open descent below FL100

[antonilo]

Hello,
I was told that in some airlines the pilots are not allowed to descent below 10000´ using open descent mode (thrust idle) and instead of that they should use a vertical speed. All that for safety reasons, but I´d like to know the exactly reason for the use of this technique. Thanks for your help...

[harry the cod]

Would seem very strange if true. Without wishing to sound rude, you may be getting a little confused or someone has fed you some duff gen!

What if you're high on profile? :uhoh: (What.....me?! :rolleyes:) Open descent with speedbrakes is the most effective means for losing height. Providing you havn't set 0000 in the ALT window, the A/C will not fly into the ground. Ironically, with Vertical speed you can fly away from a platform altitude into the ground whereas selecting FLCH would result in the Aircraft flying back up to the window selected altitude. Besides which,I could never think of a situation in which you wouldn't have an altitude selected in the MCP window anyway! Or Maybe i've been missing some threat for the last 18 years or so?

Now, I could understand if there was a restriction in using a mode such as V/S above 10,000'.

Harry

Disclaimer.
The above information is based on current Boeing Aircraft and no information contained herein has any connection with Aircaft manufactured by Airbus. Any similarities are purely coincidental and the Author takes no resonsibility for those that may be unfortunate to be flying such Aircraft at present. :{

[square leg]

I think you might be misinformed.

Companies will state something to the effect of: "Do not use OPEN DES below final approach altitude (i.e. after passing FAP or FAF depending on type of approach) but use SPEED mode (i.e. LOC-GS, TRK-FPA, LOC-FPA, APPR NAV)". Basically below 2000' agl you don't want to be using open descent unless you are looking for trouble. Trouble because of you might find yourself in a high energy state with idle thrust at low altitude and long spool-up times, possibly resulting in CFIT, so yes a safety issue.

If the airport is at 8000' elevation, what you heard is correct. And if you use VS below 10000' agl you will be safe, but run the risk of overspeeding if you don't actively control your speed (OPEN DES), when needing to get down quickly (i.e. winding up the speed to kill height=energy). Make sense?

Hope that helps.

I don't see the need for a VS restriction at any altitude. In fact more people should be using VS at low altitudes when approaching cleared levels in congested airspace, rather than relying on ALT* to do the work. There is already so much excess energy available when climbing small step climbs at low altitude. It saves you from performing potential TCAS manoeuvres, filling in reports, wasting engine life, disrupting pax comfort & scaring others. But... it is prudent to use OPEN CLB or CLB when at altitude.

[A330AV8R]

I second square leg on this one , when in a TCA in high density traffic you should ideally use V/S so as to not cause unecessarry TCAS alerts and also so that the autothrust doesnt bounce up and down . . . .

Yet at the same time if your high n hot on the 30 Open DES ALONG with full speed breaks get you there no question about it .. .

Having said that when ur below 2000 ' your in SPEED mode so as not to end up in a high energy state when so close to the ground
:ok:

[Admiral346]

Appoaching Moscow with sct TCU around a few years ago, I selected Open Descend while turning form downwind to base. We must have gotten increasing tailwind at that instant, as the plane went nose down and developed a descent rate of around 4000fpm. I'll never do that again...

Nic

[jonny dangerous]

Preference only: V/S below 10000 feet, and I encourage F/Os to do so. No SOP restriction against Open Descent at my oufit (BLO FL100).

With V/S, if you select A/P OFF, autothrust still supplying thrust for MCP spd: With Op Des (LvL Chg), much easier to forget YOU are controlling thrust (Autothrust ARM).

As well, in my neck of the woods, frequently receive "Descend to XXX, then slow to YYY": Selecting V/S after stable in Op Des (Lvl Chg), and then input the required MCP speed, and that's what you get at level off.

[tubby linton]

There is a restriction on using open descent during the final stages of the approach on the fbw.This comes from the Indian Airlines 320 that was written off doing just this many years ago.

[Capn Bloggs]

more people should be using VS at low altitudes when approaching cleared levels in congested airspace, rather than relying on ALT* to do the work.

In the 717, fiddling with the V/S near an assigned altitude is dangerous, as you may "bump" off the flare/capture and fly straight thru the altitude. Better to let the jet do the flare in speed hold mode, climbing or descending. That said, I have seen quite a times the Admiral's situation, where in speed hold, the nose drops alarmingly near the ground. If not on the VNAV path with speed hold, V/S is better.

Then again, the mighty Avro RJ would not pass thru the assigned altitude regardless of what you did on the MCP. That I liked.

Depends on the system you have...

[Dream Land]

No one I've ever flown with has impressed me with their skill using the V/S knob. :ugh:

[420]

yeah i couldn't agree more. i dont see any reason why wny airline whould restrict usage of open descent BELOW 10000. unless the elevation of the airfield is 8000 ft as stated above...haha;)

of course...perhaps maybe yr airline has imposed this rule due heavy and dense height constraints into most of yr destinations and there have been a number of times that yr pilots may have busted these altitudes or the "star steps". therefore...to stop this from happening then yr airline may have enforced the usage of the managed DES to adhere to the height constraints. not good of course as they should probably train them better rather than enforce such a ruling:ugh:

this of course is not an accusation pleaSE remember.
this is merely an assumption to add colour to this grey area:ok:

[Chuck Y]

I agree. I have no problems with using open descent. In fact I'd rather the guys use it since quite often they end up playing around with the v/s & getting high. No limitation except as is mentioned in the FCOMs.

[square leg]

I strongly recommend the use of OPEN DES at any altitude except from 2000' agl down to say 35-20' agl (depending on type and state of energy). At 35-20' agl I strongly recommend the use of OPEN DES... it might NOT say "open des" on the PFD, but something like THR IDLE... but if you don't "open descend" it, the landing might be long... and there is an inherent risk of being called a RETARD.

On a more serious note: OPEN DES is like converting to glider mode and controlling the ROD with speed through the elevator.

Rule of thumb (probably stated before):
- above profile (not glideslope) use OPEN DES
- below profile use VS (again, this depends on your energy state as well)

Everybody has their own style... some like to burn fuel, others like to preserve as much they can with the same level of safety (SA), economy, puntuality and pax comfort in that order (or possibly swapping the middle two).

small edit

[Castle Don]

I suppose this is overkill at this point....but just as a point of interest....on 2 turboprop types I have flown (note: No Autothrottles!), use of IAS HOLD was prohibited during any approach segment, and allowed only with caution below between 10000-5000'AGL....for the same reasons as mentioned already...extreme rates of descent and nose down pitch attitudes, which of course can be more acute on a turboprop due to the very high drag at flt idle.