B737 MCP - SPD Button
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B737 MCP - SPD Button
Hi PPRuNers
I am an A320 F/O with previous experience on the B733, B727 and MD11.
The other day, I took a ride on a B737NG and "jumpseated" on the cockpit.
At one point, we discussed the usefulness of the SPD button on the MCP (the one to the left of the LVL CHG button, not the SPD INTV button). I should confess that I don't remember this from the B733 anymore. So, they said that this button is "useless", because for speed interventions when on VNAV they use de LVL CHG button (on MCP without the SPD INTV button). He pressed the button when on VNAV and nothing happened.
I've checked the manual and the explanation is not precise.
So I would like to ask you more about this topic.
Is the SPD button really useless or it has some real function?
What is the reason for this button work (or not) this way?
Thank you very much.
I am an A320 F/O with previous experience on the B733, B727 and MD11.
The other day, I took a ride on a B737NG and "jumpseated" on the cockpit.
At one point, we discussed the usefulness of the SPD button on the MCP (the one to the left of the LVL CHG button, not the SPD INTV button). I should confess that I don't remember this from the B733 anymore. So, they said that this button is "useless", because for speed interventions when on VNAV they use de LVL CHG button (on MCP without the SPD INTV button). He pressed the button when on VNAV and nothing happened.
I've checked the manual and the explanation is not precise.
So I would like to ask you more about this topic.
Is the SPD button really useless or it has some real function?
What is the reason for this button work (or not) this way?
Thank you very much.
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As said above it can be used to deselect the MCP SPD mode and enable the autothrottle ARM mode. On the other hand, it can select the same mode if it is compatible with the other autoflight modes. We use it (nearly) every flight to get into the autothrottle ARM mode.
Last edited by Denti; 10th Mar 2015 at 21:30.
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Imbracable.
Guilty as charged ..my bad.deepest apolgies.
As denti said...i used to use it to deselect speed on approach,ie A/Tvin arm mode but my actual airline is all off so i obey and switch everything off
Guilty as charged ..my bad.deepest apolgies.
As denti said...i used to use it to deselect speed on approach,ie A/Tvin arm mode but my actual airline is all off so i obey and switch everything off
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As with all buttons on the MCP, they can only be deselected if they are lit. Of course it does nothing when in VNAV, as speed is managed from the FMC (speed window blank) and A/T FMA says "FMC SPD". I can't see what the captain tried to demonstrate to you? 
System knowledge?!
In other pitch modes like ALT HLD, MCP SPD or G/S, it disengages/re-engages the auto throttle or change the thrust mode.
Useful?
1) You took off with full derate, after takeoff ATC asks you to expedite climb... Press SPD to get climb thrust (FMA thrust mode changes from ARM to N1) and delete any reduced CLB-1 or CLB-2 from the FMC N1 limits page. You now have full climb thrust.
2) Raw data take off with A/T on. Press SPD to get manual throttles to perform a level off. Press SPD again to get climb thrust, when further climb clearance (lazy version, as opposed to A/T off
).
Edit:
as correctly said by Defred, both above examples of course the N1 switch. Sorry brain fade, and hereby corrected. 
3) In cruise with increasing speed due to changing wind, throttles move towards an undesirable low N1: Press ALT HLD (to exit VNAV) and SPD to get manual throttles and set them where you like. Press SPD again to resume auto throttle (alternative is to battle the auto throttles motors or again switch off A/T - I think deselect SPD is more elegant
).
4) Approach with G/S captured. Press SPD to get manual throttles, but leaving them armed for GA (can't be done in any other way - SOP in some companies
).
There are a lot of things in the auto flight that to some may be useless and to other may have a purpose. Great with choices, right?
System knowledge?!In other pitch modes like ALT HLD, MCP SPD or G/S, it disengages/re-engages the auto throttle or change the thrust mode.
Useful?
1) You took off with full derate, after takeoff ATC asks you to expedite climb... Press SPD to get climb thrust (FMA thrust mode changes from ARM to N1) and delete any reduced CLB-1 or CLB-2 from the FMC N1 limits page. You now have full climb thrust.
2) Raw data take off with A/T on. Press SPD to get manual throttles to perform a level off. Press SPD again to get climb thrust, when further climb clearance (lazy version, as opposed to A/T off
).Edit:
as correctly said by Defred, both above examples of course the N1 switch. Sorry brain fade, and hereby corrected. 3) In cruise with increasing speed due to changing wind, throttles move towards an undesirable low N1: Press ALT HLD (to exit VNAV) and SPD to get manual throttles and set them where you like. Press SPD again to resume auto throttle (alternative is to battle the auto throttles motors or again switch off A/T - I think deselect SPD is more elegant
).4) Approach with G/S captured. Press SPD to get manual throttles, but leaving them armed for GA (can't be done in any other way - SOP in some companies
).There are a lot of things in the auto flight that to some may be useless and to other may have a purpose. Great with choices, right?
Last edited by cosmo kramer; 14th Mar 2015 at 12:14.
The ARM/disconnect AT on approach is as far as I can see the big religious divide in SOPs on flying the 737.
Boeing says Manual Flight = Manual thrust, but that does not preclude the use of the AT in arm as far as I can see as you are managing the thrust manually.
In the areas where you might use the SPD button when flying manually in the climb, I prefer to disconnect it as it's just easier in the RHS than reaching for the SPD button.
Boeing says Manual Flight = Manual thrust, but that does not preclude the use of the AT in arm as far as I can see as you are managing the thrust manually.
In the areas where you might use the SPD button when flying manually in the climb, I prefer to disconnect it as it's just easier in the RHS than reaching for the SPD button.
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I've used SPD switch a lot when i was flying 737NG mainly on approach while G/S capture on the FMA.It deselect A/T servos without deselecting Autothrottles hence giving you manual control of the thrust but doesn't protect you from the gust,it gives you Alpha floor protection meaning that in case of underspeed A/T quickly recovers the speed for you avoiding you a stall.The rest is like Cosmo kramer explained in depth
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At one point, we discussed the usefulness of the SPD button on the MCP (the one to the left of the LVL CHG button, not the SPD INTV button)
Autothrottles hence giving you manual control of the thrust but doesn't protect you from the gust,it gives you Alpha floor protection meaning that in case of underspeed A/T quickly recovers the speed for you avoiding you a stall
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From the manualÇ
"SPEED Switch
Push – (light not illuminated)
• engages A/T in SPEED mode if compatible with engaged AFDS modes"
What are the AFDS modes compatible with it?
"SPEED Switch
Push – (light not illuminated)
• engages A/T in SPEED mode if compatible with engaged AFDS modes"
What are the AFDS modes compatible with it?
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The quote refers to the corresponding pitch mode.
It's really not that complicated:
Every mode where speed is managed from the MCP.
, i.e. speed window open (except VNAV speed intervention, if installed).
It's really not that complicated:
Every mode where speed is managed from the MCP.
, i.e. speed window open (except VNAV speed intervention, if installed).
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My airline SOP is A/T off with A/P off so the SPD button is never used.
From distant memory it was occasionally useful on the 737 classics, as these aircraft did not have a SPD INTV button, but it's too long ago to remember the specifics. (Edit: I think we used it if we wanted to change speed with the MCP during ALT ACQ with FMC SPD - pressing the SPD button was the only way to open the speed window).
I can see an advantage in using A/T ARM mode with A/P off. Has anyone forgotten to engage the A/T with the A/P after an all-engine G/A? I've seen it happen, but not with any adverse consequence. Also the minimum speed protection (yes it's not called alpha floor but achieves the same thing) on approach may save a particularly incompetent crew (also may have saved the Asiana 777 in SFO but I'm not familiar with the 777). However, my company's interpretation of Boeing recommendations is not to use it.
Cosmo's comment is curious:
This is a new one for me - mainly I suppose because I've never heard of anyone being asked to "expedite climb" below thrust reduction altitude in our operations.
But does pressing SPD really put the A/T into CLB thrust? I don't know, and I can't find a reference for it. According to the FCOM, if you want CLB thrust earlier than the thrust reduction altitude in the FMC, you press N1.
And you would need to ask the PM to delete the CLB-1/2 reduction prior to pressing N1 or you will get a thrust reduction before you get a thrust increase. Pressing a button that reduces thrust below 1000' would likely earn me a punch in the ear from the other pilot, and so it should...
Your other option is to press TO/GA again - above 800' thrust levers will move automatically, below 800' still in THR HLD so need to manually move the thrust levers to the reference bugs.
From distant memory it was occasionally useful on the 737 classics, as these aircraft did not have a SPD INTV button, but it's too long ago to remember the specifics. (Edit: I think we used it if we wanted to change speed with the MCP during ALT ACQ with FMC SPD - pressing the SPD button was the only way to open the speed window).
I can see an advantage in using A/T ARM mode with A/P off. Has anyone forgotten to engage the A/T with the A/P after an all-engine G/A? I've seen it happen, but not with any adverse consequence. Also the minimum speed protection (yes it's not called alpha floor but achieves the same thing) on approach may save a particularly incompetent crew (also may have saved the Asiana 777 in SFO but I'm not familiar with the 777). However, my company's interpretation of Boeing recommendations is not to use it.
Cosmo's comment is curious:
"You took off with full derate, after takeoff ATC asks you to expedite climb... Press SPD to get climb thrust (FMA thrust mode changes from ARM to N1) and delete any reduced CLB-1 or CLB-2 from the FMC N1 limits page. You now have full climb thrust."
But does pressing SPD really put the A/T into CLB thrust? I don't know, and I can't find a reference for it. According to the FCOM, if you want CLB thrust earlier than the thrust reduction altitude in the FMC, you press N1.
And you would need to ask the PM to delete the CLB-1/2 reduction prior to pressing N1 or you will get a thrust reduction before you get a thrust increase. Pressing a button that reduces thrust below 1000' would likely earn me a punch in the ear from the other pilot, and so it should...
Your other option is to press TO/GA again - above 800' thrust levers will move automatically, below 800' still in THR HLD so need to manually move the thrust levers to the reference bugs.
Last edited by Derfred; 13th Mar 2015 at 03:46. Reason: added reference to using SPD button with ALT ACQ
Also the minimum speed protection (yes it's not called alpha floor but achieves the same thing) on approach may save a particularly incompetent crew (also may have saved the Asiana 777 in SFO but I'm not familiar with the 777).
Minimum Speed Protection/Reversion is an elevator speed mode not an A/T mode. The reaction of the A/T is dependant on the position of the MCP. If the MCP is above the current altitude the A/T will increase thrust to CLB and the elevator will pitch for SPD, if the MCP is below current altitude then the A/T will close the thrust levers and the elevator will pitch for SPD.
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Interestingly it is called both alpha floor protection and minimum speed protection in my FCOM. In the airlines i have worked for it was SOP to use the ARM feature and it does work a treat if someone is really daft enough not to monitor the speed enough. It can be a nuisance in gusty weather during approach and switching the AT completely off then is of course a good idea.
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Minimum speed reversion is not available when the A/T is OFF and the AFDS is in ALT HOLD or after G/S capture. Minimum speed reversion is also not available when in VNAV PTH and flying a level segment.
Remember the THY AMS incident
Remember the THY AMS incident