Really hard to slow down...

[Tristan Marchent 3]

Hi All,

 

Very recently I have been flying the 737-800NGX alot and I have found it quite hard to actually slow the aircraft down on the descent. Sometimes on the approach, established on the ILS, I would be sometimes coming in at 200-210 kts, and even have the speedbrake extended fully and it 'slowly' slows down but not enough. I don't like putting the speedbrake in because it doesn't feel realistic to have it extended all the way down the Glideslope but to slow the aircraft down I have to! :(

 

The other day I had to go-around because I was going to fast on the approach. I even start to descend the aircraft about 10nm BEFORE the Top of Descent to make sure there is a smooth transition onto the VNAV Path! It's ok on the descent, follows the VNAV Path nicely, it's just when it gets closer the capturing the ILS the speed is still high and hard to bleed off!

 

I have also been having alot of messages, mainly when inputting route and flight data, but also throughout the flight of the aircraft being 'UNABLE 250/XXX KTS at XXXXX' and I don't think I really had this message very much in the past. I don't know what I have changed really... The only thing I can think of is I am now putting in Cruise Winds and Temperatures but I don't see how that can affect it..

 

I would really appreciate some advice, tips on anything I am doing wrong.

 

Many Thanks,

 

Tristan

[richardnobes14 13]

First of all, you might want to reconsider what cost index you use. 15 to 30 would be ideal. Secondly it is important to know that you can go down or slow down, you generally cant do both so it's important to think ahead and perhaps begin descending earlier or descend below the path a little to give yourself some extra room to slow down if slowing down is really a problem.

 

You should also insure that you have entered winds in the DES FORECAST page of the FMS. This coupled with the ISA and QNH information you enter assists in calculating the descent and gives the FMS an insight into what lies ahead.

[tazisdylan 2]

No expert here, but assuming you are talking about 'UNABLE 250/XXX KTS at XXXXX' messages on descent, may I suggest you amend the leg with 250B/ in the scratch pad as this often helps. I also find there is sometimes a need to applya bit of speedbrake during decent, but not to the extent you seem to be and not to any extent immediately before or on establishment.

 

Could be totally off course here too, but the air temps affect the abilty of the engines to provide power, do they not.

 

Any partciular approaches you are flying on and getting this with? -3.00 degree GS? Or steeper? I am guessing you are making some steep descents on approach?

 

Richard of course above correctly points out that generally speaking you cannot 'go down and go slow'. Are you filling out DES Forecast as he asks?

 

Hope this helps in some vague way.

[flyingsquirrel 6]

Monitor the descent with mental arithmetic to confirm whether the VNAV programming is sensible and whether the wind/energy assumptions remain valid. Take the altimeter reading in hundreds of feet and multiply by 3 (e.g. FL370 = 37 * 3 = 111). Take the middle digit of your IAS (e.g. 270 kts = 7) and add it on. Take the tailwind at altitude (from the map display) and divide it by 10. Add that on. If engine anti ice will be used during the descent add a ballpark figure of 5 miles per 10,000'.

 

So for FL300, 50 kt tailwind, descending at 270kts and engine anti-ice from FL300 to FL50, the required track miles is 90 + 5 + 7 +12.5 = 114.5 nm.

 

Correctly program the VNAV with the target speeds (including the desired speed over the beacon or on a radar-vectored base), assuming 180kts for flap 5 will steer you in the right direction. The descent winds and engine anti-ice have a bearing and must be entered, otherwise it's garbage in - garbage out and you've wasted 5 minutes of your life.

 

Some real world targets that work (the Boeing FCTM is a little optimistic in the descent planning section, particularly for the -800):

 

- 10,000' 250kts, 40nm track miles

- 12nm out, up speed, 3000' AGL. Continually monitor track mileage and height using height x 3: check head or tailwind on final for configuration planning.

- At glideslope intercept: flap 5 and '5' speed

 

For a low drag approach, assuming a 3 degree glideslope, select gear down at 2300', flap 15, complete the landing checklist, at 1300', select landing flap.

 

Things to catch you out:

 

- straight in approaches with long finals (>12nm) as you'll have to slow down and configure flap whilst descending. Intercepting the glideslope clean or at flap 5 but high on speed will mean that prompt action is required - depending on how far you've got left to descend. The speedbrake on the -800 is ineffective at low speed but sometimes the 1-300fpm difference that it makes will be enough. Look at the descent page on the FMC to see whether your flight path angle is greater than the Vertical Bearing - if it is then you'll be wresting control back from the gods. The real option for slow down when in the dwang is the gear and the best philosophy for avoiding an unstable approach is to grab the aeroplane by the neck and make it do what you want.

 

VNAV is a wonderful trap. It only shows you height above or below the optimum descent path: the path that you have programmed. If there are hard heights in there then they may be above an ideal glide descent profile - so temporarily delete hard heights to see where you actually want to be (useful heads-up for many STARs). What VNAV pays no attention to though is energy, i.e. you can be on path but flying 330kts - when the path was programmed for 270kts or 240 below FL100. Try and imagine the VNAV deviation as a colour, if you're fast then it's red and if you're slow then it's blue. Red needs drag or increased speed to dive below the path. Consider the path deviation as miles, using 300' per mile, so if you're 9000' high on profile, you'll need 27 extra miles or have to dive down to get below and then create sufficient track miles to slow back down onto your planned speed. 2100' below the path gives you 7 miles.

 

Last thing on slowing down: in level flight it's about .75kt per second. In the descent it's about .375kt/second. So, quick sum: 250 kts to up speed ~ 45 kts. 60 seconds in level flight, 120 seconds in descent. 250 kts is about 4 miles per minute. 250-up in level flight = 4 miles. 250-up in descent = 8 miles. So the amount to be below path if aiming to slow up to up speed and regain the path again is approximately 2400'. Use these same numbers for higher up to enable more control and also appreciation of what VNAV is actually telling you.

 

Lastly, VNAV can also bite when you turn inside the lateral path that you've programme for an arrival. Also, if the lateral arrival has a turn of more than 60 degrees then the aircraft will turn inside it. Radar vectoring may also turn you inside the lateral path. The result: a reduction in track miles. If you were 40 miles out then it's not an issue but if you're 12 miles out and "suddenly" you're 9 miles out then you're 1000' high and likely looking for localiser and glideslope intercept. The world has changed to a bright shade of red and it's fighting the energy once again - just when you think that you're home and dry. The tip for this: update the lateral path and anticipate what reduction in track miles there's going to be and be at the correct speed and correct height. These days, most approaches, particularly precision approaches, are flown as continuous descent, and this means either meticulous programming of VNAV to create a shallow descent path to decelerate and configure, and/or, flying in V/S mode and adjusting the descent rate depending on the track miles.

[trancesetter 5]

I must say that the single thing I never had problems with is the speed in the NGX. By properly planning the flight and adding this flight into the FMC (I use a combination of vatroute.net and simroutes.com) the FMC calculates the T/D quite well. Sometimes it says that the descent path is unachievable, but with some airbrakes applied you can see the vertical deviation rapidly shrink on the FMC's Descent page. It's all a matter of not wanting to fly into the 20nM range of the airport with 280kph ;-)

[NoahBryant 26]

 

 

How fast are you when you are intercepting the localizer? When are you deploying your flaps? Also, at what point are you noticing your speed is too high? I find the landing gear does a much better at slowing the plane then the speed brake if it looks like things are too fast.

 

 

 

 

 

[MED1473 18]

Tristan, can you tell me about your flap deployment schedule in these apps?

[Guest]

First of all, you might want to reconsider what cost index you use. 15 to 30 would be ideal. Secondly it is important to know that you can go down or slow down, you generally cant do both so it's important to think ahead and perhaps begin descending earlier or descend below the path a little to give yourself some extra room to slow down if slowing down is really a problem.

 

You should also insure that you have entered winds in the DES FORECAST page of the FMS. This coupled with the ISA and QNH information you enter assists in calculating the descent and gives the FMS an insight into what lies ahead.

 

Hmmm. to be honest I always use a C.I of 90 and I never experience slow descents.

 

Same speech for the DES FORECAST, I have never fiddled with it but I have never had high speed issues.

 

Despite your statements are correct I am sure it depends on something else, if I have to take a guess the STAR into the FMC might be not correct or ..silly question...do you descent exactly at T/D or later ?

[Spin737 102]

Make sure your N1 is at idle. Sometimes people have an interface issue where the engine is "stuck" at a higher power than should be.

 

At the TOD around mid-30's to FL40, your idle should be around 40%ish. It slowly decreases to around 20% on approach. If you're getting high and fast, that could be an additional problem.

[richardnobes14 13]

Hmmm. to be honest I always use a C.I of 90 and I never experience slow descents.

That seems a bit excessive doesn't it? CI's between 15 and 45 are the norm

 

Same speech for the DES FORECAST, I have never fiddled with it but I have never had high speed issues.

The DES FORECAST page is not just something that you "Fiddle" with. It is an important part of your descent planning. It assists with descent point calculation and gives the FMS an insight into what lies ahead. This page should be completed for all flights prior to T/D

 

if I have to take a guess the STAR into the FMC might be not correct or ..

STARS are generic and are the same for all aircraft that use them. Unless the STAR has very steep above or below points they shouldn't effect the aircraft speed.

 

Remember that you can GO DOWN or SLOW DOWN but not at the same time! Choosing V/S over VNAV or FLCH during the descent will cause the aircraft to gain speed regardless of what speed you enter in the speed window, because you are GOING DOWN without SLOWING DOWN.

[Spin737 102]

 

That seems a bit excessive doesn't it? CI's between 15 and 45 are the norm

 

 

The DES FORECAST page is not just something that you "Fiddle" with. It is an important part of your descent planning. It assists with descent point calculation and gives the FMS an insight into what lies ahead. This page should be completed for all flights prior to T/D

 

 

STARS are generic and are the same for all aircraft that use them. Unless the STAR has very steep above or below points they shouldn't effect the aircraft speed.

 

Remember that you can GO DOWN or SLOW DOWN but not at the same time! Choosing V/S over VNAV or FLCH during the descent will cause the aircraft to gain speed regardless of what speed you enter in the speed window, because you are GOING DOWN without SLOWING DOWN.

I complete flights quite often with a CI of 90. If the dispatchers want the jet back quickly, they'll use 90. That said, I'll usually back the descent speed off to .80/300 or less.

 

The FORECAST page isn't make or break. It'll reduce your need for speedbrakes or throttles sometimes, but I've neve had a flight where I couldn't fix the problem. It's not a must do. It would be a REALLY good idea if you're in a jet streak at your TOD.

 

V/S won't automatically increase your speed. It will if the requested descent rate is greater than an idle descent would allow.

[richardnobes14 13]

I complete flights quite often with a CI of 90. If the dispatchers want the jet back quickly, they'll use 90. That said, I'll usually back the descent speed off to .80/300 or less.

 

The FORECAST page isn't make or break. It'll reduce your need for speedbrakes or throttles sometimes, but I've neve had a flight where I couldn't fix the problem. It's not a must do. It would be a REALLY good idea if you're in a jet streak at your TOD.

 

V/S won't automatically increase your speed. It will if the requested descent rate is greater than an idle descent would allow.

 

Fly however you want to fly, its totally up to you. If you don't like my advice then don't take it.

 

I simply offered the guy an answer to his question. Without asking for a debate.

 

V/S- The simmers favourite choice.

[EGLL77W 1,508]

220knts just prior or on downwind - 180knts on base - 160knts once fully established on then ILS. Start to reduce to your VREF 6-4 Miles.

 

I use that profile every time and works like a chime. It's pretty much standard into EGLL. However if ATC give speed then I comply with that.

 

I found the 800WL which is the only model I fly, doesn't like to slow easy. The speedbrakes are my best friend! Speed = Altitude & Altitude = Speed.

[davierosoft 13]

There were problems without previous service packs where few users ( I had with one of the first service updates) had problems with the idle, idle remaining too high.

Also, ngx simulates idle control (wich in flight will never be 20% N1 if engine is not shut down) but, to simulate it it varies thrust lever position.

I found in previous revisions that at a point of descent the throttle remains at a position (and will not move from that) until you reach a restriction when AT will move them. This is not related to the joy thrust position, as I always put it in idle.

The problems with sp1c are gone.

So, if you have a problem you're probably doing something wrong.

1) Check the DES FORECAST and fill it if you have winds, it will change your TOD accordingly to the winds.

2) Are you using Anti ice? If yes, it must be forecasted as the antiice will rise the engine idle and the plane will slow much less.

3) Aircraft model, the 800 is slippery.

4) Just to be sure, put always your thrust lever to idle, when descending in VNAV and the AT is in Thrust hold, you are free to move the throttles, obviously, it will be more than idle...

5) Des NOW is useless, it will start to descend at 1000fpm, with no idle thrust, then, will start to descent at the same rates of when you don't use it, unless you use VNAV SPEED mode.

 

Now, if the problem will not be solved with these, and above advice, so you must check your idle in flight and report to us the various idle at different altitudes. (add also if AI is on or off)

[simbio 56]

And also check your Throttle hardware and your setting like FSUIPC and Calibration that can bother NGX.