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jetblues
29th Jul 2004, 06:53
Now this ones quite a talking point amongst the ranks. If you are given an RTA, what are the time tolerances ?

The two most common answers are;

1. 60 seconds early, 0 seconds late
2. 30 seconds early, 30 seconds late

What are your thoughts, and more importantly what are your Jepp/ATC references to back up your answer.

Cheers

Hempy
29th Jul 2004, 07:59
AIP 19.3.1

"During flight pilots must maintain a time reference accurate to within +/- 30 seconds."

I don't think this would change with a requirement to arrive overhead

SM4 Pirate
29th Jul 2004, 08:47
Both answers are correct... well sort of! The 60 seconds applies to ATC and the 30 secs in the AIP reference to pilots...

So if the pilot gets a time they must meet it +/- 30 seconds; however if the ATC gives you speed and vectors etc; instead of time, the ATC tollerance is 60 seconds...

There may be circumstances when the ATC gives you a time, which has already been adjusted for the ATC tolerance. So the ATC is expecting you to be within 30 seconds as they have already used their tollerance....

E.G. MAESTRO says ARBEY 30 then 250KTS, but due to traffic considerations the ATC gives you 31, if you turn up at 32 there will be a gap, if you get there at 31.5 we can probably make it up with an abreviated circuit or adjusted speed. and if you get there at 30.5 nobody will notice... well sort of, but you get what I mean.

sorry no reference included, but will try and find later.

jetblues
29th Jul 2004, 12:02
Hempy just looked at that reference in my Jepps, ATC AU 505 5.3 and the statement is correct.

The application though is very vague. Many moons ago when starting my training I had been lead to beleive that the 30 sec was primarily in reference to the watch on my wrist (or time keeping device in the aircraft).

So lets say ATC's clock expects us at Arbey at 0210 and their clock is +/- 30 sec they could expect me from 0209.5 - 0210.5.

Say my clock is consistently 30 sec different from ATC then they could expect me from 0209 - 0210 or 0210-0211.

My maths suggest they are looking at a possible 60 sec error. So with combined errors (ATC 30 sec and Pilot 30sec) could +/- 60 seconds be more correct ?

Macrohard
29th Jul 2004, 13:10
Jepp ATC AU-1001 para 3.11

A pilot shall endeavour to leave the holding point on time, but one minute ahead of time is acceptable.

About does it for me, as there is no other reference to time tolerances for RTA's.

However, I do see both sides of the fence during flight when given an RTA, and have no argument with the A/C arriving 30 secs late, due lack of reference material in the Jepp.

Tinstaafl
30th Jul 2004, 00:43
The keeping of a time reference within +/- 30 sec relates to the accuracy of the timepiece used. It's not a navigational tolerance.

There used to be a reference to +/- 2 mins for navigational accuracy. Bear in mind this applied to when ATS must be advised about a changed ETA. There was no suggestion that it was a time-on-target requirement.

jetblues
30th Jul 2004, 02:50
Macrohard agree 100%. Thats the reference I use.

Chapi
30th Jul 2004, 15:29
I do not think that there is any specific AIP reference that defines the accuracy to which pilots must meet fix times used for sequencing.

When ATC issue fix times, it is with the intent of achieving an arrival sequence by allowing pilots to adjust their speed and descent profile with minimal ATC intervention (ie minimise vectoring/holding).

If the first aircraft of an in-trail sequence arrives ARBEY 30 secs late, and the second aircraft arrives 60secs early, the 2-min sequence is reduced to 30-secs! Not good.

So ....

- try and meet the times as accurately as possible;
- it can be helpful to let ATC know how you will achieve the time (ie what speed you will be on descent)
- if you can't meet a time, let ATC know in a timely fashion;
- ATC will adjust/fine -tune with speed control and/or radar vectors to assure separation and maintain the sequence (if necessary - sometimes its critical ... sometimes less critical)

karrank
31st Jul 2004, 02:21
I'm not aware of any relevant reference, but I like the Jep one. It leaves us the most flexibility. A minute early is a lot better than a minute late. The minute early we can soak up easy, the minute late is a minute lost to the sequence. On time is brillliant, but still might not work...

The TMA & the flow machine are working in seconds, but enroute are working in minutes, and those minutes will be clicking back and forward (in a loose sequence) until you call the TMA. You may be busting a gut to meet a time that is actually 1.4 min later than we (all of a sudden) need you there.

Add in variables like you a minute early and the heavy in front 2 minutes late (bluddy A340s) and thats when the sequence is changed. Jolly good fun and boosts the canteen's coffee sales:E

Please don't use this as an excuse to drive through early to snipe somebody elses slot but, some misguided, twisted souls think these silly times are a means of achieving separation also.

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