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Question about adjusting FMC descend speed

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Question about adjusting FMC descend speed

Old 3rd Jun 2018, 18:16
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Question about adjusting FMC descend speed

Good day,

ill be starting my 737 Type Rating very soon and have a couple of questions which came up during reading and trying it out at PMDGs.

1. Changing FMC descend speed—leading to change of the T/D point:

I thought that with increasing the Descend speed from eg 280 to 350 I would have my T/D point starting later since I lvl be having a higher ROD. But trying it out the FMC T/D started even earlier. Does the extra mileage needed for reducing speed effect the T/D more the higher ROD?

2. Visual Approach: FMC distance versus DME

typing in the visual approach as backup I start the turn to base around 6 nm to go...3 for base,3 for final for 1500ft AGL...read a book now which used the rule of thumb of starting it at the ratio of 1.5:1 (AFE/Miles) ...meaning that being at 1500 after this rule I start turning base at around 2.25nm, not at 6 as before..

does this shorter distance maybe refer to DME distance and does not include the the mileage needed for the turn..?

thank you!
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Old 3rd Jun 2018, 19:28
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1. Higher descent speed you'll have your TOD later, you're correct. Btw, Vmo on the 737 is 340 kias.

2. 300 ft/nm is a good approximation up close to the runway, say the last 10 nm track miles or so. 6 miles to go, you should be close to 1800 ft AGL.. Never heard of 1,5:1.

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Old 3rd Jun 2018, 19:34
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What do you refer to when saying track miles? 6DME or 6NM (Calculated route FMC)
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Old 4th Jun 2018, 10:23
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David, I used to find it helps greatly to draw a ring and a radial from the airport. Either the Runway or the 4 letter airport code, draw a line in the opposite direction (for example landing RW 24, draw radial 062 or whatever reciprocal of runway course is) to get a final approach line. Then a Ring 5 miles (for 1500' final) or 10 miles (for 3000' final). You end up with a ring around the airport and a long approach path line. It gives you something to aim at flying a visual circuit approach. I always used to stick to a 10 mile ring, you get used to the size and shape for different map scales. Even if you get heavily pre-occupied, just keep an eye on where they cross and be sure you can be down at 3000' and (or 5 miles /1500') and speed reduced at that point and you won't make a fool of yourself! Do this on EVERY visual approach. I even used to do this on almost every single approach, but I am retired 7 years now so don't listen to me!
Always keep checking the FMS descent with 10,000/30miles plus about 8 miles slowing and correct for wind with a few miles either way- sometimes those FMS descents can do weird things. The idea is to try and develop continuous descent skills for fuel, cabin comfort and noise, as far as fitting in nicely with ATC requirements. I sat getting furious with a copilot who dragged a 747 across London at 0430 level at 3000'- despite dropping hints. VS mode becomes very useful for this.
Good luck and enjoy the course! You have a lot of work ahead!
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Old 4th Jun 2018, 11:43
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What do you refer to when saying track miles? 6DME or 6NM (Calculated route FMC)
When you've got 6 nm left to fly. (you should be 1800 AGL). You can come in on a visual from any direction so the DME, there is an on-site DME, is not good for you anymore. Except if you're already aligned with the runway. The FMC distance will be whatever is programmed in the FMC. It's getting a bit academic now, a visual approach is primarily a visual maneuver and those who are experienced with it will develope good eye-balling. It will all make sense eventually, your first experience with visual maneuvering will be your base training. It's plenty of fun!
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Old 4th Jun 2018, 18:59
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The FMC calculates a descent path based on altitude and speed constraints at the End of Descent point. If you increase the descent speed it will not necessarily change the TOD as the geo path limitations lower down may be the restriction. Now back to basics, what happens when you increase the descent speed to a figure higher than the CRZ? what will happen at Mach Transition? how about the effect of TAI in the DES Forecast page on the N2 and profile? what happens if the speed is outside the cost index limits? Before you mess around with the "advanced factors" I would enjoy the course, stick to the procedures and don't become a "child of the magenta" When you suddenly get a series of green decel dots and msg "unable next altitude" or "drag required" this is when you know you forgot to check the route DATA and you are asking the aircraft to do things it can't. The biggest change to TOD will come with changes in the STAR or removal of restrictions lower down, not speed, this just temporarily displaces the magenta deviation while it tries to figure out what you are trying to do! as a rule of thumb for every 10 kts above Econ descent you should be 300 feet below the path, so in your scenario if you change from 270 to 350 to be "on path" you would actually need to show approx 2400 below the path on the magenta deviation. Of course you could mess around in the DES page and put 350 Kts, but is that really much use? your display will be scattered in green dots and eventually the VS will go to zero while it slows to where it needs to be.

Last edited by Avenger; 5th Jun 2018 at 08:16.
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Old 4th Jun 2018, 23:24
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Thanks for all the kind replys!

very good inputs here I sure will write down.

300ft per every 10 extra knots above entered FMC speed.. This is for the extra decerlrration I guess... but in fact with descending faster I ll be below the path anyway... or is this meant to be the Minimum i have to be below?

Alright... so dme distance is not a big help for visuals? Since it gives a direkt distance only...
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Old 5th Jun 2018, 08:26
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You created the "path", therefore if you put a speed in the DES page and use VNAV it should be on the "path" you created. If you leave it in VNAV with ECON DES SPD but use speed intervention the path remains constant and the speed increases, the green decel dots appear as needed. If you use LVL Change or VS with MCP speed, and a higher speed than the speed selected in the FMC, the path will creep up. Under these conditions the 300 per 10 KTS means when you then reduce the speed to the speed selected in the FMC to comply with the STAR restrictions the path will creep back to the correct profile. Typically, you may save 1 or 2 minutes only by descending at high speed, on the other hand, if you think you will descent into turbulent clouds you would be foolish to select high speed. The Boeing FCTM and Bulletins now put Turbulence speed as .76/280 climb and .76/250 as descent... and they are the experts..
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Old 5th Jun 2018, 09:14
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If you have an end of descent constraint that includes both alt & speed then alt is priority at the expense of speed.
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Old 5th Jun 2018, 09:48
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Whatever happened to LDW ( Lookout Da Window) approaches? Are todays pilots incapable of flying a visual pattern/approach without a map page full of lines/circles, have we forgotten about 3 times distance/altitude?
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Old 5th Jun 2018, 10:31
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Originally Posted by RAT 5 View Post
If you have an end of descent constraint that includes both alt & speed then alt is priority at the expense of speed.
VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots. In this case VNAV will fly a level segment, the path drops down and the message "unable next altitude" will appear.
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Old 5th Jun 2018, 11:14
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Originally Posted by zlin77 View Post
Whatever happened to LDW ( Lookout Da Window) approaches? Are todays pilots incapable of flying a visual pattern/approach without a map page full of lines/circles, have we forgotten about 3 times distance/altitude?
this is what I want to achieve very soon.

The 3 times distance rule works for me..but what I don’t get is that with the DME not helping here, since it is the direct distance to field only... I somehow need to have the Approx track miles being displayed to compare it with my alt... here again I need a proper path having entered in the FMC before again?
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Old 5th Jun 2018, 12:05
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David, you are overcomplicating a simple procedure, its a visual pattern, doesn't matter if there is no DME, in fact when circling visually the DME may be at the wrong end of the runway! if you must have "backup" simply put a 2.5 DME arc from the landing runway in the FIX page. Aim to be 1000 at 4 miles to stabilise and be fully configured wings level by 500 latest (ideal 1000). If you can't find 4 DME the simplest way is to start the base turn abeam the arc, then with the radius of turn by the time you roll out you will be about 4 miles, judge the turn and descent to finals using the HDG track line placed over the radius of the arc. If you make the arc larger your downwind will be too wide and you may not remain visual. Selecting a "DIR ICPT" in the legs page will not show the correct DME as when you are downwind you will be outside the capability of LNAV and the path/distance is nonsense. IF YOU REALLY NEED an accurate DME then select RW EXT in the FMC and a distance/ glide angle. This is a visual pattern, but all too often guys rely on the FMC for navigation and profile..not good airmanship.
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Old 5th Jun 2018, 12:34
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The 3 times distance rule works for me..but what I don’t get is that with the DME not helping here, since it is the direct distance to field only... I somehow need to have the Approx track miles being displayed to compare it with my alt... here again I need a proper path having entered in the FMC before again?
Yes, the approximate track miles is just that.... approximate. Think geometry. What direction are you coming in from? What's the runway alignment?. Distance to the onsite DME is one factor your mental computer, your brain, uses. As you're coming closer, you'll be visual with the field. Now think you're back to flying a C172 around the pattern. The difference is a B737 has better a glide angle than a C172, it's harder to get rid of excess enegy. If you feel you're high on speed and/or altitude - extend the downwind, head for a wider base or whatever is necessary.. If you feel you're good, keep it tight. It's fun! Our manual says wings level at 500 ft.

It'll come to you eventually. You wont be an expert from the get go. The base training will be a good introduction. Depending on airline you're about to join you may get many, or not so many, opportunities to try it out in the future. For the Type Rating you'll have other things to focus on.
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Old 5th Jun 2018, 13:03
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For visual approaches, I usually put the FAF/ALT in the DESC page so I can get a vertical bearing. No worrying about DME and Field Elevation + Planck's Constant/Rho - Relative Humidity blah blah blah.

If I can get to the FAF from any direction with a guide to the 3:1, then I can get to the runway at 3:1.
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Old 6th Jun 2018, 01:23
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Originally Posted by David1991 View Post


this is what I want to achieve very soon.

The 3 times distance rule works for me..but what I don’t get is that with the DME not helping here, since it is the direct distance to field only... I somehow need to have the Approx track miles being displayed to compare it with my alt... here again I need a proper path having entered in the FMC before again?

This seems a bit backwards.

I would be aiming to learn a proper visual approach first, then when you've got that nailed you can start adding bits and pieces to your nav display to aid the situational awareness you already have. DME is useful to have, but you must understand what it is telling you. A DME may be a couple of miles from the runway threshold so even if you are flying straight in, you can't just use 3 x DME distance for a profile. Remember also that you only need to get yourself to a final position, from there you should be descending on the PAPI or possibly the ILS glide slope if there's one available.

If you learn to fly a visual approach by programming and following an FMC track, what's going to happen when the visual approach gets sprung on you with little notice? You can't be heads down programming the FMC at 2000', you need to be able to transition smoothly to just looking out the window and using judgement.

More generally, I'd suggest that given you are starting a B737 type rating soon, you should probably not be trying to use a commercial flight sim to practice in advance. At best you will have better awareness of where the cockpit switches are but at worst you will learn a heap of bad habits that just need to be unlearnt. Your B737 endorsement is designed to teach people to fly a B737, you're best to let your trainers do their job at the appropriate time. In my opinion it would be more valuable for you to learn the SOPs that will be used in the sim. If this is your first jet transport rating you will soon find out that if your SOPs are no good, everything else falls apart as well.
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Old 20th Jun 2018, 12:38
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Higher descent speed, shouldn't the TOD moves up earlier since you would now need more track miles to lose the additional speed?
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Old 20th Jun 2018, 17:33
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Tango you are right. That's why in Airbus managed descent speed varies +/- 20kts. If the aircraft is high on profile it will dive at +20kts to get back.

Last edited by vilas; 21st Jun 2018 at 07:56.
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Old 21st Jun 2018, 03:51
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The 73 in Vnav will play about with the speed within reason to maintain the path.
In a properly designed aeroplane it will stay on the VNAV path AND stay on speed.
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Old 21st Jun 2018, 03:56
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Sure! There is a different terminology used. Descent and open descent. In descent it will fly the planned profile. Open descent is idle thrust descent at fixed speed which can be varied.
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