Conversion switch on MCP for selected IAS/Mach

[Hardy Heinlin]

Good morning,

I have a question that is probably not 747 specific. I guess most jet flight decks have a little switch that converts the selected IAS to a selected Mach and vice versa (I mean for the airspeed bug on the PFD).

(The official term is "IAS/Mach select switch", but I'm calling it "conversion switch" to avoid confusion with the "IAS/Mach selector" which is the rotary knob that winds the values up and down.)

On the 744, it seems the MCP receives a conversion factor value from the FMC; the MCP just has to apply this factor to the selected IAS or, reversed, to the selected Mach. It has to apply this factor only once: in the moment when the switch is pushed. The MCP itself obviously does not know the required air data (pressure altitude, temperatures) to compute any Mach number for any IAS value on its own.

Maybe the MCP can ask the FMC "compute the Mach for my selected 321 KIAS with reference to the current pressure and temperature, then send it to me". Or the MCP could ask the FMC "what's your current conversion factor for the current  aircraft speed? Send it to me."

If the latter is the applied method, the conversion switch works accurately only when the selected MCP speed is near the current aircraft speed. This is because the conversion factor cannot be applied linearly to any speed value. Mach is the ratio of TAS to the speed of sound; and the MCP displays IAS, not TAS. IAS includes compressibility effects and other non-linear errors, as we know.

Is there anybody on the forum who can remember any special effects regarding this conversion switch? For example, was your current aircraft speed at Mach .3 and your MCP was in IAS display mode, and you tried switching to Mach but it stayed in IAS mode, although the selected IAS was -- theoretically -- high enough to convert to a Mach number higher than .4? (Mach .4 is the lower limit for the Mach selector on the 744.)


Cheers,

|-|ardy

[Lasse]

Hi Hardy

For the 737NG we call the switch for IAS/Mach Cross/Over switch in short C/O or cross over.

As I read in the deeper manuals then we have M0.6 as lower limit for the cross over and to me it seams that the digital signal comes from the flight control computer, not from the FMC. The MCP however sends data to the FMC.
As I remember then when I use the switch then I will get the current Mach or KIAS rounded up or down to the closest number... E.g Mach 0.756 = 0.76

Hope that helped just a little..

/L

[Hardy Heinlin]

Hi Lasse,

thank you.

So, if M.6 is the lower limit for your MCP crossover (737), have you ever tried to select a Mach number when your aircraft was significantly below M.6 while a KIAS value was selected?

I guess most pilots will never get into such a situation; typically, the crossover happens when the aircraft speed is at the selected IAS/Mach -- be it with or without VNAV.

A rare situation could be:

During departure, aircraft speed is 270 KIAS, maintaining FL150 due to ATC restrictions. Then ATC says or sends: "Climb to FL310, Mach .82." At this time, the crossover switch may or may not work (the big question). If it won't work, you'll have to guess a corresponding KIAS value, increase it more and more, and play with the crossover switch while increasing the selected KIAS -- or maybe even the aircraft KIAS -- until the KIAS value generates a Mach number at or above 0.6.

In PSX, the crossover switch works all the time. In a previous version, it was inhibited only when the aircraft speed was 0/0.0 KIAS/Mach. Recently, I removed this inhibit too. I don't know if this is realistic.


Cheers,

|-|ardy

[Lasse]

Hi Hardy

Accoring to the manual then the switch is inhibited until reaching the criteria. M0.6, I would imagine its the same for the 744.
Yes, in real life it would be unrealistic to be asked like that, however since programming a simulator also has to cater for the imagination of the user then yes, you would have to accelerate until above M0.6 to whatever that KIAS that would be and then transition .. I would say very very few crews know the minimum C/O speed so they would just press now and then until it would convert. However I think M0.3 is equal to a low KIAS anyway so its not really going to be a problem for the 744 or 737.
The Mach is stated below the speed tape shifting from GS to M at M0.4 for the 737NG.. so you would have some guidance if they would say fly M0.5 now at 3000 feet.. :-D

/Lasse

[Hardy Heinlin]

Hi again, Lasse.

Accoring to the manual then the switch is inhibited until reaching the criteria.

"until reaching" -- I think this is the typical standard AOM text. It is not clear to me whether "until reaching" refers to the rotation of the speed selector, or to the current aircraft speed. For years I've been interpreting this text like this:

Crossover switch
- Switches to Mach display if IAS is displayed and selected IAS corresponds to Mach 0.4 or higher.
- Switches to IAS display if Mach is displayed and selected Mach corresponds to 399 KIAS or less.

(M.4 / 400 KIAS are the limits on the 744.)

Normally, the aircraft flies at the selected speed. This means, normally, the text would be correct in either case: In case it refers to aircraft speed, and in case it refers to the selected speed.


Cheers,

|-|ardy

[Lasse]

Hi Hardy

Well that wording was actually mine.. :-D
When I read the AMM then for the display it will show IAS from 100 to 399 in 1 knot increment and for Mach from 0.6 to 0.89 in 0.01 increment. (We do not have the option of getting 3 digit MCP as the 744)
Then for the IAS/MACH Change/Over Switch they write:
"When you push this switch and the airspeed is more than mach 0.6, it changes the display from IAS in knots to mach or from mach to IAS in knots. If the airspeed is less than mach 0.6, the display will only show knots and the change over switch does not do anything."
Ahh well ok after reading you post 10 times I might get your drift now.. (Maybe, else just correct me :-D )
What will the speed display show if selected speed on the MCP is 300 KIAS and the IAS on speed tape is 250 kts at FL150 and you press C/O? Well then since the 250 KIAS converted to TAS and the TAS converted to Mach would be less than M0.6 then I would expect nothing to happened when I press the C/O switch. During descend then flying M0.4 on the 744 then I would expect the MCP speed window to change to actual IAS when TAS would be equal or less then M0.4

/L

[Hardy Heinlin]

Hi Lasse,

yes, you see what I mean :-)

I understand it was your wording; nevertheless the original AOM texts are vague; in this 737 example, they just say "the airspeed". -- I wonder which airspeed, and under which conditions? :-) Nobody knows. One can only guess what that means. Sometimes the term for the aircraft's airspeed is "current airspeed", and the MCP speed is the "selected speed" -- to distinguish the terms. In a certain 747 AOM I read "when within range". That's just as vague :-)


Cheers,

|-|ardy

[Lasse]

Hi again.

Oki super then there is only one way to find out. ;-)
Ill be back in the air the 9th then Ill let you know.

Have a nice weekend.

/L

[IefCooreman]

"Open speed windows" and actual IAS-M number crossover is completely independent of FMC values, they are related to fixed values. When you open the speed window, it shows the current IAS/M-number (PFD value) depending on (777 values):

In the climb, M is shown if actual M-number is at or above 0.84
In the descend, IAS is shown if actual IAS at or below is below 310kts

So for the MCP speed window, crossover is fixed. For the FMC, it can ge changed on the VNAV page.

If you push the crossover switch after changing the speed with the speed window open and the aircraft lagging behind in acceleration/deceleration, the target speed bug on the PFD will not change position (unless a tiny mm because of rounding off the values).

So where the conversion -calculation- happens, I'm not sure, but it must be some kind of air data computer value (but then one can ask what happens to the switch when you have ADC problems, or ADIRU/SAARU problems for the 777). (BTW, on the 777 ie, the speed window selectable values are also depending on flap limits etc but that's another story.)

The example of high speed climb is pretty close to real life, although it happens more frequently for a descend. For climbs/descends, ATC generally used IAS requests, never M-number. We are not always able to do that at high altitudes, and simply follow the request from the crossover point on. Above/below that, we do what we can (accelerate/descelerate based on M-number as good as we can). In cruise, speed requests are always M-number.

The crossover from IAS to M-number through the switch is inhibited below M0.4, which coincidently is also the mach number where the M-indication on the PFD comes alive. Logical, because how would you be able to confirm your M-speed if it isn't displayed on the PFD?

Hope this helps...

[Hardy Heinlin]

Thanks, Ief.

If you push the crossover switch after changing the speed with the speed window open and the aircraft lagging behind in acceleration/desceleration, the target speed bug on the PFD will not change position (unless a tiny mm because of rounding off the values).

That's clear; the switch converts the MCP speed, not the aircraft speed. The switch will not cause the speed bug to jump to the current aircraft speed.

The only mystery to me is this:

(Referring to 744 limits: M.4 / 399 KIAS.)

Example: The current aircraft speed is 250 KIAS / M.3 and the MCP is 350 KIAS.
Question: Is the MCP conversion switch at this time inhibited or not?

Assuming the current pressure level and SAT are such that the MCP selected 350 KIAS are equivalent to a Mach number way above 0.4.

Will the system accept the conversion (as the converted MCP Mach number would be within the allowed range)?

Or will the system inhibit the conversion (because the current aircraft Mach is below 0.4)?


Cheers,

|-|ardy

[John H Watson]

So where the conversion -calculation- happens, I'm not sure, but it must be some kind of air data computer value (but then one can ask what happens to the switch when you have ADC problems, or ADIRU/SAARU problems for the 777).

Looking at the maintenance manuals, it's not clear where the MCP gets the mach value from. The ADCs do not talk directly to the MCP and I don't think the MCP has sufficient brains or inputs to compute the value for itself.

So, a mach value has to come from another box. Manuals list what comes from the FCC and FMC. Only CAS is listed (not mach) as coming from the FCCs. This may be a simplification though(?). FMC "target mach" is listed as one of the parameters going from the FMC to the MCP, but, in this case, I don't think target mach is the same as MCP selected mach.

[IefCooreman]

(Referring to 744 limits: M.4 / 399 KIAS.)

Example: The current aircraft speed is 250 KIAS / M.3 and the MCP is 350 KIAS.
Question: Is the MCP conversion switch at this time inhibited or not?

We (777) have the same limits, so I did some testing this morning during the acceleration after takeoff.

On the ground the conversion switch is inhibited, whatever the speed you select.

In flight, the converted "MCP-IAS" has to be above M0.4 for the M-conversion to show on the speed window. Current aircraft speed can be below M0.4 (so no M-number indicated on the PFD). I tried it out during for different speeds during the acceleration.

Hope this helps,

Ief

[Hardy Heinlin]

Great. Thanks a lot!

Yes, this helps :-)


Cheers,

|-|ardy