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Pro Member Trainee
CP16 Trainee

Hi
When I'm in APP, with VNAV mode on, I start lowering the flaps, but should I also slow my speed down? 😕
thanks

7 Responses

Pro Member Chief Captain
CrashGordon Chief Captain

While lowering the flaps will slow you down, you don't want to lower them at a speed higher than your aircraft's specs call for. There are then two things to be concerned with as far as speed is concerned. You don't want to stall short of the runway and you want to be just above stall speed just before touchdown. Actually, there are more than two things, but messing up either of these will make your landings,...umm...more interesting than they need to be. 😉

Pro Member Trainee
CP16 Trainee

If I going at 250kt in the VNAV mode and I lower the flaps, the speed dont go down....the aircraft mantain 250 kt....

Pro Member Captain
Bindolaf Captain

Hi

Flaps are used not to shed speed (at least not primarily), but to generate extra lift at lower speeds. If you deploy flaps (at least more than a couple of degrees) at 250 indicated, chances are you're going to lose them (at least damage them).

If I may offer some advice, by far not the best out there:

1) Start your descent early enough. A rule of the thumb is:

[(Current Altitude - Desired Altitude)/1000 *3]+10 (result in nautical miles)

So if you are flying at FL230 and want to descend to FL090:

[(23000 - 9000)]/1000 *3 = 42
Add the 10 nm and your grand total is 52 nautical miles. So you start your descent 52 nm out. If you're unsure, start even earlier.

2) Keep a reasonable descent rate. Depending on your aircraft, of course, but for jets somewhere between 1500 and 1800 feet per minute. That will ensure that your airspeed won't increase out of any control. Remember to throttle back enough, so you keep a constant descent speed. If you *absolutely* have to, you can deploy some airbrakes (spoilers). I hate doing it, but in some heavy jets it's inevitable, even necessary.

3) Check the aircraft specifications for the maximum flap speeds. For example in the ATR 72 (yea, I fly the 'props a lot 😛 ) you descend and as your speed approaches 180 knots you can deploy 15 degrees (1 notch). You know you can and you need to, because your aircraft starts feeling "heavy". It starts wanting to point the nose down, you have a hard time keeping your rate of descent.. all this means you need some flaps. Then, as you pass 170 knots you lower the landing gear and below 150 knots you may deploy the next notch, 30 degrees flaps. You see the airspeeds are low.

The speeds for the 737 are comparable, mutatis mutandis of course. Passing 200 knots you can start 5 degrees or 10 flaps, but not more (737 flyers will correct me here). At the minimum approach speed, around 134 knots I think, you have 30 degrees flaps. But not at 250 indicated.

4) Start flying Cessnas. Fly the lessons, read them. Get a feel for how "heavy" the aircraft feels when too low and no flaps. Get a feel for how airspeed increases a LOT when your rate of descent is too high. Get a feel for how fast your airspeed drops when you level off with throttles closed. Fly around a lot and try things. You will be rewarded 🙂

crosscheck9 Guest

Well Bindolaf, I haven't flown in a while, (waiting on a VGA card) but I would like to mention one thing. You said in your post that jets will often descend at 1500/1800 fpm, but I've never been able to do that - For example, flying the 777, I'm cruising at 0.84 Mach. It comes time for descent, so for the beginning of my descent, I slow my speed down to 0.72/0.70 mach, (depending on the aircraft weight, wind speed, etc..), but even then, the aircraft requires a 1900/2200 fpm descent. What am I doing wrong? I gradually reduce my speed, and then at about 14,000 feet, travelling at about 270 IAS, I disengage the autothrottle, and slowly and manually reduce and increase thrust to get below 250 IAS as time and altitude permits. I find that I can't keep the autothrottle engaged, because they don't slow down as fast as I require them to do so sometimes. Please reply with you suggetsions/comments.

Pro Member Captain
Bindolaf Captain

I'm not sure I understand. What do you mean "the aircraft requires"? What happens if you set A/T 0.74 for example and descent 1800 fpm? I think the aircraft should be able to keep the speed around 0.74. If not (and it goes higher) you can just make the descent shallower?

Or do you mean that the aircraft slows down? But I 'm not sure that's probable =) Let me say again, I am by no means an expert and I don't fly the heavies - don't like them very much. 737 is about my limit.

Pro Member Captain
John Hodges (originalgrunge) Captain

As an update on the descent rate for heavies, you're right, when you're at lower altitudes keep the descent rate in the 1500 ft/min range to ensure that you can make all the necessary flap speeds before you're on final (otherwise it can be a trick to slow down enough!)

But don't worry too much on the initial descent. When the ATC/FMC/Charts usually indicate the start of a descent, 2200-2400 is about average, although any jet can descend fairly comfortably all the way up to 4000+ thanks to cabin pressurization! I've seen a number of cockpit videos and photos that have the v/s guage in the 4-5000 region when still in the high altitude region, and it's not really an issue. Even most of the FMC's in flight sim aircraft will only give you a "high speed disconnect" on descents that start going over 5000 fpm. It's not even unreasonably uncomfortable for the passengers either.

(Although, with a descent like that you have to make sure you'll have reasonably long periods of level flight (like around 10,000 ft) where you can slow the plane back to 240 kts!)

It's okay to expect higher descent rates the heavier the plane is too, because generally they operate at higher speeds than 737s/Turbo-props, meaning you need to descend faster to cover the same height in the same distance!

Pro Member First Officer
Canyon (NoWorries) First Officer

One thing, if you lower flaps at mach 2, they will not go down, the same as if you lower flaps at 250 knots...the aircraft is equiped with hydraulic pressure relief valves, and the wind pressure on the flaps will open that valve, and the flaps stay up.

You will not "lose" them, as many people think.

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