Circling Calculations

[Stone_cold]

I am curious to know where in the FCOM or FCTM it states that there is a wind correction for circling . As far as I see there is an "approximate" 3s/100' because it is a visual manoeuvre . That seems to be the only timing mentioned on downwind .

[Capn Bloggs]

Timing 3seconds per 100' Agl +\- wind

I reckon this is nonense and have done for years. What does it matter how high you are going abeam the threshold? The critical issue is going Downwind the correct distance so you will roll out of final by 400ft (or higher if you have to be stable higher up). By all means start descending passing abeam but the turn must occur at the same point over the ground. Headwind/tailwind on Downind? Adjust turn point by 1/2 second per knot of H/T.

[RAT 5]

Bloggs: Your 'circling' technique is not that approved by Pans Ops if circling at minima. Yours might work if flying a visual circuit above minima, but not on a manoeuvre conducted with cloud base sniffing minima.

[avoka]

Hi Vilas
I did check fcom and fctm during reading that thread and was so surprised that these docs had not recommended to do wind correction.Anyway it's good method to check distanse from active threshold ☝ �� �� and I think it's not so critical to have some extra distance due to tailwind on the base turn!
Safe flights

[vilas]

I am talking about end of down wind time. Blogs, your procedure seems to be different. Airbus wants you to time from abeam threshold as corrected for wind and start a level turn for finals, when you are about 90 degrees to the runway if correctly vertically positioned only then commence final descent. Purpose of timing is to place you at the same spot on ground for final turn. Basically a visual manoeuvre timing is for guidance. It has to be altered as required to remain visual and keep runway in sight.

[Capn Bloggs]

I am talking about end of down wind time. Blogs, your procedure seems to be different. Airbus wants you to time from abeam threshold as corrected for wind and start a level turn for finals, when you are about 90 degrees to the runway if correctly vertically positioned only then commence final descent. Purpose of timing is to place you at the same spot on ground for final turn. Basically a visual manoeuvre timing is for guidance. It has to be altered as required to remain visual and keep runway in sight.

Precisely as I said, Vilas. You can be at an MDA of 1500ft or 500ft, or have gone down to 400ft (PANs ops Cat C terrain clearance). It doesn't matter. Turn on time, corrected for wind. Use a Fix if you need a crutch. The crunch is, as you mention, keeping Visual with the threshold or "runway environment" on the latter part of Downwind. Very difficult if you want to roll out on Final at 1.5nm/500ft ("Stable"). And you will almost certainly exceed the circling min vis on the first part of the base turn. Never see the runway then...

And Yes, you do commence final descent only when intercepting the "normal" 3° path. At 500ft, that means a level turn all the way round.

[avoka]

Vilas
Let's think about the weather conditions during circling.It is not visual maneuver when good vis and clouds so high.It is option for pilot when weather conditions are lower than for visual pattern,but tail wind component little bit greater then wind for straight in approach.On default You have 10 kts min or greater tailwind at point abeam threshold,ergo -1 sec,but I think that it's not so bad to just keep in mind it,but to perform base turn in according to fctm recommendation and You are at point with extra distance from point in calm conditions-You just have to decrease Your v/s , the same time pay more attention to circling radius for cat C,D planes
Safe flights

[vilas]

avoka
When you turn 45 degrees and fly for 30seconds that also needs to be adjusted if winds are strong. That takes care of circling radius. Below from airbus documentNot for those who consider everything other than what they know as abstract document)


5.4. Timing for Circling:


The timing Airbus recommends is 30 seconds from wingslevel, adjusted for strong Head or Tail wind, by reference to the ND windindicator. However, this is a visual exercise: Timings are approximate only.


Blogs, I have to disagree with you about going down to 400ft. It is not a question of terrain clearance but obeying the circling minima. You cannot descend below minima to fly level. You can only descend below minima during the final turn

[Capn Bloggs]

The timing Airbus recommends is 30 seconds from wingslevel

Vilas, is that from abeam the threshold? Sorry, not familiar with Airbus circling procedure.

You cannot descend below minima to fly level.

Regulator's rules: we can descend to the minimum terrain clearance altitude in daylight.

[vilas]

When you break from the NPA at circling minima and turn 45 degrees either left or right then once the wings are level fly 30 seconds from wings level adjusted for wind for suitable displacement on downwind.

[vilas]

Bloggs
Do you operate to TERPS or PANS OPS?
PANS OPS says
Descent below MDA/H should not be made until: (1). Visual reference has been established and maintained. (2). The pilot has landing threshold in sight. (3). The required obstacle clearance can be maintained and the aircraft is in a position to carry out the landing.
Can the last condition be assured on downwind?

[avoka]

Vilas
agree-RECOMMENDS!!!! just recommended!!!
There are not such recommendations in documents,just magazine
Safe flights

[Capn Bloggs]

Vilas, PANS Ops. As I said, our CAA allows descent below MDA to terrain clearance in daylight prior to final descent.

Still confused about how far Downwind Airbus says. You could turn off the 45° degrees anywhere. But what defines the Base Turn point? 30" from abeam the threshold?

[Stone_cold]

Bloggs : approximately 45" to the landing threshold .

Vilas : could you say where it states that the downwind timing is adjusted for wind from the abeam point?

[Checkboard]

Capn Bloggs is obviously in Australia, where you can descend to 400' obstacle clearance in daytime (but must maintain MDA until you can make a constant descent to land at night).

In Australia it is also typical for the final approach for an NPA to not be runway aligned - it may approach the runway at any angle.

Bloggs, the Europeans typically only consider circling from a runway aligned approach - with the circle to the opposite direction runway - so the whole 45º for 30 seconds bit is how they obtain their downwind spacing. Not a good procedure, but Europeans never circle as a rule (so they are always nervous about it), which is why the OP wants an aid for it rather than looking out of the window.

[vilas]

Stone_cold
There was a instructor seminar held by airbus in 2012 which I myself attended in which one of the topic discussed was circling approach. What I quoted is from that. Since it is a mechanics of approach and a technique I consider it as valid. Otherwise even FCTM has no legal validity like FCOM. Since lateral downwind spacing is determined by ground speed in strong crosswind the head/tail component needs to be catered for. As stated timings are for guidance only.

[vilas]

Blogs
If you descend to terrain clearance 400ft on downwind do you consider approach stabilised on downwind? What is the stabilised approach requirement for your regulator?

[Stone_cold]

Vilas : I am not questioning the downwind spacing of 45° / 30 secs. I was asking about +/- corrections to determine the base turn , i.e. timing from the abeam position ( 3 secs/100' ) .

[Capn Bloggs]

Originally Posted by Vilas

If you descend to terrain clearance 400ft on downwind do you consider approach stabilised on downwind?

Not until steady on final commencing descent on the PAPI. We were approved for 400ft, then it changed to 500ft to "fit in" with everybody else, and now visual circling is not permitted as we have straight-in RNAVs everywhere. I note Airbus comments about using a 400ft stable height for circling approaches in it's Stabilised Approach note.

Back to the OP's issue, while this is supposed to be a purely visual manoeuvre, I think there is a case for using "the system" to help. I'm definitely not suggesting a building a full LNAV/VNAV min wx circuit, but using Fixes makes life easier, with no need for 45/30. My preference is to place a fix at the end of the Downwind leg ie at the Base turn point, then put a "radial" line back along the Downwind leg. When you get Visual, you then simply fly over to the line in TRK, follow it to the fix and turn Base. Having an app that would calculate that Fix point (taking into account spacing out from the runway/crosswind effect and distance/head/tailwind) would be helpful. The stopwatch would be used as a backup.

The Fix position would be based on the threshold waypoint. You could input the runway QFU, the wind velocity, the bank on Base and the speed, and it'd spit out the location of the base turn point WRT to landing threshold.

For normal circuits, I just use 40° off at 3.4nm.

[vilas]

Flying circling approach in nav mode is not a good idea and is forbidden in airbus after Air blue crash in Islamabad. Even in India circling approach is forbidden. Why a 3.4nm circuit isn't it too wide?