Distance or Time before TOPD??

[Mamakim]

Hey Folks,

Just wondering if there is a preferred method when it comes to calculation of top of descent point. I tend to work it out by calculating how many feet I have to descend (say 5000') and then estimating my rate of descent (say 500' FPM) and after a few more quick calculation come up with a distance from arrival point to descend at and then I add a few more miles onto that for a buffer.

Other people I speak to work out there TOPD in minutes....fair enough but if your 1-2 mins off then this could be a problem coming in high and fast.

What method do most of you use out there. One or the other or both?? Interested to hear people's views.

Thanks

MK

[Capt. Queeg]

Preferred by whom, exactly? You the man now, do what you want.

Since velocity is a function of distance AND time, you are already using both of the two factors. It's just a matter of how you choose to express the solution.

Personally I look at where I want to land and point the nose that way. If it looks like it's gonna disappear underneath, I push the nose down lower. Sometimes I chuck the gear down to help out. I've never been criticised for this technique.


What i really want to know is, does power equal airspeed or ROD....... :rolleyes:


PS:then I add a few more miles onto that for a bufferLove it.....! :ok: :ok:

[Capn Bloggs]

does power equal airspeed
Depends! :} :{ :E :ok:

[scrambler]

-Use 500 ft /min ROD.
-If you are flying an average lighty (100 to 120 kts) 4 NM for every 1000 ft works (plus a 5 NM buffer).
-With GPS these days 2 Min per 1000 ft (plus a 2 min buffer) is also a good rule of thumb.
-Both work well.
-But as the Capt says, whatever works for you.

[Grease]

I don't know. Just press the managed decent button and the airplane does it by it self. New airplane no brain required. Profile as computer wants it.:sad:

[Chimbu chuckles]

Pressurised aircraft....3 x altitude = Distance out of TOPD.

Unpressurised aircraft....4x or 5x if aircraft is fast...whatever it takes to keep your ROD around 500-700'/min.

Add in a buffer for slowing/configuring....that is type specific but 1nm/10kts works for the 767.

When close in change to 3 x dist=alt.

It gives a slightly different figure which is more accurate...3x6000'=18DME or 3x18nm=5400'..3 x alt has a little buffer which stops you being low in a jet....low is bad....too high is worse but you can manage that as long as you recognise it early.

3 x dist should also be adjusted for position of DME or waypoint...base that on a 300'/nm profile (all aeroplanes...actually to be perfectly accurate should be 320'/nm...too hard so just use 300).

6000' (6080'=1nm)runway with DME site at far end would be 3x dist-300'...GPS waypoint midfield would be 3x dist-150'...all assumes msl airfield....add elevation to your position correction. For instance airfield elevation 800' with GPS waypoint half way along a 2500' airstrip would be 3xdist-100' + elevation. so 3xdist+700'

Simple when you get the hang of it....and easier to keep track of on descent....every 1000' simply multiply by 3 and check you have your buffer intact...in the 767 at say 15000'/320kts...55 track miles required....3 x alt + 1nm/10kts (slowing from 320 to min clean). 14000' would be 52 track miles (note it ends up being 3nm/1000'....at 6000'/250kts you'd be looking for 21 track miles on the FMC.

Without an FMC you just use you DME/GPS and adjust a little to allow for the fact that the dist reading is present position direct the aid...and you are not doing that...with experience you can eyeball it and be surprisingly accurate....that's the difference between DME/GPS distance and track nm to run to touchdown.

To really look clever in the 767 and do a proper CDA with thrust levers at idle it works better if you do 3xdist+.8nm/10kts....this is fine tuning when you're used to the aeroplane and not so ****scared of ending up high and fast....15000' would then be 53nm instead of 55...makes the difference between spooling up in level flight right near the end around GS capture or capturing the LOC and GS almost together and in a constant low drag, flight idle descent. :ok:

[turbantime]

I don't know. Just press the managed decent button and the airplane does it by it self. New airplane no brain required. Profile as computer wants it.

Most GPS units these days have VNAV profiles that you can plug in and it all works nicely. :p

[Chimbu chuckles]

Great if you like being a passenger....the 767-300 has a LOT cleverer VNAV than you'll see in any GPS but if you don't do the math and adjust accordingly you won't end up looking very clever.

[rmcdonal]

Depends if freight or pax. Freight i just push the nose down so it lines up with the target (normaly the runway) and use the turn onto downwind (load up some g) to slow down. With pax I work out a 500fpm ROD + 2 miles S+L for slow down. :ok:

[halas]

Used to fly with a bloke who used the PAX head count as top 'O' the drop in DME distance. Some times it even worked out!

halas

[VH-XXX]

I prefer to chose my descent point then leave it 1-2 minutes later. That way I'm pretty sure I'll be at VNE when I either overfly at 1,500ft or join cross-wind at 1,000ft.

There's nothing quite like an overfly at 1,500ft at 200+ knots, then a quick tight! turn onto crosswind. I love to nail the crosswind threshold join at 1,000ft. Looks good at fly-ins!

[Sunfish]

I use an even better technique. I just arbitrarily select an arrival time and push the nose down till the ETA on my wrist GPS says the same time. Of course you have to have the GPS set to read in 24 hour time mode, or you could be out by twelve hours:E

Actually I like to be at overfly height a few minutes early with everything put away, fly zipped up, ERSA read, frequencies set, ATIS, etc.so that all I have to do is think about traffic, joining, landing checks and how I'm going to go around if I stuff it up and brief any passengers accordingly. 500 feet per minute in a cruise descent seems to work.

[Capt Fathom]

...and as the manifold pressure increases on descent, rather than pull back on the throttles, just increase the RPM to keep it in check. That way, you'll hit the circuit area with everything firewalled.. :ok: :E

PS...better make that 6nm per 1000 feet!

[18-Wheeler]

In the 747 we take into account the cruise altitude, weight, head/tailwinds, any limitations for the descent ...
... then go down at 110 miles. :)

[puff]

Used to fly with a bloke who used the PAX head count as top 'O' the drop in DME distance. Some times it even worked out!
halas

halas this bloke wasn't on 146s at AN was he?. I was upfront with this bloke once and he calculated the TOD on the POB, he said that on the 146 it worked great guns until he had to revise when he had less than 50 POB :)

[rmcdonal]

Capt Fathom You could make a game of how many Red lines you can exceed before reaching the circuit. Oil, MP, RPM, Airspeed etc. And then try and beat that score the next day. :ok: :E :}