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How to know if you stop before relevant RVR's?

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How to know if you stop before relevant RVR's?

Old 11th Jun 2021, 03:41
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How to know if you stop before relevant RVR's?

Greetings,
I'm just wondering we have relevant RVR's when an aircraft speed is above 60kt. So how do you know at 'which point' you would decelerate below 60 kt when planning in the air and before briefing the approach?
For example, if you fly A320 in CAT II weather, and the MID RVR is below minima, let's say 75m, but the Touchdown RVR is fine, 300m or so. Would you still continue to land? and how do you know that you would reach 60kt or below before the MID RVR?
Many thanks.

Last edited by Aceostrat; 11th Jun 2021 at 10:01.
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Old 11th Jun 2021, 05:24
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What is the relevance of the 60kts you mention?
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Old 11th Jun 2021, 05:37
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The RVR values are controlling in those runway segments where the speed is above 60 knots. However, I can't think of any way to calculate where you will reach 60 knots. If the last third of the runway has RVR out of limits, you should be OK if your landing distance falls entirely within the first two thirds, which are within limits.
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Old 11th Jun 2021, 08:51
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You know from IFLD calcs how much runway you might use and you could subtract an arbitrary 100m as 60kts to zero (works out at ~0.4g which is close enough). Divide LDA by three, then compare with the number you came up with earlier to decide whether itís relevant or not. Simples.

Now for the Human Factors bit. Is it sensible in terms of risk? What could go wrong and what mitigation strategies do you have? Why do we use all these different minima anyway? Will the airport file a report that might find its way to the regulatory authority, who may be interested in your decision making process?

In 30yrs of flying, Iíve never come across the situation described but itís not an impossible scenario. However, given the Vref of most jet transports, the factoring up of landing distance for autoland, the LDA of all but the most massive runways and the likelihood of encountering a stable 300/75/-- while shooting a Cat II, itís something that realistically youíd only encounter in a technical questionnaire...
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Old 11th Jun 2021, 10:26
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Cat II autoland: MID required 75 w/ rollout
Cat II manual: MID required 125 same as LVTO.

T or F? The memory is failing.

Never found a way to calculate what is being asked, and the airline was always happy to use the approximations already stated. CAA too, as well for the reasons above, never made this into a subject of oral competency review.
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Old 11th Jun 2021, 17:49
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Keep it simple.
Just calculate the distance needed until you stop and see if it falls within the first, second or last third of the runway. I donít play around with trying to adjust it to 60kts.

And if your gut feeling says itís still not sensible, trust it.

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Old 12th Jun 2021, 02:51
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Aceostrat

If you don't have the required RVR's you cannot continue the approach assuming you are above your company "approach ban" altitude. My company uses 1000', so if RVR's not correct at 1000' GA. However, if the RVR's were good at 1000' but subsequently went below minima when A/C below 1000', we would continue to minima and if the required visual reference was available Land, if not GA.
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Old 12th Jun 2021, 08:32
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EcamSurprise

Sounds good to me..

Run an IFLD, consider the runway and met conditions and using company suppled data (e.g. QRH) see if you can find and if so use an autobrake setting that brings you to a halt in no more than two thirds of the runway length....job done.

Do anything else and you are just guessing and in the worse case what you have done might not stand up to scrutiny at a subsequent board of inquiry.
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Old 15th Jun 2021, 02:09
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Thank you everyone
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Old 15th Jun 2021, 12:57
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EcamSurprise

Probably the most sensible approach. I’ve had it where the midpoint is below minimums on a couple of occasions but have been confident that I can stop in half the runway length. Therefore midpoint not needed - in theory.

Probably not going to look great if you have a runway excursion and subsequently need to explain your actions, so it’s down to personal attitude to risk, experience, runway conditions and wind. It’s probably not a good idea to take the risk if it’s 20kts across a wet runway; calm and dry - crack on!
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Old 15th Jun 2021, 13:49
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Why would you need to know; any specific requirement for landing?
The approach RVR limits (first third) are primarily associated with having the required visual reference at DH, which will be sufficient for the type of operation; auto-land, or manual-land reversion, etc.
Lower RVRs (mid, end) generally consider the ability to maintain directional control when on the runway; e.g. auto rollout revert to manual, or manual, etc. (As per FlightDetent #9)

The different tasks are reflected in the RVR values, and also that during landing the aircraft is decelerating.

There may be a different interpretation for takeoff, e.g. having to maintain directional control for an RTO.

Most airports limit operations to 75m RVR due fire rescue limitations.

Use the highest autobrake setting.

Last edited by alf5071h; 15th Jun 2021 at 14:38.
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Old 15th Jun 2021, 15:51
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Originally Posted by alf5071h View Post
Why would you need to know; any specific requirement for landing?
I'm assuming the question was posed by the OP probably out of academic interest because of (as I recall it) the requirement in some (?all) jurisdictions that the stop end RVR becomes controlling if the groundspeed on entering that portion of the runway is above 60 kts...
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Old 15th Jun 2021, 16:30
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wiggy, thanks.

So for academic interest, if the landing were to be made according to the - ‘stop within 60% of the runway length’ (the plan), the remaining 40% should be greater than the last third of the runway (33%); i.e. stop end RVR never relevant.

[ ] ‘Yes the distance can be calculated, but we never do.’ / [devilment]

If the less than 60kts is a requirement, then the question ‘how to ensure this’ should be directed to the regulator. Lest this be another ‘good idea rule’ lacking practicality.

My quandary suggests that I don't understand the issue, nor the regulators point of view.
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