Slowing down to 250kts

[Guest meyomyx]

Fantastic!Not going to argue with that - how can I? Input from you 737 drivers is worth a lot. Well to me anyhow.So you're probably the best to judge - I'v never considered slowing the PMDG 737 to be a problem - but then I've always reckoned on VNAV (or any othe descent method) holding on to a very low rate of descent at 10,000 feet until 250k is pegged.My Uncle (God rest his soul) was a senior Captain with BA (BEA) flying Trident 3. He was one of the few to be authorised to fly into Gibraltar in the Trident and always said you needed to get the reversers up and in motion before you got the thing on the ground.Actually - I've just studied the Trident 3 training manual and interestingly, they make reference to speed brakes and lift dumpers - though the operation and principal seems identical to that of the 737.Ho hum. "Speed Brakes" on the lever? Well, Boeing are American so I wouldn't take TOO much notice of that hehehehe. Go on. Tell me you're American. :)Thanks - keep it coming

[Guest boxjockey99]

Nick Alas I am not american but flying B737's for an airline based not a million miles from where you live although I am actually based in Leeds (have you guessed who we are yet? LOL)The B737 is slippery but no more so than my previous aircraft (the Fokker F27 - another clue as to the operator). The key is proper planning of the descent, VNAV is great but you may be relying on it a little too much, airmanship dictates you keep in mind what the computer is trying to acheive and then monitor that so if the VNAV profile is wildly off the mark then be wary of it.On our descents on the -300 series aircraft we often see the descent rate drop from 2500fpm to 500fpm as the aircraft slows through 10,000ft. The trick to flying this aircraft is energy management, live by one rule - never go down and slow down - you'll get naff all of either if you do. If you want to slow down then level off and the speed will bleed off quickly (assuming this is OK with ATC). We often use speed in the descents to help us, for example when cleared to a level say FL70 and you get there and have to actually level off when your VNAV is telling you to go down, wind the speed back to 210/220kt then when recleared further wind the speed back up to 250 or if cleared for high speed all the way up to 310kt, putting the speed brakes out and accellerating in LVL CHG causes a high descent rate thus allowing you to chase the profile where needed. This also is more economical as the thrust levers are back at idle more. There are many tricks to the B737 and you simply have to know which ones to use in each circumstance but the best advice is to ensure you plan ahead and know what is going on before the computer starts to get ahead of you. if you know that there is an ATC alt restriction then put it in so you have a more realistic VNAV profile, if you know you'll be given a shortcut try to pre-empt the descent slightly and thus you won't find yourself too high for the approach!From a technical point of view the B737 speedbrake/spoilers fulfill 3 roles in normal operation. As discussed they act as speed brakes in the FLT Detent and lift spoilers when fully open in the GND DETENT but they also provide high speed roll assistance to preven an aerodynamic peculiarity called adverse yaw which as I recall is found in the transonic speed ranges (M0.6 to M0.99ish) in which the ailerons alone would actually produc a turn in the opposite sense ie rolling right would cause the aircraft to actually turn left...wierd I know. The spoilers assist in this problem and so you will see the spoilers deploy even in normal flight with the speed brakes DOWN. Something to bear in mind in a crosswind takeoff however if you put the aileron into wind too much you will get the into wind spoiler deploy also which will make control once airborne rather more sporting than normal! Finally I have to underline my expeience is on the 3/4/500 series of the B737 and so I cannot accurately comment on the more intricate flight characteristics of the NG. I know the NG has a lower profile more critical wing which would make it slightly more slippery but one assumes that the VNAV profile takes that into account! Happy trailsKris PS of on holiday Sat so apologies if I don't get back to the forum for a while

[Guest meyomyx]

>Alas I am not american but flying B737's for an airline based>not a million miles from where you live Thank the Lord!>VNAV is great but you may be>relying on it a little too muchActually, Kris, the reverse. Brad (Zapper) advocates extensive use of VNAV (but then he flies 'em) whilst Tim M advocates judicous use. I favour the latter for a variety of reasons.>your VNAV is telling you to go down, wind the speed back to>210/220kt then when recleared further wind the speed back up>to 250 or if cleared for high speed all the way up to 310kt,>putting the speed brakes out and accellerating in LVL CHG>causes a high descent rate thus allowing you to chase the>profile where needed. This also is more economical as the>thrust levers are back at idle more. Interesting>assistance to preven an aerodynamic peculiarity called adverse>yaw which as I recall is found in the transonic speed ranges>(M0.6 to M0.99ish) in which the ailerons alone would actually>produc a turn in the opposite sense ie rolling right would>cause the aircraft to actually turn left...wierd I know. The>spoilers assist in this problem and so you will see theIndeed. I once had a big adverse yaw problem on a scale PIK20 sailplane. I ended up modifting the aileron travel so that only the upgoing aileron moved. It worked - though the roll rate was less than amusing!>PS of on holiday Sat so apologies if I don't get back to the>forum for a whileBe Mostly Inactive on your hols Kris - after all, harmless acronyms are more comman than you think! :)Cheers!

[Guest boxjockey99]

When I said you may be relying on it a bit much I meant that some people take what VNAV tells them as gospel and don't actually check what it is doing manually. We make more use of VNAV on the 300 series than any other airline in the UK but we are heavily advised to keep a mental track on descent profiles simply because 'garbage in = garbage out'. unless you catch an error early then you will find it very difficult to fix and that can get quite embarrassing not to mention bringing the 2 winged master race into disrepute .. LOL!I intend to do as little as possible on hols but I have booked a PADI dive course so I am going to have at least 2 days of over-exertion! Happy trails allKris

[Guest cfm56]

Has anyone ever though of solving this problem by simply begin descent a little earlier. Ive seen a few videos where the pilot actually begun decsending bfor the top of descent by hitting the "des now" key in the descent page of the fmc. Also heres a cool formula that is used in the real world: Crusing Altitude-Target Altitude=Amount of ft to descend....Amount of feet to descend x 0.003the 1 in 3 method is a useful rule of thumb. '000 of feet to descend/100 x 3 = nm from destination to commence let down. So to go from 31000 to 1000, you would start at 90 nm. Jeff And for descent RATE use groundspeed times 5. 400 knot ground speed = 2000 feet per minute.Im not sure, but the far/aim says that a useful rule of thumb is that u want to be a 10000 some 30 nm out so i guess we can all put 2 and 2 together and see what we get.

[Guest boxjockey99]

CFM56I think you are supposed to sign your name after every post but I am not a moderator so I will hold off the firing squad this time! As to your post, you are correct although you have perhaps overcomplicated things a touch. The way we do it on the -300 is:actual alt - target alt = amount of ft to descend as you said3nm per 1000ft = number of miles requiredNow add 10nm to that to slow down! Descent rate is not really needed as the computer will help you out here! bear in mind that V/S mode is not really a mode we like to use on the B737 as it has no speed protection as LVL CHG and VNAV do. Simply use the green bannana on the Nav display to see where you'll be levelling and if it is too short of the desired point reduce the speed if it is beyond the desired point increase the speed! The key is to know what the modes do and they are much simpler than people think! LVL CHG - closes the thrust levers in descent and pitches for the target speed! (thats all it will ever do!!)VNAV PTH - Closes the thrust levers in descent and pitches to maintain the VNAV PATH regardless of target speed (will not exceed VNE/MMO though)VNAV SPD - Closes the thrust levers in descent and pitches for the VNAV target speed in the DES page. So you see bearing this in mind you can decide what you need to do:eg. If you were at FL330 and told to descend to FL100 by a certain point 90nm away: you know you need to loose 23,000ft = 69nm +10 = 79nm In VNAV PTH the profile will show a descent point a few miles up the road so you can either go DES NOW and descend at 1000fpm till you intercept the profile then VNAV will pitch to chase the descent profileIn VNAV SPD the aircraft will wait till the T/D point then desend at that point pitching for the Target speed and ignoring the VNAV profile. In LVL CHG the thrust levers will retard and you use the speed window to vary the descent rate ie faster for a higher descent rate, slower for a lower descent rate. simply adjust the speed till the green bannana hits the desired level point then adjust as required. In V/S use the V/S window to place the bannana on the desired level off pointThis is all trial and error I'm afraid but it will come if you stick at it. just know what the modes are actually trying to do and you are laughing!Hope that helpsKris

[Guest autothrottle]

I've seen in a video (Just Planes Hapag Lloyd Express 737) that the pilot started the descent at the T/D but he used LVL CHG with mach speed 0.80.He said that it's to catch up some time.