RTA Progress: Go faster to go slower?

[MRFarhadi]

Greetings,


I was using the "Less Restrictive" CPDLC in central asian airspace and resceived a RTA message to pass position "GOSPA" at or after 1103z, instead of the initial ETA of 1102z.
I was flying at FL400 with Cost Index of 50, resulting in Mach .87 cruise speed. Thus, I entered the RTA into the PROG page of the CDU but recieved anomalous numbers regarding RTA Speed; seemingly, I had to go faster, at MAX RTA SPEED of .88 just to make the 1102z ETA a minute slower to 1103z.
While I was playing with this odd phenomena, I reduced the RTA MAX SPD to .86 but the FMC still returned .88 as the RTA mach number to fly.




Best,

[Hardy Heinlin]

Greetings,

to analyze your situation we need a situ file of your situation.


Regards,

|-|ardy

[MRFarhadi]

Dear Hardy,

Unfortunately I haven't been able to produce such .situ file as I've been very busy lately. I'll send one when I possibly can, probably in the next week or so.


Best,