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smokeyupahead

About mcp speed...

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Hi guys and gals,

 

does it make sense and is it possible to set and engage a speed of 165 by pressing the mcp speed button shortly after takeoff with the autopilot off and autothrottle on while N1 is still commanding thrust?

 

I'm trying to perform the initial climbout from runway 26 at Innsbruck all the way up to the ADILO1H transition.

This is what I do:

 

"I take off by pressing the TOGA button, I notice N1 comes alive in the air and at this point I am maintaining V2 + 20.

At climb thrust I set 165 in the mcp window as per the speed restriction and then try to engage the speed button but it wont engage."

 

I can't even engage vnav on ground or in the air to have it control the speed if I wanted to try that.

As I understand N1 will not hold speed unless I use picth to control my speed in this phase.

 

I hope I was clear enough and thank you in advance.

 

Cheers,

David

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Don't touch the SPEED button.

 

I'm not sure exactly what you're trying to do, but simply changing the selected speed in the MCP should do the trick. VNAV would probably be your first choice.

 

Do you have a chart for the SID you're doing you can post? 165 seems like an odd restriction.

 

Make sure you've completed your PERF INIT and you might be able to get VNAV.

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VNAV sounds like the best option. Try to figure out why you can't get it running.

 

Otherwise, you'd simply need to dial in 165 from whatever your V2 was.

I can't even engage vnav on ground or in the air to have it control the speed if I wanted to try that.

As I understand N1 will not hold speed unless I use picth to control my speed in this phase.

N1 is simply giving you a constant climb power - the engines don't care what speed you want, they're just pumping out the power. You're controlling airspeed by pitch.

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If your issue persists, could you post FMC and MCP screen shots?

 

Also ensure that you are inputting your takeoff runway in the FMC (or am I confusing that with LNAV requirements?).

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Wow guys thanks for your replies!

Anyway I have everything setup in the fmc as it should be.

I know vnav would be my best option but I would have to fly with the autopilot on; I want to hand fly the departure with the only assistance of the autothrottle to mantain 165 knots.

I don't know if it can be done tough nor if it is a correct way of flying this departure...?

I've hit the sac already but I will post a video tomorrow.., stay tuned please.:-)

I tried to use the mcp speed mode with the ifly 737 and it worked!

My question at this point is to know which one is right.

 

Thanks again guys,

 

David

 

Don't touch the SPEED button.

 

I'm not sure exactly what you're trying to do, but simply changing the selected speed in the MCP should do the trick.

 

I tried that too but it still keeps me in the N1 mode.

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165 is correct, IIRC. You gotta make sure you make that 180 degree turn in the narrow Inn valley.

 

Oliver Branaschky

 

There is a minimum required bank of 25 degrees. Very interesting departure, so many things going on...

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I know vnav would be my best option but I would have to fly with the autopilot on

 

No, that's incorrect. Generally, you can select an AFDS pitch or roll mode irrespective of whether the A/P is engaged or not (with A/P FLARE and ROLLOUT being obvious exceptions, but that's not important right now). In other words: you could very well select VNAV and fly the aircraft manually. In this particular scenario of the eastbound departures out of LOWI runway 26, you've got a plethora of options:

  • pre-arm VNAV on the ground and set a 165 speed restriction covering the initial turn on the FMC LEGS page (if it isn't in there already as part of the SID)
  • pre-arm VNAV, don't set an FMC speed restriction, but use SPD INTV to set a speed of 165 knots on the MCP before entering the turn, and then click SPD INTV again after exiting it to resume normal VNAV SPD mode during the climb
  • select LVL CHG after take-off, and dial in 165 knots on the MCP, proceed through the turn, and then select flaps up manoeuvring speed and accelerate as normal (and switch to VNAV thereafter)
  • or... technically you could stay in TOGA, select 165 knots on the MCP until you've completed the turn, and select a different pitch mode afterwards.

Choices, choices... They'll all do the job just fine (disclaimer: I Am Not A Pilot); which one to pick will depend on your airline's SOP or, in the case of a simmer, one's whimsical mood of the day.

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I tried VNAV but unless I engage it togheter with the autopilot, the autothrottle will not hold the fmc speed of 165 knots.

Could it be because the first leg is actually a vector? Would that cause VNAV not to engage at times? Going crazy!

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I tried VNAV but unless I engage it togheter with the autopilot, the autothrottle will not hold the fmc speed of 165 knots.

 

The autothrottle will not do that because in the take-off and climb phases of the flight, unless told otherwise, the A/T is commanding a set amount of engine thrust (N1) and does not try to maintain any particular airspeed. The latter is up to you by following the flight director pitch cues on the PFD, i.e. to vary the pitch angle of the aircraft so as to achieve and maintain the desired speed of, in this case, 165 knots.

 

In order to gain a better understanding of these AFDS-related matters, I'd suggest you study FCOM Vol. 2, Chapter 4 ("Automatic Flight"). It will likely clarify a lot of the issues you're currently struggling with.

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Uhm...I think I should still be able to use the mcp speed...

Let me do some other tests tomorrow, I don't want to come to conclusions yet.

 

Thank you for your help so far!

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:unknw:

 

165 in the mcp - FD on - AT on. Speed selected after takeoff. Held speed regardless of what i done, pitch up or down.

 

 

 

You want to hand fly the departure. set 165 in the mcp. it may be that you cant arm LNAV/VNAV on the ground because you dont have the FD on so make sure the switch is on. If the FD is on and both nav modes are select the flight director bars will move and you have to hand fly to follow them. I took off with TOGA, gear up and hit speed.

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Uhm...I think I should still be able to use the mcp speed...

Let me do some other tests tomorrow, I don't want to come to conclusions yet.

 

Thank you for your help so far!

I'm confused here. Are you trying to use VNAV or speed hold. Both do two completely different things. VNAV is going to hold your thrust and controls your speed by varying pitch. So you would have to engage the autopilot to use VNAV, otherwise VNAV wouldn't work since it controls your pitch to maintain speed.

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Autopilot is not needed for VNAV. Any mode except autoland works without the A/P.

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I want to hand fly the departure with the only assistance of the autothrottle to mantain 165 knots.

I heard that Boeing does not recommend use of auto throttle for manual flight. At least at some airlines what you are trying to do would go against recommended procedures.

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If you are going too fast then increase your climbrate. If you are going too slow then reduce your climbrate. You generally want to have a very high EPR during your initial climbout to get you away from the ground as quickly as possible. Reducing the EPR during a climbout to be able to adhere to a restriction sounds like poor piloting, I seriously doubt that they designed the procedure with a throttle reduction in mind

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Hi guys and gals,

 

does it make sense and is it possible to set and engage a speed of 165 by pressing the mcp speed button shortly after takeoff with the autopilot off and autothrottle on while N1 is still commanding thrust?

 

I'm trying to perform the initial climbout from runway 26 at Innsbruck all the way up to the ADILO1H transition.

This is what I do:

 

"I take off by pressing the TOGA button, I notice N1 comes alive in the air and at this point I am maintaining V2 + 20.

At climb thrust I set 165 in the mcp window as per the speed restriction and then try to engage the speed button but it wont engage."

 

I can't even engage vnav on ground or in the air to have it control the speed if I wanted to try that.

As I understand N1 will not hold speed unless I use picth to control my speed in this phase.

 

I hope I was clear enough and thank you in advance.

 

Cheers,

David

In real operations, for such performance critical departures, we don't use VNAV because acceleration is not based on an altitude, but a point on the departure and the condition of the aircraft. The normally adopted procedure for speed limited departure turns is, when above thrust reduction height, we select LVL CHG and set the speed as required, hold the speed until the turn is complete and acceleration is required, then accelerate by manually setting flaps up speed.

 

BTW, the 165kts mentioned above is a maximum speed, In this case I would stay at V2-V2+20 until the turn is complete as then you are achieving the best angle of climb throughout the turn and thus ensuring maximum performance. if V2+20 is more than 165kts, then I'm guessing you're using an improved climb, double derate, flap 1 departure. For this kind of departure, probably best to use full rated TOGA thrust and Flap 5. Engine bleeds off takeoff supplementary procedure would also be prudent.

 

Not being able to engage speed mode isn't documented in relation to the TOGA pitch mode, but as it is like LVL CHG in that speed is controlled by the AP and not the AT, then it makes sense that you cannot select SPEED. If you enter a mode that requires variable thrust, where the AT alters thrust to maintain speed and the AP commands a specific pitch like VS and ALT HOLD, then SPEED is automatically selected, but then you can deselect it and put the AT into ARM.

 

Incidentally, I have noticed that NGX does not allow the N1 button the be selected to manually command CLB thrust. I haven't used the N1 button since moving from the Classic, but with two engines, for some reason I expected it to engage CLB thrust manually regardless of the FMC TO PAGE 2/2 setting.

 

I heard that Boeing does not recommend use of auto throttle for manual flight. At least at some airlines what you are trying to do would go against recommended procedures.

Correct, except for T/O and climb, because the thrust setting is expected to be constant and not varying as it would be any other phase of flight.

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:unknw:

 

165 in the mcp - FD on - AT on. Speed selected after takeoff. Held speed regardless of what i done, pitch up or down.

 

 

 

You want to hand fly the departure. set 165 in the mcp. it may be that you cant arm LNAV/VNAV on the ground because you dont have the FD on so make sure the switch is on. If the FD is on and both nav modes are select the flight director bars will move and you have to hand fly to follow them. I took off with TOGA, gear up and hit speed.

 

After several tests I found out that the only way to engage mcp speed is the following:

 

N1 PB Off

VNAV PB On

VNAV PB Off

Now you can select Mcp Speed PB.

 

Wacth the video I am about to post...

 

In real operations, for such performance critical departures, we don't use VNAV because acceleration is not based on an altitude, but a point on the departure and the condition of the aircraft. The normally adopted procedure for speed limited departure turns is, when above thrust reduction height, we select LVL CHG and set the speed as required, hold the speed until the turn is complete and acceleration is required, then accelerate by manually setting flaps up speed.

 

BTW, the 165kts mentioned above is a maximum speed, In this case I would stay at V2-V2+20 until the turn is complete as then you are achieving the best angle of climb throughout the turn and thus ensuring maximum performance. if V2+20 is more than 165kts, then I'm guessing you're using an improved climb, double derate, flap 1 departure. For this kind of departure, probably best to use full rated TOGA thrust and Flap 5. Engine bleeds off takeoff supplementary procedure would also be prudent.

 

Not being able to engage speed mode isn't documented in relation to the TOGA pitch mode, but as it is like LVL CHG in that speed is controlled by the AP and not the AT, then it makes sense that you cannot select SPEED. If you enter a mode that requires variable thrust, where the AT alters thrust to maintain speed and the AP commands a specific pitch like VS and ALT HOLD, then SPEED is automatically selected, but then you can deselect it and put the AT into ARM.

 

Incidentally, I have noticed that NGX does not allow the N1 button the be selected to manually command CLB thrust. I haven't used the N1 button since moving from the Classic, but with two engines, for some reason I expected it to engage CLB thrust manually regardless of the FMC TO PAGE 2/2 setting.

 

 

Correct, except for T/O and climb, because the thrust setting is expected to be constant and not varying as it would be any other phase of flight.

 

Hi and thank you Vagabondo for you the insights for the departure procedure, while I am pretty familiar with the AFDS modes and how they work, I wanted to get some more information on the mcp speed used alone. You are right when you say it is not documented but I did a lot of tests today and I realized that the only way to use it by itself is by deselecting N1, selecting and deselecting VNAV, then you can select mcp speed alone and it will control mcp speed.

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Here is a video showing the Pmdg and Ifly mcp speed engagement behaviour:

 

 

 

This one shows the only way I could engage mcp speed control during the takeoff:

 

Link: I will upload tomorrow...sorry.

 

 

These are not aimed at showing correct procedures or good flying but were done to show the problem I have. I did do the setup the same way for both aircraft.

I wonder which aircraft models the correct behaviour?

 

Cheers,

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I fly the plane, and I'm totally confused by what you're doing. :lol:

 

Remember that the MCP is what those in the schoolhouse call a "wishlist." You need to look at the PFD for the modes, not the MCP to really see what's going on.

 

Stop touching the SPEED button. Just hit LVL CHG.

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Forgive me, what's wrong with using mcp speed to hold a speed during climbout? I'm not challenging you here, i'm merely curious.

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Forgive me, what's wrong with using mcp speed to hold a speed during climbout? I'm not challenging you here, i'm merely curious.

That's what LVL CHG is for. The SPEED button could be removed from the jet. I have seen that button pushed maybe 5 times in 6 years of flying, and that was only to enter ARM mode.

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Forgive me, what's wrong with using mcp speed to hold a speed during climbout? I'm not challenging you here, i'm merely curious.

 

Well, even though I Am Not A Pilot™, I think the problem lies in the fact that he is trying to engage the A/T in MCP SPD mode, whilst the AFDS pitch mode remains unchanged at TOGA. Now ask yourself this: what would the pitch angle commanded by the FD be in such a configuration? Should the plane go up, or down, and if so, at which pitch angle... or remain level? It boggles the mind, doesn't it? Therein lies the issue, in my humble opinion.

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Surely that is dictated by the vert speed selector, as in the case of certain SIDs you may have to cross a certain point at a certain altitude and if the speed is dictating the pitch, and thus the vert speed, the proper altitude may not be attained before the point at which is has to.

 

Say i had to takeoff and cross a VOR at no less than 2500ft, i engage LVLCHG at the appropriate speed and the vertical speed achieved is not sufficient to reach that altitude. What happens in these situations? Does the LVLCHG take into account vert speeds?

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Surely that is dictated by the vert speed selector, as in the case of certain SIDs you may have to cross a certain point at a certain altitude and if the speed is dictating the pitch, and thus the vert speed, the proper altitude may not be attained before the point at which is has to.

 

Say i had to takeoff and cross a VOR at no less than 2500ft, i engage LVLCHG at the appropriate speed and the vertical speed achieved is not sufficient to reach that altitude. What happens in these situations? Does the LVLCHG take into account vert speeds?

You really don't use V/S for takeoff. You could use VNAV or LVL CHG, and you'd be in N1 climbing at your selected CLB power and appropriate speed. Your rate of climb would be a function of this, not the primary.

 

If you had a close in climb restriction, that would probably be part of your TOW for the runway.

 

Down low, you'd typically use V/S to smooth out a low level-off or to prevent a RA from TCAS.

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