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ADS-C and Indicated versus True Mach
My understanding is that ADS-C reports indicated mach (among other parameters of course). Our Falcon equipped with ADS-C has indicated mach of .84 for a true mach of .82.
Would this cause any concern to an oceanic controller, should he/she continually see a reported ADS-C mach of .84 knowing that the assigned mach was .82? This was posted in Tech Log here; https://www.pprune.org/tech-log/6183...true-mach.html however I would appreciate a controller's perspective Thanks, Hawk |
Gosh, no one here with any thoughts?
Hawk |
Hey. I believe the ADS-C reports show the indicated mach number as cleared rather than the true mach number you might actually be flying. Don’t quote me on that though but I would believe that it’s similar from an ATC perspective as IAS vs. TAS, you fly a certain TAS to comply with a cleared IAS and that changes with altitude etc etc. You’re cleared M.083 and you are a true M.0812 to fly an indicated M.083 as input into the FMS then you are considered and shown as doing a M.083. ADS-C reports show if there is a discrepancy between actual vs. cleared route for instance. If a different fix/route is inserted into the FMS than is cleared a message promts and advises the controller. Would probably be the same action if a report shows a different mach than cleared. |
Originally Posted by SThor
(Post 10397026)
Hey. I believe the ADS-C reports show the indicated mach number as cleared rather than the true mach number you might actually be flying. Don’t quote me on that though but I would believe that it’s similar from an ATC perspective as IAS vs. TAS, you fly a certain TAS to comply with a cleared IAS and that changes with altitude etc etc. You’re cleared M.083 and you are a true M.0812 to fly an indicated M.083 as input into the FMS then you are considered and shown as doing a M.083. ADS-C reports show if there is a discrepancy between actual vs. cleared route for instance. If a different fix/route is inserted into the FMS than is cleared a message promts and advises the controller. Would probably be the same action if a report shows a different mach than cleared. ADS-C I believe sends the mach indicated by the air data system, in this case it would be the .76 value. Hence the question I have for our new Falcon 2000, an assigned true mach of .82 would be .84 indicated mach for this aircraft. So ATC would continually receive .84 via ADS-C when .82 was assigned. Does this make sense? |
I use it every day and, to be honest, I have zero idea if it is indicated or true and never considered the difference. All I know is, if I assign you a Mach, I am expecting that to be your indicated Mach which, if I had to take a guess, would be what is reported to me in the ADS-C report. I will take a look at the reports my next shift to see if it says.
But generally I treat it exactly like when I assign indicated airspeeds when I work a low altitude radar. When I tell a pilot to maintain 280 kts, I expect that to be indicated. When I tell a pilot to maintain Mach 80, domestic or non-radar oceanic, I expect that to be indicated. Only time I assign True is for a true airspeed, not Mach, for low and slow aircraft in Oceanic. But again, I have no idea what the report gives but based on almost never getting reports that have Mach numbers much different from what I assign, I would guess indicated gets reported. |
Checked today....it did not specify indicated vs. true in the report my system receives.
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Mach Number Technique
Us pilots are told that it is true mach we must fly when assigned a mach number, eg crossing the Atlantic, as in the link above. This is since longitudinal separation is based on it. So I don't know what else to think about this question Thanks for your reply JustAverage Hawk |
Correct, MNT is used extensively and even though satellite technologies are slowly being relied upon more and more to allow for distance based longitudinal separation rather than time based, my oceanic area in particular will always assign a mach number to aircraft entering.
That link you provided clarifies the confusion. As a controller, I just assign a Mach and the pilot does what they do to comply. It sounds as though most modern aircraft will have the indicated mach shown to the pilot be the actual true mach. Some aircraft however will require the pilots to do the conversion. That makes sense because if we were to profile an aircraft for knots instead of mach, we use true and not indicated. |
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