After take-off speed selection

[Marcel_MPH]

Wondering about the following, especially for B737NG aircraft.

At take-off, the engines are initially spooled up to about 40% N1, after that the TOGA mode is engaged. Which speed has to be remained after getting airborne and is it controlled by the FMC or is it set by flight crew. I understand the speed setting after t/o has to be around V2 + 15 kts.......

Thanks in advance.

Marcel_MPH

[IRRenewal]

Which speed has to be remained after getting airborne and is it controlled by the FMC or is it set by flight crew.It's calculated by the FMC based on weight and flap setting and confirmed and selected by the flight crew.

I understand the speed setting after t/o has to be around V2 + 15 ktsYes, you keep this speed until acceleration altitude (usually 1000 feet above airfield elevation), then the commanded speed is increased and flaps are retracted according to the flap retraction schedule as the aircraft accelerates.

In case of engine failure on T/O, if below V2 you target speed becomes V2, if between V2 and V2+15 you maintain whatever speed you have, if above V2+15 you trade speed for height and your target speed is V2+15.

[Marcel_MPH]

Hey IRRenewal,
thanks for your reply. It's much more clear to me now.

But there's another thing.
If the TOGA mode is engaged the aircraft will accelerate with a certain N1 setting (let's 99,8%). So after V2 you most probably overshoot the V2+15 kts speed without any crew throttle input. So what is the take-off procedure after TOGA mode engagement?

Marcel_MPH

[The Greaser]

Power setting is reduced to climb thrust at a nominal altitude, normally 1500' above airfield elevation. As with any aircraft in a climb, or descent, pitch attitude is what controls airspeed not power adjustments. ie if airspeed starts to increase, pitch is increased, if airspeed decreases, pitch is decreased.

[Easy226]

Hi everyone,
Does level change on the MCP link in any way to the descent/climb profile of that programmed into the FMC and if it doesn't, is it possible to use LVL CHG to replicate the climb/descent profile of that in the FMC?
Many Thanks
Dan

[IRRenewal]

You really need to select VNAV for that.

Level change just goes up at the selected speed at a thrust setting calculated by the FMC based on various inputs (like temperature and cost index) and goes down at flight idle at the selected speed.

In VNAV the FMC will do a bit more work for you. Say you are in the cruise at 30K, and you have programmed an arrival with an altitude limit of 5000 feet at a certain point and a speed limit of 250 knots below FL100. Provided you gove it the right wind information, the FMC will calculate for you at which point you start the descent. Provided you select 5000 in the MCP it will reduce to flight idle, descent to FL100, reduce speed to 250, and continue the descent to 5000 all at flight idle. This is a VNAV speed descent, which could be done manually with LVL Change to 10K, a speed reduction to 250, and a lvl chg to 5000, but that would be harder work.

Another option is the VNAV path descent. Say you are at 10K, and you have selected a point where you want to be a 5K. Based on the ground speed the FMC calculates the required rate of descent. What you do is select 5K in the MCP and the aircraft works it out for you. You could do the same by using V/S. You select 5K in the MCP and dial up a certain ROD. The FMC dsplays a green arc to show the end of the descent, and you adjust the V/S to put this arc over the waypoint with the 5K restriction.

All this assumes you can plan your own descent totally. In reality, unless you go to some remote island without radar, this is hardly ever the case and ATC will dictate your descent and speed.

Marcel, ref your question about N1 setting.

When you reach acceleration altitude (see my previous post) you don't only select 210 knots (or whatever you use on the day), you also select N1 on the MCP. This reduces N1 to climb thrust, which is calculated by the FMC.

Hope this helps.

Gerard

Ps: Marcel, waarom al deze vragen?