B738 Descent Planning

[Johnny Tightlips]

Hi all,
I am a third of the way through my line training on the B738. Rotations, landings and everything else is improving the whole time and the LTC's are happy but one thing that is bothering me is descent planning. I just find it a small bit complicated and I would just like to know how other pilots do it. How do I use VNAV, LVL CHG, speedbrake etc to get a good profile while saving a bit of fuel and time as well? I would appreciate any feedback.

Thanks.

[BOAC]

Are your training Captains qualified on type

[go around flaps15]

If you want to be lazy use VNAV. If your high on profile speed intervent and increase speed sensibly. If your in amongst the balls on a procedure(not approach) use the speedbrake, again sensibly.

Level change roughly 3 miles per 1000ft. If you stick as close enough to that "rule of thumb" and try and be as ahead of the aircraft as you can it makes life an awful lot easier.

I think the most important thing is if you see that you need to do something, do it early. Don't around waiting to be hot and high and then decide to act.

[Ireland105]

The B738 was always a great one for a late change on approach, similar in essence to the B727 - speed brakes and flaps full really help slow it down quick even as near as 4 miles final, it is not the norm of course but has been done. Its a robust machine for sure.

[BOAC]

PURRLEEZ do not drag this one out. We have someone "1/3 of the way through line training" and he/she is asking an internet chat room how to fly a descent? Isn't it obvious to all?

[ImbracableCrunk]

I'm currently working on a rule of thumb for FO's descent planning:

No matter which method you choose, the Captain will tell you, "No, I don't like that method."

I think this may only apply in certain hemispheres/latitudes/attitudes.

[ballyboley]

Johnny, expect a few smart comments on here for asking this question - its part and parcel of posting on pprune!


All I can say is that decent planning is something that comes with time, some people get it quickly, others take longer. I am still setting myself targets whereby I dont want the thrust levers to open from TOD until I put the gear down at 4 miles, nor do I want to have to use speedbrakes to achieve that. It doesn't always work and I've seen myself with Flap 10, Up speed, speedbrakes and maybe gear out if it all went pear shaped or was cut in tight. Nevertheless it is rewarding to get it. The key is that YOU (not the FMC, the path indicator, the banana or anything else) should know where you are on the profile and making sure you fix it. As has been said - If you know you are high and do nothing about it, it will bite you. Sort it out when you are 500ft high, dont wait until you are 2000ft high and being asked to slow down. Similarly, blasting down too quickly and doing 100fpm for 10 miles with flap 5 out isn't really what a CDA is either.

I struggled with this at the start of line training too, for me the key was ignoring the path indicator which has made some far too lazy. I stuck a little post-it sticker over that part of the screen to force me to think about it myself. VNAV, LNAV, Magenta lines, are all great tools - but they are garbage in, garbage out and it WILL lie to you at times. You are flying the airplane.
If you are expecting a 6 mile final, 4 miles out, then 6 miles to where you are downwind then you want to be around 5000ft at the end of the downwind. If you are still doing 250kts, then you need another 5 or so miles at least to slow down. If you have a tailwind you will probably either need to be lower, or use speedbrake which you need to anticipate. If it goes badly wrong, remember you have the gear as an excellent speedbrake. The 3x Altitude never goes wrong. Remember if you have selected a STAR or transition in the FMC it could more than likely be much longer than the radar vectors so will show you low or on profile when you are in fact high. The ND is excellent for "eye balling" a 6 or 7 mile final and the distance you have to get there.
Further up the descent is alot easier to sort out - speed intervention or lvl change depending on your SOPs and 330kts will soon get you back on path! Remember VNAV does this automatically and will move the speed bug down but pitch for the profile but beware of it at high level as it may just hit the barbers pole.
Sorry to ramble on, you will get a million different views from Training captains on this, I would encourage you to do whatever works for you!

[STBYRUD]

The miracle on the 737-800 is that especially when you're new to it you will probably be (assisted by ATC) be high on profile - the key is really to play with the modes, which takes practice and a couple of messed up descents as well just to find out what doesn't work I for myself calculate a 3° TOD, subtract half a mile for each 10 knots of headwind, add a mile for each 10 knots of tail wind, add a mile for each 10 knots to be slowed down. Gives me a rough idea, but at some airports the MRVAs dictate that ATC will keep you high according to your descent plan, sometimes considerably. Lets assume you don't use VNAV - you could start your descent using V/S at about 2200fpm, speed at 280 - monitor the trend, if the A/T kicks in increase your rate of descent after checking the banana - the great thing about V/S is that the banana will more accurately show you where you will reach the MCP altitude than in LVL CHG (due to the variable vertical speed). If ATC gives you a speed restriction or you reach FL100, change to LVL CHG - monitor the vertical speed trend, if ATC wants you to slow down expect to go high on the descent path - might be a good chance to use the speed brake if necessary. If you level off due to ATC and you don't have a speed restriction, reduce the speed as much as possible (and allowed by the local regulations) to bleed off some energy, once you are cleared to descend don't forget to increase the speed again to resume a high rate of descent if necessary. If you notice you cannot lose energy fast enough, don't hesitate to use the speed brake, if you're within 25NM of the airport consider setting flaps 1 or even 5. If you're able to reduce to 220 knots 5 NM before turning final you're in great shape. If you end up high on the glide path, gear down usually does the trick - speed brake below 230 knots really doesn't do much except shake the airframe.

Hope this helps in some way, best of luck!

[Callsign Kilo]

I understand your predicament Johnny. When I was a 1/3 way through my line training I was asking myself the same question. I continued to ask it all the way through my line training and well in to my time on line. Like in many walks of life, there are 'many ways to skin a cat' so to speak! Descent planning in the 738 is no different. If any pilot is perfectly honest, it all comes with both experience and practice. There are different methods for different scenarios, which you have probably witnessed already. Understandably this plethora of approaches to descent planning and energy management may be leaving you scratching your head. Added to the fact that different LTCs will either demonstrate or 'patter' different methods. They may also interject with something you didn't already know or hadn't already been shown. Added bewilderment when everything is coming thick and fast during your training!

If I am guessing the airline that you are flying for correctly, then I am aware that the preferred mode for descent planning is VNAV. VNAV is a good bit of kit, however it is a mode that functions best when left alone (i.e. without ATC track manipulation). If it isn't managed accordingly, it will give you duff information. 'Sh1t in, sh1t out' so be careful. This is very evident with guys who become fixated on the lateral deviation indicator. Simply flying a different descent speed using LVL CHG in opposition to the descent speed in which VNAV is using to calculate the path will put you out. This along with no or inaccurate information in the forecast page of the FMC (especially when conditions vary considerably from standard atmosphere) and failing to remove level and speed restrictions in the LEGS page (when they aren't applicable) will put you off the path.
The training department in my airline (and possibly yours!?!) focus on VNAV training and ask LTCs to demonstrate its use to cadets. This is because a VNAV PTH descent will allow long periods of the descent to be conducted with idle power (hence saving fuel). Route modifications such as speed changes or vectoring by ATC during descent may see you doing a little inflight adjustment by using LVL CHG or V/S, updating your LNAV track and/or changing your target descent speed in the DES page and then reselecting VNAV. The mode is also preferred because it ensures all level and speed restrictions within the LEGS page are met. Especially important and useful when flying lengthy arrivals into airports like Madrid or Gatwick where ATC expect published speeds and altitudes to be met for their own planning purposes. This is where VNAV is worth it's weight in gold.

VNAV is merely a mode, and like all modes has it's pitfalls. Always back it up by calculating your own track miles from the various sources of information available, whether it be from the FIX page, PROGRESS page or plain old DME. Again, remember the downsides of each. Neither one will give you an accurate figure of miles to run, especially if ATC are positioning you; but it’s always best to have an idea of your own profile rather than fixating all you attention to the lateral deviation indicator! Another piece of advice I would add would be look for cues from air traffic. If you are passing FL180 in the descent and the approach controller clears you immediately to FL60 and starts to tighten up your arrival then he/she is giving you a big hint that they want you down tout suite! If they are taking you away from your preselected arrival and in turn slowing you down then relax a little. Also keep an idea of what other traffic are doing, especially the aircraft immediately ahead. Keep your eye on the wind direction on the ND and consider how it’s direction will affect you as you are positioning for final approach. A thirty knot crosswind whilst downwind can turn into an unhelpful tailwind when on base. Allow for this and manage your profile early, because often ATC won’t. Also consider aircraft weight. If you are heavy you will carry more momentum and require a greater number of track miles for your descent. Plus never attempt to intercept the G/S in clean configuration when over 60 tonnes – you will need the gear early which unfortunately results in all your fuel saving for that sector heading south!

For CDAs, I like to use the altitude capture range arc (often referred to as the ‘green banana’) It is very useful, however it is based on current v/s and groundspeed. So allow for slowing up and configuring. With a bit of practice you will be able to ‘eyeball’ this as previous experience will give you an idea of what is required.

That’s about it. As I say, descent planning is all about practice and experience. Some times it works perfectly, some times it doesn’t. If you find yourself high on profile and above FL100 then use the aircraft’s energy and increase you speed in descent to come back to where you want to be. Don’t always go for speedbrake, especially when 250kts and below. It just makes a lot of noise and vibration to be honest! If it really goes to a ball of sh1t then 220kts at Flap 5 works well or below 210kts at Flap10; both with LVL CHG or VNAV & SPD Intervent. Although you may see from Vol.1 that using flaps as brakes isn’t a widely encouraged method! I say if you need it do it; as will the majority of LTCs.

Above all enjoy and experiment. When on line some Captains will be happy to let you try things out with a little bit of patter or quite simply let you get on with your own thing. Others will happily fly the aircraft for you, regardless of how much experience you may have. C’est la vie!

[Chesty Morgan]

What's wrong with 3 times your height and 5 times your ground speed plus a bit of slowing down room? And keep recalculating it as you descend.

Failing that use FPA, put the line where you want it and manage your energy correctly to meet any speed restrictions.

Lets face it, you can plan all you like but ATC will invariably muck up your planning anyway (no disrespect to ATC but that's the way it is), slow down, speed up, maintain your level, descend early or late etc..

Anyway, how do you know you are a third of the way through your line training?

[theshed]

Because they know how much the paid for!!

[Mikehotel152]

Good advice from Callsign Kilo and I wish I had similar advice during my line training.

Once you get on the line you'll find that every Captain has a different method yet it's amazing how almost all of them get the aircraft to 10 miles at 200 knots and 3000 feet. Sometimes it's uncanny.

[cirr737]

Same here - Took me about half a year of line flying to figure out how a good descent works - now, 4 years later, I do not really plan the descent any more or think about it, and it works out fine (well, most of the times...)

At some point in time your brain will subconsciously integrate all the information you need like DME, green banana, wind indication and so on, and combine it with the "stored" data on how the aircraft performs and just tell you "Now it's time to descend" or "Better use the speedbrakes now"

What I did in my training and worked for me, was to calculate my descend with 290 knots (that is around 3NM per 1000ft in LVL-Change), so I have a reasonable margin to adjust speed between 250 and 330 if I'm too low or too high. Add 3 miles for reducing from 290 to 250, another 3 miles from 250 to 210.

Another trick is to use the banana:
If you are on a straight in approach just put the banana 3NM ahead of your glideslope intercept when you are at 210kts, 6NM when at 250kts and 8NM at 280kts (assuming no wind conditions). That will give you a reasonable margin to slow down. If you are on a downwind you have to anticipate at what point you are 3/5/8NM from intercept and just put the banana there. When you get more experience you can reduce those distances, but that might require some cheating like going above the glide to configure if you made a mistake.

And always remember the 3 problems of a 737 pilot:
1. Too high
2. Too fast
3. Too close

Edit:
Almost forgot: If you are hot&high on the glide FLAPS 10 is a wonderful flap setting - with 200knots the best speedbrake you can get on the 737 - but do not use it that way when instructors are watching you

[uchy]

All the above explanations are really very useful. I have just start know to fly on the 737, and I would like to ask you an other thing. How long does it take to decrease speed? I read 10kts for nm, but this is true only in level flight. So if you are in lvl change and you star to reduce, it will assume a ftpm of 500/1000, and how long will it take to decrease? I suppose less than 10kts for nm? Somone can help me?thanks a lot

[STBYRUD]

Level change is fastest of course, but at low altitude I prefer using the vertical speed to maintain a reasonable descent while slowing down in a controlled manner - level change is a bit too brutish and unpredictable in its speed.

I was wondering the other day - one captain refused to extend flaps to 1 at 240 knots and -800fpm vertical speed, saying that there is some Boeing document out there that recommends not to use the flap placard speeds but instead use a maximum of 235 knots for flaps 1, and correspondingly less for the rest to increase actuator life - I hope thats a load of tosh? I couldnt find any such reference. Also, whats wrong with using flaps to slow down within say 25 miles of the airfield? Again, even the FCTM doesnt mention the matter...

[STBYRUD]

Well naturally that makes sense - nonetheless its obviously not a hard limit - of couese there is no need to push the envelope unnecessarily.

[FullWings]

Also, whats wrong with using flaps to slow down...

Flaps: Used to increase the lift coefficient of the wing so you can fly more slowly.
Speedbrakes: Used to increase drag.

Yes, the last few stages of flap, normally used for landing, are there to produce extra drag as well as a bit more lift. Going from clean to the first stage of flap on a 737 is not really going to help much if you want to lose energy as the wing is still fairly efficient in that configuration. What it will do is stress the airframe more than usual, make the ride worse by lowering the wing loading and require a larger pitch change to stay on the same glidepath. Plus, when you're flying at limiting speeds, all it takes is a gust or positive shear and you've got an exceedance, which is generally taken quite seriously with flaps as the aerodynamic loads increase very rapidly with airspeed.

If you're too high and/or fast, use the speedbrakes; if that still isn't enough, drop the gear as that is much tougher (built to withstand landings like mine!). If you're *really* high, slow down to landing speed, deploying the flaps on the normal schedule - when you're configured, you can do >1,000' per mile.

[ImbracableCrunk]

Does any operator out there not have some pseudo-placard for flaps? Like "don't select F5 until F1 maneuver speed plus 10."

My last operation also prohibited Flaps and Speed Brake at the same time. They really babied the flaps. Still, they were in the news monthly for flap malfunctions. . .

[On Guard]

Boeing says select flap pref 20kts+ below placard speed. Your Capt was correct why did you need F1 at 240 kts?

Descent planning. Keep it simple.
3 times profile plus 1 mile for every 10kts you need to decelerate to 200kts. IE 280kts needs 8 miles decel. 5* GS = ROD required so monitor this if you are not achieving this then you will eventually end up high.

No S/Brake until below transition unless you have really got high or have a tailwind. Panic slowly I say, at FL300 why take S/B when 500 high fighting a 100kt hw, sure enough you'll be pouring on the power down lower.

If you are high within 25nm you need to slow down and get drag out, prior to that you can use speed and parasite drag to assist.

I use VNAV but monitor the profile so I know where I should be at all times. Strong HWind des late and cut into the profile , vv, strong tailwind des. early.

[FR8TDOGii]

The precision solution

The following is from my PPAC [Precise, Predictive, Aircraft Control] training systen for upgrading airline pilots.

FIRST: We want to know where to point the airplane to accomplish a given alitude loss within a specified distance. The simple rule is: ±One degree of pitch change [DP] = ±100 feet/nm. Therefore altitude change in100s of feet [DAlt] divided by NM to go = DP in degrees.

Examples: Alt to lose[DAlt] NM to go DAlt (100s of feet)/NM = DP
9000 30 90/30 3°
12000 40 120/40 3°
16000 40 160/40 4°

Thus if you decrease the pitch 3°, you will lose 9000 feet in 30 miles.If you hold 3° you will lose 300 feet/NM, REGARDLESS of airspeed because it's a matter or Trigonometry NOT aerodynamics. The faster you go the faster you have to descend to remain on your 3° flight path.

Now for the second step - acounting for the wind. If we have a head or tail wind our Air Mass flight path will have to be adjusted to maintain our desired flight path with relation to the ground.

Fortunately, there is a simple formula to correlate speed, Rate of Descent, and pitch change:

Speed(NM/min) X 100 X DP = Rate of descent.

Where speed is Ground speed for ground-based reference and TAS for Air-based reference.

Take the 1st 2 digits of your speed, divide by 6, multiply by 100 then by DP from above to get rate of descent.

Some examples:
GS (NM/min) X 100 X DP = ROD
420 7 700 3° 2100
360 6 600 3° 1800
420 7 700 4° 2800

The most beneficial application is the inflight (air-to-air) pre-computing of level off pitch changes from climbs/descents.

To do this we simply reverse the second step above to determine DP. This method eliminates the BG's [By Guess & By Golly] and provides for VERY precise aircraft control.

Let’s assume we climbing at 1800 ft/min at 180KTAS with a +10° pitch attitude and we're about to level off. What will be our final pitch attitude, assuming we maintain 180KTS (3NM/min)?

DP
= Rate of climb/descent /[KTAS(NM/min) X 100]
= -1800 /(180/60X100)
= -1800 /(3X100)
= -1800 /(300)
= -6°
New Pitch = 10° - 6° = 4°

Now you're thinking just like the autopilot/flight director !

Sorry if the formatting is off