# Convert TAS cruise speed to Mach number for the FMC?

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A flight plan has N460 (460 knots true airspeed). Do I have to convert this TAS to MACH NUMBER in order to insert it in the FMC?

Thanks

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Usually you don't convert TAS to Mach or vice versa.

The TAS on the filed flightplan is calculated with your cost index, which when entered in the FMC gives you a Machnumber at cruise closely to your filed TAS.

However, if you want to calculate TAS to Mach you use this formula:

TAS = Mach No x LSS local speed of sound)

LSS equals 38.94 multiplied by square root of temperature in degrees kelvin

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Usually you don't convert TAS to Mach or vice versa.

The TAS on the filed flightplan is calculated with your cost index, which when entered in the FMC gives you a Machnumber at cruise closely to your filed TAS.

However, if you want to calculate TAS to Mach you use this formula:

TAS = Mach No x LSS local speed of sound)

LSS equals 38.94 multiplied by square root of temperature in degrees kelvin

Chris is right of course.  You would normally go with whatever the cost index calculates for you.  I read the question more as, "I'm trying to replicate this flight plan I saw, is there a way to input the true airspeed in knots so I don't have to do the math?"   :lol:

Incidentally, that formula is great for engineers, but a little impractical in flight.  Were we to ever have a need to actually figure it out for ourselves, we would normally just use a flight computer of some kind.  Believe it or not, most pilots hate math, lol.

Jeff

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Usually you don't convert TAS to Mach or vice versa.

The TAS on the filed flightplan is calculated with your cost index, which when entered in the FMC gives you a Machnumber at cruise closely to your filed TAS.

However, if you want to calculate TAS to Mach you use this formula:

TAS = Mach No x LSS local speed of sound)

LSS equals 38.94 multiplied by square root of temperature in degrees kelvin

My question about this is the reverse of the original. How would I calculate the TAS from cost index? Obviously I could take a typical cruise Mach and convert to TAS, but that's not quite the same.

Also does anyone know why airliner flight plans are filed with TAS and not Mach? Mach No is what the aircraft speed will actually be controlled by, not TAS.

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I can only guess, because I'm no dispatcher or whatsoever.

A costindex gives you a rather constant TAS over the whole planned route, whereas your machnumber (while on econ) will change depending on environmental conditions. And with your constant TAS and windspeeds you can calculate ETAs of waypoints, FIR-boundaries etc.

As for claculation of TAS from CI, Iguess you need special flightplanning software.

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A flight plan has N460 (460 knots true airspeed). Do I have to convert this TAS to MACH NUMBER in order to insert it in the FMC?

As has been mentioned, you don't actually need to put that in there (unless you're flying a constant speed segment, like a NAT, but in those cases, you'd see Mach anyway).  See below for why.

Also does anyone know why airliner flight plans are filed with TAS and not Mach?

(Kevin, I attempted to answer your question and then got caught in my own extra clarification for the general audience, too, so not all of this is aimed at you:)

You can file any of them on an ICAO flight plan, actually.  The ICAO flight plan form uses the same mid-route format you see in a lot of routes for the "cruise speed" field.  If you're looking at the old FAA flight plan form (they've adopted ICAO 2012 formatting now), TAS was the simplest to use as it's essentially ground speed in a no-wind environment.  This made things easy for position and traffic management calculations (originally of the human sort), which are the only true users of that data now.  Even though the field says "KTS" on the FAA form, you've also always been able to file Mach speeds by filing M## in that field.

This may come as a shock to people, but radar controllers rarely look at that data, nor do care that it's there.  If you're off-speed, they aren't going to give you a phone number to call or fuss at you.  The exception to that, of course, is when you're in segments of procedural control (controlled by non-radar controllers and procedures), where it's control based on last known position, time elapsed, and your filed airspeed.

Since these procedural control items were originally done by human controllers, it was a lot easier to calculate positions using TAS and not Mach.  The why behind that can be answered simply by the fact that this thread exists.

Last known position + (Elasped Time * TAS) = New position

[Of course, the ET * TAS is dependent on a direction, but it's assumed that you're along your route, so only ET * TAS is necessary]

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TAS is not fixed if flying ECON (Cost Index). It will vary with your winds aloft.

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Hi,

Another simple method to convert:

1st you have to know how much is sound's speed (it depends on outside air temperature i.e. 15 °C = 288 Kelvin =1225 km/h, 661,09 knots, 340,3 meters per second ecc.)

2nd sound speed varies according with the square root of ratio (Kelvin degrees) between outside air temperature (static, naturally) and standard 288 Kelvin.

Example you're travelling at 35000 ft in ISA conditions (15°C = 288 K at medium sea level decreasing of about 2 °C every 1000 ft=218 Kelvin at FL350=-55°C) Mach 1 is equal to 661,09 knots *(square root (218/288))=661,09 knots *(0,87)=575,16 knots so i.e. if you're travelling at 450 knots your mach number is =450/575,16=0,782.

Ciao

Andrea Buono

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Did not want to start a new topic... I am unfamiliar with the 737 FMC but am planning an etops route. I have a point on my legs page I need to be at mach .78

I am at FL380, outside temp -23C

Can I not enter mach speeds in the fmc like you can on the 777? It always gives me invalid entry, even though the legs page now show .779/FL380

-David Lee

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Did not want to start a new topic... I am unfamiliar with the 737 FMC but am planning an etops route. I have a point on my legs page I need to be at mach .78

I am at FL380, outside temp -23C

Can I not enter mach speeds in the fmc like you can on the 777? It always gives me invalid entry, even though the legs page now show .779/FL380

-David Lee

Put your desired cruise Mach on the CRUISE page.

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Put your desired cruise Mach on the CRUISE page.

Does the FMC not have an option to plan different legs at certain speeds for navigating tracks?

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Does the FMC not have an option to plan different legs at certain speeds for navigating tracks?

Hmmm. Haven't tried that. I think speeds would only work on the Legs in climbs and descents. For cruise you'd use the ECON, LRC or just write in a set speed.

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Does the FMC not have an option to plan different legs at certain speeds for navigating tracks?

Along with Matt, I'm unsure of this option in the 737. The 777 has a vastly different market, and therefore flight profile, in which it serves. The NG is less likely to have to fly though constant speed segments, to the point where I'm not sure that multiple segment speeds would be supported in advance. As Matt mentioned, you would just change the value on the MCP, or set it on the CRZ page.

Full names - first and last - in the forum, please.

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Along with Matt, I'm unsure of this option in the 737. The 777 has a vastly different market, and therefore flight profile, in which it serves. The NG is less likely to have to fly though constant speed segments, to the point where I'm not sure that multiple segment speeds would be supported in advance. As Matt mentioned, you would just change the value on the MCP, or set it on the CRZ page.

Full names - first and last - in the forum, please.

Thanks. I added my name to my post before did not know you had to do it every time. Ok.

So if I am flying NATs or other segments requiring set speeds I have to do this manually once I cross over the waypoint, I go to CRZ and select the assigned speed? Thought the FMC would have an option like the 777 with setting desired cruise speed at each waypoint. Guess not!

If any NGX update is planned it would be great to have auto tuning VOR's like the 777 has I believe the 737 has this in real life

Thank you for the help. I just started flying the NGX again, found many fun routes that the 737 flies so am not that familiar with all the inner workings of the systems, it's been so long.

David Lee

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Thanks. I added my name to my post before did not know you had to do it every time. Ok.

Yep - every post. You can configure an auto-signature, like the one I use, by going into your user settings.

So if I am flying NATs or other segments requiring set speeds I have to do this manually once I cross over the waypoint, I go to CRZ and select the assigned speed? Thought the FMC would have an option like the 777 with setting desired cruise speed at each waypoint. Guess not!

As I mentioned in my last post, the frequency that the 737 traverses constant speed segments is pretty limited, when compared to the 777. The 777 and NG, though both made by Boeing, are vastly different aircraft, made for entirely different purposes.

If any NGX update is planned it would be great to have auto tuning VOR's like the 777 has I believe the 737 has this in real life

Have a specific reference for this? I know there is some autotuning (which ours already has), but only on a DME level for crosschecking and updating the computed IRU position, supplementing the GPS. You can see this on the NAV STATUS page.

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If any NGX update is planned it would be great to have auto tuning VOR's like the 777 has I believe the 737 has this in real life

The 737 Classic had VOR auto-tuning. The 737 NG does not.

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The 737 Classic had VOR auto-tuning. The 737 NG does not.

The NG has dual-channel DMEs, which are each capable of tuning two DME ground stations (on separate frequencies) simultaneously, and calculating distance to each.

The primary channel of the DME transceiver is linked to the VOR or ILS frequency of the onside NAV control head, and provides the cockpit display of distance to the facility. The primary channel remains fixed on whatever VOR or ILS has been manually tuned by the flight crew.

The secondary channels of the DMEs are auto-tuned by the FMS to provide DME-DME distance updates for updating the IRU position (in addition to GPS updating).

The auto tuning of the DME secondary channels takes place in the background and is not normally visible in the cockpit, though the crew can see which secondary DME channels are currently in use by selecting the appropriate page on the FMS CDU.

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So if I activate VOR1 and VOR2 switches the stations do not get auto-tuned to closest stations in your flight plan like they do in the 777 and 747? Obviously my 737 systems knowledge is pretty basic I figure it would be like other Boeings.

David Lee

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So if I activate VOR1 and VOR2 switches the stations do not get auto-tuned to closest stations in your flight plan like they do in the 777 and 747?

Not sure what you mean by "activate VOR1 and VOR2 switches." I'm guessing you're talking about the display switches for the ND. These only toggle the display of the currently selected VOR, and do not prompt the aircraft to automatically tune them (or not tune them). The 777 and 747 are always doing this by default, but you may not see it if the VOR/ADF switches are not switched to VOR to display this.

The 737 is doing this as well, but it just doesn't show you on the ND, because it is only tuning the appropriate DME frequency, without affecting the basic VOR tuning.

Obviously my 737 systems knowledge is pretty basic I figure it would be like other Boeings.

Define "other Boeings," though. The 744 and 777 are both on the same 'generation' so to speak. The 737 has been lagging behind mostly for reasons of commonality. It's somewhat the "odd man out" when it comes to aircraft offerings, as it's a design from 1964, with roots back to 1958. The 737MAX is where I think you'll see it get up to the 744/777 'standard' of things. The 744, by comparison, was an extensive update of a design from the same time period, but the elimination of the FE meant that the increase in automation, and extensive re-design were easier to justify to the industry. Interestingly enough, the original 767s delivered to UAL had an FE position because of pressure from the operator/union to not eliminate the position - aircraft development is a tricky business, which is partially why the 737's changes have been piecemeal. The 777, coming post 744 updates, of course, reaped these benefits, and entered into the product line without the roots holding it back from the "new Boeing" if you will.

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So if I activate VOR1 and VOR2 switches the stations do not get auto-tuned to closest stations in your flight plan like they do in the 777 and 747? Obviously my 737 systems knowledge is pretty basic I figure it would be like other Boeings.

David Lee

Although the 737 is an older design than the 747/777, the avionics in the 737 NG are actually of a newer generation than what is found in either the 747-400, 757/767 or earlier 777s. DME transceivers capable of tuning and tracking two simultaneous DME ground stations (one for the cockpit display, one for the IRU updating) only became readily available in the mid 1990s when the NG was introduced.

The 747/757/767 use older, single channel DME's which is why the primary NAV receivers have to be auto tuned, which is visible to the flight crews on the RMI and Nav displays. This does not happen on the NG because there is no need for it. It happens in the background.

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The NG has dual-channel DMEs, which are each capable of tuning two DME ground stations (on separate frequencies) simultaneously, and calculating distance to each.

The primary channel of the DME transceiver is linked to the VOR or ILS frequency of the onside NAV control head, and provides the cockpit display of distance to the facility. The primary channel remains fixed on whatever VOR or ILS has been manually tuned by the flight crew.

The secondary channels of the DMEs are auto-tuned by the FMS to provide DME-DME distance updates for updating the IRU position (in addition to GPS updating).

The auto tuning of the DME secondary channels takes place in the background and is not normally visible in the cockpit, though the crew can see which secondary DME channels are currently in use by selecting the appropriate page on the FMS CDU.

I didn't mention DME auto-tuning and I don't disagree with any of what you said. I referred to VOR auto-tuning as per the OP's question.

nice info thanks

Edited by rsneha

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19 minutes ago, rsneha said:

nice info thanks

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You do need to remember that the 737 has always been a parts bin special. It may use the same GATES or various tuning units on the pedestal, and have similar screens to other more modern Boeings but there is a fundamental difference that even the MAX shares.

It is not a digitally integrated data bus design. Thus all the systems are discretes and owe more to the 707 and 727 than the 787!

A lot of stuff in there has no idea what other bits are doing. You select VORs from the tuning head, and the boxes under the floor do as they are told.

Yes, we have dual channel DMEs, and the FMCs do talk to the NAV boxes as a result. but without the integration you will never get EICAS and other system information on screen.

Never overestimate the simplicity of the 737! It's not big and it's not clever...

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