Calculating 'Meet Time'
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Calculating 'Meet Time'
Do any of you procedural radar guys remember how to calculate a basic meet time? Was it just the average of the speeds of the converging a/c to get a miles per minute, then double that, divided by the number of miles apart? I vaguely remember some exotic formula...
Canoehead
What do you mean by a basic meet time? Whatever the explanation, why would you want to ascertain an average speed and then double it (two calculations)?
2 s
What do you mean by a basic meet time? Whatever the explanation, why would you want to ascertain an average speed and then double it (two calculations)?
2 s
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There were formulae which we D-Men used to use, mainly to determine the time during which vertical separation need to exist. Alzheimers or something prevents me from disclosing them. T1 + T2 divided by something rings a bell. I can, however, remember that it was over 40 years ago. If anybody can remember it would be nice to have one's memory jogged...
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Well, first radar as opposed to tower people. Should have said Center controllers.
But I seem to remember from another life that in a non-radar environment, you could discontinue vertical separation ten minutes after aircraft on converging tracks had passed each other, just can't remember how we actually calculated that 'meet time'.
2s - If both a/c have a G/S of 8 miles per min, then you double it because they are closing on each other at 16. And if you know the distance between them, you divide the dist. by the speed and get the time at which they will 'meet'. Or do I have it all wrong?
But I seem to remember from another life that in a non-radar environment, you could discontinue vertical separation ten minutes after aircraft on converging tracks had passed each other, just can't remember how we actually calculated that 'meet time'.
2s - If both a/c have a G/S of 8 miles per min, then you double it because they are closing on each other at 16. And if you know the distance between them, you divide the dist. by the speed and get the time at which they will 'meet'. Or do I have it all wrong?
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Always used "Time of passing" in my previous life performing the "black arts". The phrase had that lovely sense of "impending doom".
The method of calculating "time of passing" is only revealed to those who have been initiated into the "brotherhood".
10 minutes before or after "time of passing" is all I'm allowed to reveal to novices.
OTB
The method of calculating "time of passing" is only revealed to those who have been initiated into the "brotherhood".
10 minutes before or after "time of passing" is all I'm allowed to reveal to novices.
OTB
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Ah, yes. Those good old "Black Art" days. 
I prided myself being quite good at it when most aircraft didn't have DME and we had no radar. It was always satisfying to hear the acknowledging mic blips, right on the calculated time as the aircraft passed, just prior to the pilots reporting the visual passing. Unfortunately, I too have forgotten the formula, but it was based on the ETAs/ATAs at reporting points either side of the crossing point. Anybody got an early 1960's (green) 1st edition of the NZ MATS?
I prided myself being quite good at it when most aircraft didn't have DME and we had no radar. It was always satisfying to hear the acknowledging mic blips, right on the calculated time as the aircraft passed, just prior to the pilots reporting the visual passing. Unfortunately, I too have forgotten the formula, but it was based on the ETAs/ATAs at reporting points either side of the crossing point. Anybody got an early 1960's (green) 1st edition of the NZ MATS?
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A quick search on PPrune archives reveals this for reciprocal tracks and more.....
zkjaws
5th May 2009, 09:40
There is a simple formula to work out the Estimated Time of Passing (ETP)
ETP = ETO1 + EET1 (ETO2 - ETO1)
EET1 + EET2
EET = Estimated Elapse Time between two points
ETO = Estimated Time Over one point
It can be found on a pray wheel (nav computer) by:
Set EET1 + EET2 on the outer scale over EET1 on the inner scale;
Opposite ETO2 - ETO1 on the outer scale read X on the inner scale
Add X to ETO1 = ETP
zkjaws
5th May 2009, 09:40
There is a simple formula to work out the Estimated Time of Passing (ETP)
ETP = ETO1 + EET1 (ETO2 - ETO1)
EET1 + EET2
EET = Estimated Elapse Time between two points
ETO = Estimated Time Over one point
It can be found on a pray wheel (nav computer) by:
Set EET1 + EET2 on the outer scale over EET1 on the inner scale;
Opposite ETO2 - ETO1 on the outer scale read X on the inner scale
Add X to ETO1 = ETP
Last edited by qwerty2; 3rd Oct 2012 at 13:05.
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Never did it for real (the "cowshed" doesn't count) but using IAS in the diagram above doesn't seem quite right.
TAS might be ok, but you could be caught out with the wind.
Ground Speed (GS) would be better methinks...
BEX
TAS might be ok, but you could be caught out with the wind.
Ground Speed (GS) would be better methinks...
BEX
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Never did it for real (the "cowshed" doesn't count) but using IAS in the diagram above doesn't seem quite right.
TAS might be ok, but you could be caught out with the wind.
Ground Speed (GS) would be better methinks...
TAS might be ok, but you could be caught out with the wind.
Ground Speed (GS) would be better methinks...
It might be GS.....
BEX; changed diagram to reflect GS. Thanks!
Last edited by LoserGill; 5th Oct 2012 at 20:23.
The correct formula (IIRC) is posted by qwerty 2 above.
The time of passing was as good as the relevant ETA's at each of the reporting points concerned, and was usually within a minute, on the relatively short sector lengths in the airspace I worked.
One of my trainers on Area insisted that as procedural controller, I write the ETP for every potential confliction on the radar controllers' strip before passing it to him/her. I became quick at the calculation. I wouldn't remember how to do it now. I haven't even seen a circular sliderule/nav computer for years.
Use of airspeeds is irrelevant. GS is the only measure that can provide the correct figure, hence the use of a formula based on the actual/estimated time at fixed points.
Using IAS will provide the correct value only in nil wind, and if the two aircraft are at similar levels (obv not the same level).
There weren't many times I had to use it in anger. You'd have to be a bit unlucky and have a DME station or two inop. Using 10DME after passing is usually a bit less restrictive than using 10 min.
Back in the day, the guys working oceanic were very good at it. Had to be.
The time of passing was as good as the relevant ETA's at each of the reporting points concerned, and was usually within a minute, on the relatively short sector lengths in the airspace I worked.
One of my trainers on Area insisted that as procedural controller, I write the ETP for every potential confliction on the radar controllers' strip before passing it to him/her. I became quick at the calculation. I wouldn't remember how to do it now. I haven't even seen a circular sliderule/nav computer for years.
Use of airspeeds is irrelevant. GS is the only measure that can provide the correct figure, hence the use of a formula based on the actual/estimated time at fixed points.
Using IAS will provide the correct value only in nil wind, and if the two aircraft are at similar levels (obv not the same level).
There weren't many times I had to use it in anger. You'd have to be a bit unlucky and have a DME station or two inop. Using 10DME after passing is usually a bit less restrictive than using 10 min.
Back in the day, the guys working oceanic were very good at it. Had to be.
The best formula I have been given works for reciprocal tracks
Aircraft 1 reports A at 0510 and estimates B at 0550
Aircraft 2 then reports B at 0515 and estimates A at 0558
Add all the minutes 10+50+15+58 = 133
divide by 4 = 33.25
Time of passing is 0533.25
If any time is in a different hour then add 60 to that min
ie if aircraft 2 estimates A at 0610 (slow G/s) then addition is
10+50+15+70 = 145. Divide by 4 give cross time at 36.25
Simple and quick and works for any ground speeds.
Just don't ask me HOW it works!
Aircraft 1 reports A at 0510 and estimates B at 0550
Aircraft 2 then reports B at 0515 and estimates A at 0558
Add all the minutes 10+50+15+58 = 133
divide by 4 = 33.25
Time of passing is 0533.25
If any time is in a different hour then add 60 to that min
ie if aircraft 2 estimates A at 0610 (slow G/s) then addition is
10+50+15+70 = 145. Divide by 4 give cross time at 36.25
Simple and quick and works for any ground speeds.
Just don't ask me HOW it works!
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During the short time I actually practiced the 'black art', there was a guy who figured his meet time, then cleared an A/C through another's altitude at that time. Problem was, he forgot to add the required 10 min as per the sep standard. So, on the freq comes LH, 'now leaving 350 for 390, just passed our traffic.... btw, you guys are really good!' . Needless to say, that guy was taken out to the woodshed....