Boeing VS mode: 2nd segment ROC selection

[rhovsquared]

what's up have a question thats been really difficult to resolve with the info i have:
What is the "best" way to select a vertical speed in the afds in order that: 1. high enough so that pitch attitude is compatible with selected climb power so that you don't lose speed dangerously? 2. what data in the afm/fcom etc is useful in making this selection or is it a simpler matter? 3. Would there be negative performance at any time if an engine is lost and too high a value were selected and is this even a major consideration in the second segment?4. what about climbs at high altitude if buffet boundaries etc are compatable with that intention and the pilot doesn't wish to use FLCH so as to avoid a repeat of the Pinnacle Airlines-type disaster any guidance?

I know i've asked a mouthful and any replies would be most helpful..still saving for that 757 TR one day...one day ...not even to be a pro just to have a big jet TR on the ticket:ok:

[Jet Man]

Are you talking about a low level alt capture so the pitch mode changes to ALT ACQ and the thrust is reduced? If so just bug up to min clean speed. Otherwise leave it in TOGA or MCP SPD (if autopilot is engaged).

At high alts I always use level change (= Mach hold) - I would not use or condone the use of V/S at high altitudes even with the inbuilt protection provided.

Hope this helps.

[None]

3. Would there be negative performance at any time if an engine is lost and too high a value were selected and is this even a major consideration in the second segment?4:
How to accomplish a takeoff and departure is in your Flight Crew Training Manual.
You refer to 2nd segment climb, which is immediately after takeoff. At this point vertical speed is not being used. Also at this point, the autopilot is not being used.

[rhovsquared]

None: the fcom gives this as the following guidance under "automatic flight procedures "The Autopilot is certified to be engaged above 500ft. When the initial climb is established, ensure the aircraft is established on the desired speed and select an autopilot to command (CMD). This causes the autopilot to engage in the Vertical Speed (VS) and Heading Hold (HDG HOLD) modes.

If the vertical speed at the time of engagement has not been established and is momentarily high, it may be necessary to select a lower, more suitable figure in the VS window in order to prevent a loss in airspeed" i hope that clarifies my reason for posting.
Jetman: i did had a hunch that VS mode is not the best thing to use at high altitude i think some say VS makes for smoother climbs and descents, but it must be watched and the manouver skillfully executed

thankyou all rhov....more please:D

Jetman i know the boeing afds's are similar that's why i posted as such, but i think ALT CAP is automatic on the 75 and doesn't have to be selected on the MCP sorry I forgotten to respond earlier but very intersting reply...thanks

[None]

None: the fcom gives this as the following guidance under "automatic flight procedures "The Autopilot is certified to be engaged above 500ft. When the initial climb is established, ensure the aircraft is established on the desired speed and select an autopilot to command (CMD). This causes the autopilot to engage in the Vertical Speed (VS) and Heading Hold (HDG HOLD) modes.
If the vertical speed at the time of engagement has not been established and is momentarily high, it may be necessary to select a lower, more suitable figure in the VS window in order to prevent a loss in airspeed" i hope that clarifies my reason for posting.
:D
Are you flying a desktop PC?
It would be a rare event to see any pilot engage the autopilot at that low of an altitude. While the aircraft is certified to allow engagement at low altitude, it is not flown this way on the line.

[Check Airman]

Are you flying a desktop PC?
It would be a rare event to see any pilot engage the autopilot at that low of an altitude. While the aircraft is certified to allow engagement at low altitude, it is not flown this way on the line.

I've seen a number of vids, where the AP is engaged shortly after gear up. Hardly before 1000ft.

[rhovsquared]

no i can't even taxi a piper warrior on a flight sim program and i almost invariably get lost and crash land. And except on strict ILS can't land. but believe me the real airplane presents no problem for me; i actually can fly i want a jet type rating, to heck with a simulator other than a level D six axis simulator fully certified within the bounds of a quality TR course from a good program is my goal. but i'm injured( herniated disc C6-C7)for now don't have the cash for that right now nor acess to all of the proper data. SOPs,oeisids, RTOW's, QRH's, MELS etc so i come here TO LEARN. furthermore i have proper AFM's for boeing 727-200 and 747-200, but am relying on an internet source for 757 info unfortunately I can't buy a real one right now, i haven't yet posted a question about 747-200 or 727-200. lastly the boeing-757-200 source is from the afm stated elsewhere about min-acceleration altitude and restated basically the same autopilot thing i know such engagement is forbidden by most SOP
"At or above the Acceleration Altitude (Aa) reduce the climb rate by selecting VS mode on the MCP and a vertical speed (normally between 1,200 - 2,000 fpm, depending on aircraft weight, altitude and temperature) which will allow the aircraft to accelerate through the flap retraction schedule to the appropriate climb out speed.

Acceleration altitude is normally 1,000ft AGL. Should obstacle clearance, noise abatement etc., necessitate a higher Aa, a note will be found in the Performance Manual on the take-off page for the airfield and/or runway in use. "
hence my question i want good guidance for selecting a value in the VS mode i find the above entry a little vague i also feel that there may be pitfalls as there always are in aviation with this that i'm ignorant of and i love details in the afm:8
once again thanyou for replying

[mbcxharm]

Well if you're talking 757s then now I feel I can post!

The only time our SOPs call for the use of VS mode is for acceleration and flap retraction after an engine failure, i.e. at min accl. height we call for VS +200fpm or lower depending on weight.

As you seem to imply for your original post you are aware of the limitations of VS with regard to speed control etc, which is why it's really not appropriate just after take off.

The standard take off profile from the 757 flight crew training manual (FCTM) is as follows:

400ft AGL: Select roll mode, e.g. LNAV, HDG SEL, HDG HLD.
Acceleration height (2 engine): Select VNAV
Acceleration height (1 engine): Select VS 0 to +200fpm

The autopilot is then normally engaged at some point after these events have occured and so VS mode is never selected by engaging the AP.

I have heard of some company SOPs which called for the selection of FLCH at fairly low altitude so as to enable to autopilot to be engaged soon after the 500ft that you mention. The selection of FLCH (and subsequent speed selection on the MCP) would avoid the problems associated with VS at that stage.

[rhovsquared]

thankyou mbcxharm: i find that reply extrodinary helpful i must say that VS mode on all of the boeing with which i have some familiarity seems to be the most enigmatic mode i figured it was best not to to fool around with it too much.

rhov

[junior_man]

Only way it would go to VS upon AP engagement is if the takeoff was without using the flight directors. If you take off with the FDs on, and engage the AP at 500' you will be in TOGA mode, not VS.

To accelerate you bug the speed up. Use of VS instead of that will allow the A/T to reduce power from TO thrust which is not what you want in an engine out situation.

Vertical speed is seldom used in jets. About the only time is if ATC gives you a small altitude change like 1000 or 2000 feet and you don't want it to go the climb thrust for a small climb. Also used on non precision (dive and drive) approaches to descend to MDA.
Use of VS in most transport jets will have the airplane attempt to maintain the VS at the expense of maintaining speed. Not a good thing.

Most climbs you want to climb at climb power and a certain speed, Level change (737) Flight Level Change (757, 767) or Open Climb (A 320) will give you climb thrust and use pitch to maintain the selected speed. VS will maintain the selected airspeed. If below what can be acheived at climb thrust, it will be at reduce thrust and selecet speed. If the VS exceeds the climb rate the aircraft can achieve at climb thrust limit, it will trade airspeed in an attempt to maintain the VS, NOT a desirable thing.

[rhovsquared]

thanx junior man ----sorry been away from the computer a while didn't want anyone to feel their words were wasted on me as they weren't...love this stuff:8

[richjb]

Interesting discussion. I have a related question

I fly the Dassault Falcon 2000. At heavy weights, selection of FLCH or VNAV programmed speed after takeoff results in a very shallow climb rate during acceleration to set altitude, down to the minimum limit of 250 FPM. This especially true when departing an airport in Class B airspace and accelerating immediately to 250 kts. The resulting low altitude acceleration can get quite exciting if selection is made less than 1,000 ft.

With that in mind, may I inquire as to:

1) What is the normal airline all-engine acceleration height?

2) Does selection of FLCH or VNAV mode at that altitude provide sufficient rate of climb for terrain clearance or is V/S mode needed to get the aircraft climbing faster.

3) Do some airports require different procedures? For example Las Vegas.

4) What altitude is the A/P normally engaged in the FLCH or VNAV mode?

As a corporate pilot, we're not always aware of these standard procedures.

Thanks,

Rich Boll
Wichita KS

[None]

1) What is the normal airline all-engine acceleration height?

1,000 AFL

2) Does selection of FLCH or VNAV mode at that altitude provide sufficient rate of climb for terrain clearance or is V/S mode needed to get the aircraft climbing faster.

Yes. Climb power and +10 degrees pitch gives 1,000-2,000fpm depending on weight.

3) Do some airports require different procedures? For example Las Vegas.

Yes, and special pages are published for non-standard clean-up altitudes.

4) What altitude is the A/P normally engaged in the FLCH or VNAV mode?
Depends on the pilot. The lowest is around 10,000, while some fly up to cruise altitude. The average would be around 18,000.

This is what I see at my company. Techniques at other airlines may vary.

[Pilot Pete]

Vertical speed is seldom used in jets

I think that is a bit of a blanket statement and depends largely upon SOPs. We regularly use V/S to prevent unwanted TCAS RAs (by reducing a rate of climb or descent to <1000fpm) when proximate traffic is around and regularly to fly a CDA. Nothing wrong with it if you understand it's limitations.

PP

[alexban]

Selecting VNAV-----Above 3000' AGL (Boeing FCTM)
While in heavy traffic,or sometimes in bad weather,we select ap on as low as 500'. The ap engages in LVL CHG ,not in VS. At least on the 737.
We use VS when approaching cleared level,to avoid RAs..
Regarding acceleration altitude,well,not bellow 1000'.Depends on noise procedure,or airport characteristics.First select climb power,then flaps up speed.This will produce no less than 1000'/min ROC,while accelerating and retracting flaps.After clean configuration VNAV is selected (above 3000' AGL),the plane is accelerated at 250 kts,ROC >1000'/min ,in about 3-5 NM,after which it climbs with ROC 2500-4000'/min (depending of weight).

[SR71]

While in heavy traffic,or sometimes in bad weather,we select ap on as low as 500'. The ap engages in LVL CHG ,not in VS. At least on the 737.


Alex,

The version of the 733/4/5 FCTM I have suggests the AP is only FAA certified to allow engagement on TO above 1000 AGL.

Has that changed? Or its just that your SOP's allow that?

I've never tried engaging the AP before clean up (although I suppose on a Noise Abatement departure it is quite legitimate) but I imagine the annunciations go from ARM or THR HLD/TOGA to MCP SPD/LVL CHG with the speed window opening at V2 + 20?

A reduction in climb thrust is prevented if LVL CHG is selected (or AP engaged) until 2 1/2 mins has elapsed from TO unless N1 is selected, VNAV engaged or you ALT ACQ/ALT HLD.

Anything else would make a mockery of the performance...

Is that correct?

[Jet Man]

On the B737 NG the autopilot can be engaged at 400' agl. Engaging the autopilot will change pitch mode from TOGA to MCP SPD (=LVL CHANGE) and the roll mode will default to HDG SEL unless LNAV has been previously armed (a roll mode should be called for/confirmed at 400' agl).

Engaging the autopilot has no effect on thrust. Thrust will change from takeoff thrust to climb thrust when N1 is selected OR the height entered in TAKE OFF REF 2/2 is reached - UNLESS a low altitude is captured in which the thrust will maintain the commanded speed.

[SR71]

Ah OK.

NG differences.

[alexban]

SR71--
You are correct,on classic the minimum ap engagement alt is 1000'.
Jet man is also correct. Furthermore,when selecting lvl chg or vs,as you said ,the Automatic thrust reduction is inhibited for 2.5 min.