Here's a start. It takes a Start time and a Finish time and gives the answer in the format of 02:30:45 for 2 hours 30 minutes and 45 seconds. I assume we're talking about RC planes, since you speak of flight durations in terms of minutes/seconds, but I included the hours for doing totals of the flight time then displaying the results.
 Private Sub Finish_BeforeUpdate(Cancel As Integer)

TotalSeconds = DateDiff("s", Start, Finish)


HoursLapsed = Int(TotalSeconds / 3600)

If HoursLapsed = 0 Then HoursLapsed = "00"


SecondsLeft = TotalSeconds Mod 3600


MinutesLapsed = Int(SecondsLeft / 60)

If MinutesLapsed = 0 Then MinutesLapsed = "00"

If MinutesLapsed <> 0 And Len(MinutesLapsed) = 1 Then MinutesLapsed = "0" & MinutesLapsed


SecondsLapsed = SecondsLeft Mod 60


If SecondsLapsed = 0 Then SecondsLapsed = "00"

If SecondsLapsed <> 0 And Len(SecondsLapsed) = 1 Then SecondsLapsed = "0" & SecondsLapsed


Duration = HoursLapsed & ":" & MinutesLapsed & ":" & SecondsLapsed


End Sub

As a strategy towards totalling flight times, I'd store the variable
TotalSeconds in a field for each flight, then take Sum of
TotalSeconds and run it thru the same formula as above!
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