Descending...descending...descending

[sayersb]

Hello all, apologies if the question is a silly one!

I file an IFR flight plan from Heathrow to JFK using GPS in the default 747. I obey all of ATC’s instructions during taxiing, take-off and climb until I reach my assigned altitude at FL350. I travel at 250 KIAS to 10,000 feet, 340 KIAS to 25,000 feet and then MACH 0.85 to cruising altitude. At this point the autopilot is engaged, 35,000 is tuned into the ALT window, NAV/GPS is set to GPS and I have tuned MACH 0.85 as my cruising speed. So far so good. Sit back and relax.

As you will know the major part of this flight is boring, save for acknowledging ATC handoffs along the way and doing some research for the approach!

About 125nm out ATC tells me to descend and maintain, say, FL260. I tune in 26,000 to the ALT window and the aircraft begins to descend at -1800 feet per min. no problem. I don’t adjust speed at this point. Once FL260 is reached ATC throws a few turns at me so I punch them in to the HDG window, that’s fine. Almost immediately ATC tells me to descend and maintain FL140 – this is where it gets interesting. I program 14000 in the ALT window and the aircraft does descend but incredibly slowly, even though -1800 is still showing in the vertical speed indicator. After a few seconds I get the dreaded message from ATC “Please expedite your descent”. Now I’m a bit lost because sure I can increase the rate of descent but of course my speed correspondingly increases – I really want to get the speed down not up!

The end result of all of this is that I arrive either too high or too fast (or both) to intercept the localizer. I must be doing something very fundamentally wrong so hopefully you guys can point me in the right direction. Is it my speed? If so, how should I configure this through the autopilot to control my descent properly? My aim is to fly the takeoff and landing but get the AP to deal with the climb, cruise and descent.

Love this sim by the way.

Many thanks.

[NikeHerk67]

Hello all, apologies if the question is a silly one!

I file an IFR flight plan from Heathrow to JFK using GPS in the default 747. I obey all of ATC’s instructions during taxiing, take-off and climb until I reach my assigned altitude at FL350. I travel at 250 KIAS to 10,000 feet, 340 KIAS to 25,000 feet and then MACH 0.85 to cruising altitude. At this point the autopilot is engaged, 35,000 is tuned into the ALT window, NAV/GPS is set to GPS and I have tuned MACH 0.85 as my cruising speed. So far so good. Sit back and relax.

As you will know the major part of this flight is boring, save for acknowledging ATC handoffs along the way and doing some research for the approach!

About 125nm out ATC tells me to descend and maintain, say, FL260. I tune in 26,000 to the ALT window and the aircraft begins to descend at -1800 feet per min. no problem. I don’t adjust speed at this point. Once FL260 is reached ATC throws a few turns at me so I punch them in to the HDG window, that’s fine.

You must have the easy settings and ignore crashes and stresses and unlimited speed set in the realism settings as .85 mach is too fast for FL260 (26000') when descending. When you start the decent from FS350, slow to about .78 Mach. If you have auto throttle engaged it would automatically throttle back to just about idle. Keep your eye on the KIAS as well, and if it's approaching 340 knots you will soon be over speeding. At .78M, as you descend, the IAS will increase because of the density altitude change. At FL260 you're definitely over speeding at .85mach. Change the read out to IAS at about FL290 and keep an eye on the IAS. I'm not sure what version FS you're flying, but sometimes when the aircraft starts to descend very slowly, if you just tap the vertical speed selector it will a adjust to the correct that. It seems to hand up at times in some aircraft in some versions.



Almost immediately ATC tells me to descend and maintain FL140 – this is where it gets interesting. I program 14000 in the ALT window and the aircraft does descend but incredibly slowly, even though -1800 is still showing in the vertical speed indicator. After a few seconds I get the dreaded message from ATC “Please expedite your descent”. Now I’m a bit lost because sure I can increase the rate of descent but of course my speed correspondingly increases – I really want to get the speed down not up!


Controlling Rate of Decent:
The 747 is heavy and does want to over speed on decent if you've not real careful about descending. Until you get used to descending in the 747, I'd recommend an IAS of no more than 300 Knots below about FS270.

Speed brakes (spoilers):
When descending you can always slow down by using the speed brakes by pushing the / key. The speed brake control is also located on the throttle control quadrant if you want to see what happens when you push the / key. Also on the outside view, check the topside of the wings. The wing view will make sense when you view it.



The end result of all of this is that I arrive either too high or too fast (or both) to intercept the localizer. I must be doing something very fundamentally wrong so hopefully you guys can point me in the right direction. Is it my speed? If so, how should I configure this through the autopilot to control my descent properly? My aim is to fly the takeoff and landing but get the AP to deal with the climb, cruise and descent.

In order to maintain <=250 knots under 10,000 feet, you will probably need to use the speed brakes from time to time. By the time you're ready to acquire the localizer, the speed shooed be no more that about 220 knots or even 200 knots. On the kneeboard, check the reference page (bottom button) and check the maximum speeds for the flaps. Also the VREF for landing configuration. As you set up to access the ILS (localizer then glide slope) arm the spoilers by hitting (Shift/). Once you touch down they will automatically deploy.
Now that you're at a good speed check the landing speeds for the 747 as well. I'd say at first, land at around at or above 150knot minimum until you get used to it.

I hope this helps, it's a bit long winded.

[sayersb]

What a great response, thanks very much. And no, it wasn't long-winded it was just what I needed.

By the way I run FSX, forgot to mention in the original post.

Once again, I'm very grateful for your time.

Happy flying!

[alexm]

Descent rate from cruise level should definitely be more than -1800 fpm. 3000-3500 fpm initially at idle thrust, using speedbrakes if necessary. You will want to decrease the descent rate somewhere around FL240 to 2200-2400 fpm.

FS ATC tends to wait a long time before giving descent instructions. One way to get around this is to plan your descent yourself. Use the 3:1 rule. Example: if you are at FL320, and destination is 2,000, you need to lose 30,000 feet, so start your descent at least 90 miles out (3 times 30 thou. = 30). You can request flight level changes from ATC to ensure you have plenty of distance to get down to where you need to.

Be at 240kts about 12,000 agl, and keep speed there (below 250 for sure) until approach. Initial approach at 225 kts, adding flaps when appropriate, then continuing as Herk says.

[sayersb]

Another great response, thank you.

Following the excellent advice from my friends I successfully negotiated a descent into JFK in the default 747, intercepted the localiser and captured the glidepath. It's just a pity that I was subsequently responsible for a number of fatalities via a horribly botched landing - but I can deal with that, it's just an issue of more practice being required...

[alexm]

Feels good when a plan comes together, doesn't it? Congrats on learning to manage your descents!

[NikeHerk67]

Feels good when a plan comes together, doesn't it? Congrats on learning to manage your descents!

Thanks Alex,

I second that....couldn't have said it better! :)

Yes!!! sayersb

You'll most likely always remember that 1st successful flight!

Regards both

[vanjah25]

Hi, my name is Vanjah and i just read ur statement and i know your answer. ok let say you are at a cruising speed of 35000 feet (FL350) and you've been given an order by ATC to descend to 23000 ft. lets say that you are using AP and you punch in 23000 ft. it will take you down to 23000 feet at a vertical speed of 1800 feet/min. about 600 feet before reaching 23000 feet, the AP will slow your descent down to about -700 ft/min because you are reaching 23000 feet (which is what u punched in). so lets say when AP reaches 23,500 feet it will slow down descent. you dont want to descend too fast when you are near ur aquired altitude. the closer AP gets to 23000 feet, it will slow down descend more. now, if it's slowing down descent and you punch in another altitude before it sets on 23,000 feet, it'll continue descending at about 700 ft/min(though it says -1800 ft) until it reaches the newly punched altitude. so to make it go faster just change the vertical speed from the -1800 to -1700 and back to -1800 and notice how it changes from about -700 feet to -1800 ft/min. u just have to refresh it's memory. if you know what i mean.... end of topic............................--------------



Now, let me help you plan your descent. you are at a cruising altitude of fl3500 ATC says to descend to 26,000 and you're 17 nm from destination and you're cruising at 515 knots which is the cruise speed of the boeing 747 you are flying. when you are descending to 26,000 feet just reduce speed to about 410 knots. and when the ATC says to descend again to 14,000, reduce speed to 280 knots, beause you have to be close to 250 which is the speed below 10,000 MSL. now the ATC says to descent to 4500 feet, you reduce power to 240 knots and attflaps at about 5 degrees and then the ATC says to descend to 2300 feet you reduce speed to about 180 or 200 knots and etc until you land.