Circle to land approaches

[Groundbased]

Hi,

How does this type of procedure work? I look at quite a lot of approach plates but don't see specific plates for circling approaches.

Is it the case that they inherently carry more risk than other types of approach?, certainly a number of accident reports indicate circling in poor wx conditions has been a factor.

I'm also wondering how often you have to perform this type of procedure.

Many thanks for any answers/information.

GB

[saudipc-9]

GB,
A circling approach is one where the instrument approach is flown to one runway but then the actual landing is done on another. The only information to be found on the plates will be higher minimums on the MDA/VIS due to the fact of obstacle clearance and that it really is a visual approach at the time you are doing it. The different mins are dependant on the category of aircraft being flown ie the faster your approach the greater your minimums.
If flown properly this procedure can be quite safe but as with any other approach it does have it's draw backs ie losing sight of the runway being flown to and not electing to carry out a proper missed approach.
I guess the number of times you need to fly the approach depends on the type of airport being flown to. The bigger ones will have a precision approach to all runways where a smaller one might not and have a requirement to circle more dependant on weather and winds.
I fly one quite often but more for local operations than anything else.
cheers

[BOAC]

R22 Nice (LFMN) has a chart showing circling. Being a touche noise sensitive:{ , this one is NOT a standard 'circle to land'

[Tinstaafl]

Circling approaches are usually depicted on the same plate as the runway approach that uses that navaid.

Some instrument background info:


To be able to land a pilot needs to perceive several things:

* Position of the a/c relative to the runway
* How that position is changing laterally, vertically & longitudinally WRT to the runway + the trends of these things
* A/c orientation WRT to the runway + trend
* Wind effects
* A/c airspeed + trend
* A/c attitude + trend
* Decide how & how much to adjust/correct/maintain/compensate for all of the above.
* The a/c also needs time & space in which to respond to the pilot's corrections in response to the above.

The transition from non-visual flight to visual flight & landing as well as being able to ascertain those bits of information takes time. The more accurately the approach can position the a/c, the more consistantly it does so and the 'richer' the visual information that is available to the pilot to assist in gaining that information then the less time will be needed to make those judgements.

If the positioning of the a/c due to limitations of the approach type is less accurate then more time & room to manouvre will be needed from when/where the pilot can see the runway to when/where the a/c would land. That results in higher minimas being specified for the less accurate types of approaches ie higher cloudbase & greater visibility.

A precision approach eg an ILS or MLS provides a means of positioning the a/c accurately & consistantly and, in concert with standards systems of runway markings/lights & approach lights, provides a rich source of visual information to the pilot. This means that less time is needed for the pilot to perceive the necessary information AND the a/c will be placed accurately within a small volume of airspace. End result is that the minima can have a low cloud base and a low visibility and still have sufficient time/room for the relatively small corrections that will be required. The minima will be printed on the chart (except in old style UK charts where you're expected to calculate this vital piece of data yourself.... :rolleyes: )

Next step 'down' in approach accuracy would be to substitute precision glideslope guidance with step down height vs. distance tables. That would be a Localiser/DME runway approach. It retains the left/right precision of the ILS but the a/c may arrive a bit high/bit low to the ideal profile. To compensate the minima will be a bit higher than for the ILS. The ht./dist. table is generally printed on the same plate as the ILS approach along with a separate minima that applies to the LOC/DME approach.

Next step 'down' would be something like a VOR/DME runway approach. Now the precision left/right guidance is substituted with a rather less precise left/right guidance. This means that not only could the a/c arrive a bit high/bit low but also further displaced from the runway centreline. Ergo, higher minima again to give more time & room to perceive & correct the deviation. Since it's a different navaid then a different plate would be printed.

Next step down would be to remove the availability of DME, making the VOR/DME ht./dist. table superflous. This means the a/c could arrive in a quite wide & relatively uncertain range of altitudes. So, even higher minimas to allow for this. The approach is still aligned with the runway centreline to the same level of accuracy as the VOR/DME approach. This can still use the same plate, but with a separate minima specified on it.

Next step is to lose the runway alignment requirement all together ie a circling approach. Now the position of the a/c when it arrives at the end of the instrument part of the approach is a lot more variable. There will almost certainly be a fair amount of manoeuvering needed once visual to fly the a/c to a position from where a landing can be made. So, the minima goes up again. The approach can still be depicted on the same chart as any of the other approaches as long as the higher circling minima is specified + any particular restrictions are specified eg no circling to the south of the runway etc etc.

The key with these is to brief for the approach before it's commenced so that everyone is clear about exactly what will be done AND what information is relevant or not.


NB: There are other considerations to how the minima are determined such as terrain, missed approach requirements, means of establishing on final, does final involve flying towards the navaid or away from it (towards ==> increasing azimuth accuracy) etc. The above is just a broad outline.

[BOAC]

Wow! TINSTAAFL seems to have covered that pretty well! Think I'll print it out and stick it on the dash for my briefing in my next sim check:D

One more point for discussion - my understanding of the g/round from a circling is that you carry out the MAP for the INSTRUMENT approach you started with, which as TINSTAAFL says, can make for a bit of a head scratch, hence HEED his advice to brief properly!

[Pilot Pete]

Well said BOAC,

just for those who haven't realised the full implications of that last statement; if for instance you flew an ILS to rwy09 and then circled at the circling minima to land on rwy27, and as on your left/right base for 27, or even on final you disappeared into some 'mirk' and lost sight of rwy 27 you would then follow the missed approach for the ILS rwy09! As you will appreciate you could be facing the opposite direction to where you need to be going. A good point to remember is to call ATC say, downwind circling and clarify whether a NON STANDARD MISSED APPROACH to rwy 27 would be available.......makes life easier in the event of a miss, but remember some of the dark dirty airfields in some places of the world will have trouble understanding what you are requesting so as ever, make the judgement call early.

The way we actually fly the approach/ circle may also be interesting to you Groundbased. In the 75/767 and many other types as well I would guess, using the above example of rwy09/27 we would lock onto the localiser in the usual manner using the automatics (no time for proving your manhood on approaches like this!). The procedure then is varied in that we do not want to capture the glideslope as we want to level off at circling MDA instead of flying all the way down to touchdown. So what we do is use the vertical speed mode to start a descent when the glideslope indicator shows us on 'glide'. We use the Groundspeed to work out our V/S and use the glideslope indicator cross checked with DME hieght/distance to stay on the glide. The MCP altitude window will have the circling minima dialled up (to the nearest next largest hundred) to capture and level off, or, in some companies the pilot will leave the MCP set at the missed approach level off altitude and press ALT HOLD as the a/c reaches the circling minima.

You then fly level in the hope of popping out of the clag to see the airfield and as soon as it is visual you then carry out your manoeuvring to land on the opposite rwy. There are different methods of manoeuvring to get yourself downwind the right distance out from the rwy. We use a 45 degree turn, starting the stopwatch at the start of turn and timing for 53 seconds +/- 1 sec for each knot of head/tailwind (yeah complicated!), this gets us the right distance out, either L/R downwind to turn back to parallel the rwy.

Oh, I forgot to mention, we would be gear down/ Flap20 on the ILS and around the circuit.

Once abeam the touchdown end of the landing runway we would again start the stopwatch (after remembering to reset it to zero first!) and now a bit more maths will get us to the right point to begin our turn to base/final. We use the radio altimeter height multiplied by three to give the number of seconds to fly extended downwind. So, if RA is 900ft we drop the zeros and x3 = 27 seconds. This is for still air, so we then adjust by half the tailwind component in seconds (remember, it should only be a tailwind as you are circling to land into wind, otherwise I would seriously question my judgement in trying this manoeuvre!!!). With a 10kt tailwind downwind we would adjust our 27secs by 5 secs, obviously subtracting the 5secs as the tailwind will be extending you downwind.

At that point we begin the turn to base leg, keeping the field visual all the time (yeah right) and once your position visually is confirmed on base (maybe a continuous turn to final, depending on the wind) or as you go through 90 degrees to the runway, we take flap30 for landing and reduce speed to our approach speed and then use VERTICAL SPEED mode again to start our descent to the runway. Once stabilised we disconnect the automatics, turn the flight directors off and visually manoeuvre to make the landing. Easy. Well, not too hard when all is going well, but certainly one of the more challenging aspects of flying a big aeroplane because as Tinstaafl has pointed out so eruditely, you are tending to have to make bigger adjustments to rectify any slight errors due to the non-precision nature of the approach, especially from base to final. Big power changes lead to big pitch changes and destabilisation of the approach; remember you may be around 700ft AGL as you turn final to see 4 whites on the PAPIs! Due to it's more challenging nature and the fact that it is not done as often as the good old ILS it is therefore inevitable that there is a greater incidence of bad approaches being flown whilst circling. So much so that my company has put it into every sim check. it all sounds very complicated, but with a bit of practice you can get quite reasonable at it. Flown correctly it is no more dangerous than any other type of approach, it's just the points outlined above which mean it's a bit of a bigger deal for us up front.

Hope this helps answer your question.

PP