drag is incorrect technique and is at no stage mentioned as a recommended technique in the Boeing 737 FCTM.

ZAZOO
14th Jul 2007, 05:13
Hi Tadex, yup your quite right the weight matters and that affects the rate of descent ! The lighter they are the faster the rate of descent.

On my aircraft 737-200 with 50 pax or less she will come down at a greater rate of descent so basically you might find it reasonable to cut down on the distance needed for speed loss to configure for the approach or you may find yourself way out and low. About 5nm is ok but remember this depends on if you have a continuos rate of descent down to FL100 and below. If you are heavy take an early descent.

At top of descent maintain 300kts to 310kts as you descend and try to keep a rate of 2500ft/3000ftmin depending on her weight all the way, now then here is the point you need to grasp and if you get it right you should have no problems, as you approach FL110 plan to be about 50nm from your initial approach fix then reduce rate to 500ft/700ft per min and decelerate her to 250kts (she will cover about 10miles to reduce speed nicely depending on how fast you pull her nose up)) at this point you should be 40nm from your initial approach fix and coming out of FL100, now increase rate of descent to 1500ft/min not more she should hold 250kts and work your way now by monitoring and having a mental picture of whats going on around you and the airport that your going into traffic xther etc ..... then start slowing her down to configure and as you get on the final approach you should be right on the money, speed distance height and all.

Remember 40nm-10,000ft-250kts my friend.

The thing is how much distance did you chew up while slowing her down still high up there!!!

Its crazy but the -200 amazingly will actually hold these numbers once you get it right and quite nicely too.

Enjoy Sir,

Zaz

bubbers44
14th Jul 2007, 05:59
Most of the time in the US flying the 737-200 we were required to be at 30 NM at 10,000 and 250 knots so we aimed for that point plus seven miles for slow down. We made it a game to leave FL350 at idle, hit 30 NM at 250k and spool up at 1,000 ft. To not need any speed brakes you needed another three miles or so depending on wind and weight. If you were heavy it took a few more miles, if light, less. That is because your best L/D at heavy weight is closer to your normal 300K descent speed than light weight descent speed. 250K below 10,000 works that way too because light weight L/D best glide is around 220K. Most of the time with small speed tweeks no speed brakes or power were required until 1,000 ft.

FCS Explorer
14th Jul 2007, 09:46
since the first 737(-100) in 1968 (?) that thing has only gotten bigger but never seen any real development or improvement. that leaves me on the B738W with speed brakes that are (almost) completely useless at speeds 250 and less. on the other hand i -could- use flaps 5 up to 250kts (see placard next to lever).
i don't know if the FCTM forbids using flaps 1/5 for drag (i doubt it) but i won't drop the gear 20NM out if i can help it.
(YES, i know extending flaps at speed close to max placard will sooner or later f*ck the fairings and my company programmed the acars to send a report if i do so above 230 IAS:=)

A37575
14th Jul 2007, 10:50
Word of advice. If planning on straight in in VMC it is prudent to stop the 250 knot idle descent on reaching 3000 ft agl and fly level at idle thrust until 210 knots. Then, depending on the situation, you can select flap 1 and then flap 5 still in level flight until flap five manaoeuvre speed - typically 180 knots block below 53T. Then resume further descent at 180 knots (cross-over speed) By doing it that way, you have more flexibility if caught high or close in and by extending the landing gear you can get yourself out of trouble with plenty of altitude to play with rather than belt down fast to 1500 ft straight in and find yourself too fast and close in for comfort. For the 737-200 at max landing weight use three times the height in DME plus 15 miles for top for descent and use o.70 into 280 knots until 10,000 ft. Then level flight at 10 until bleeding back to 250 knots then idle descent at 250 knots to 3000 ft and so on.

14th Jul 2007, 11:46
Well, I've always used as a good rule of thumb:

When more or less descending straight in (inbound course to the airport from top-of-descent close to that of the runway course), try to be 35 miles out at 10,000 feet above airport elevation and at a speed of 250 kias.

If you're more or less making a base-leg entry to your arrival runway, the 30 miles out at ten and two-fifty works pretty good.

If you're descent path is going to take you on a downwind...that is to say, you're landing in a direction more or less opposite the course inbound to the airport from top of descent... then, you can do 10,000 feet and 250 kias, 25 miles from the airport.

As someone else stated, it takes about one mile to slow ten knots...so, for example, figure on ten miles to slow from 300 to 200 (in level flight, thrust idle). This rule of thumb must be modified with strong winds...headwind or tailwind....but, with a few years practice, you'll impress even Boeing test pilots..:D:D:D (joke...)

Using slats/flaps as a speed brake is contrary to Boeing...beats the hell out of the slats/flaps...and adds little drag until you have a bunch of flaps out (i.e. Flaps 25 or more). Boeing has charts and graphs that will illustrate this... Best to use the speed brake first, then gear...or if you really need drag, use both speed brake and gear....and work out your slats and flaps on a normal speed schedule. While some swear by Flaps 5 at 250 kts...there is no aerodynamic basis for this technique. Boeing is correct...After over 12,000 hours in the left seat of those Guppies, I'm convinced Boeing is correct.

Yes, the 737 requires a little more descent planning than many other aircraft, but it's a joy to hand fly...an honest machine....takes a licking and keeps on ticking.

Have fun flying one of the most popular airliners in the world.

27th Jul 2007, 23:09
Hi Folks,

thanks for your input, has proven very helpful so far ;) There's one thing to shed some light on left though:
I calculate my TOD and leave it at idle thrust (LVL CHG). Now, as far as I know, during descent the "spare mileage" will increase as the flight path angle at high speeds is quite neat (i.e. i'm at FL250 and have 80NM to go... 25x3= 75... 80-75=5 miles spare....later on I'm at FL120 and have 43NM to go...12x3= 36...43-36=7 miles spare). How do you include that effect in your calculations? Adding 5 or 10NMs at TOD doesn't really work ou then, does it? In my opinion i want to have about 10NMs "spare" reaching FL110 to have 10NMs spare for speed reduction.... but how do i figure out whether or not I am in slot at FL250 or FL300???

It's not rocket science..i know....I'm just curious ;)

regards

Just another student
23rd Aug 2007, 17:12
I have to admit that descent planning does my head in at times aswell. I know it is simple in theory etc but when you get different people sitting next to you advising you to try it in different ways I get confused.

I love it when the Captain allows you to experiment and learn for yourself which is the best way.

I like to try and keep it simple by inputting as accurate winds as possible into the descent forcast page and descending in Vnav, starting descent in the ball park of 25nm before calculated TOD.

Vnav the other night was not proving too accurate so I used Level Change and had the same issue, the plane was just gaining speed even with speed brakes at 50 percent. It does not happen often, but each plane flys differently.

I try to avoid using V/S as much as possible, only using it for small changes, but some pilots swear by it and use it 95 percent of the time, even though you have no speed protection........ that always gets me. Each to their own.