Crosswind approaches

[Groundbased]

Hello,

I am very interested in commercial jet aviation and would like to understand the procedure for planning a crosswind approach. How do you calculate heading and drift in a crosswind to get the correct track? From articles/reports I have read I understand that, depending on operating procedures, being above or below the glideslope by more than a certain amount may mean you have to go around. Does this also apply to the Localiser course?, if so do procedures for crosswind approaches make allowance for this?

Incidentally, where are the worst?, I understand Funchal is very tricky.

Thanks in advance for any answers.

[Bluejet]

Grief there's a peculiar question.

Right I'll give it a shot. It is almost inevitable that most approaches to a landing will involve an element of crosswind. It is very unlikely that the wind ever blows straight down a runway. For example Birmingham Airport Uk (BHX) has a North South runway, but the prevailing wind is East West, so all approaches tend to involve a crosswind.

Planning as such is normally restricted to determining whether the wind is within the limits of the aircraft or the limits of the crew, whichever is least. For example a new First Officer making an approach may have substantially reduced crosswind limits than more experienced Crews. To determine the Crosswind component there are numerous charts in performance manuals, which are carried in the flight deck. However most pilots use a 'rule of thumb' which basically amounts to getting the wind driection and for every 10 degrees off the runway centreline that the wind is then take 1/6th of the wind as crosswind. So for example a northerly facing runway Rwy 36 and a wind of 030/15kts would give your 30 degrees off the centreline, so thats 1/6th of the wind per 10, so its about half the wind. In this case about 7 or 8 kts crosswind. This continues upto 60 degrees off which is considered to be all the wind as crosswind. From 60 to 120 degrees it's all crosswind, and after 120 degress there is a crosswind component calculated as above and obviously a tailwind as well.

Having assessed the crosswind you make sure it's within your limits and fly the approach. Assuming it's IMC conditions you are looking at your ILS indications. These, as you have rightly said, comprise a glide slope indication (vertical) and a localiser indication (horizontal). The simplest way to do it and the least professional is to point yourself straight down the runway and fly the correct rate of descent and heading. Inevtably the wind will start to blow you off the localiser, so you correct a few degrees and keep watching. if the wind continues to blow you off, correct a few more degrees etc. etc. Eventually you fly a wobbly localiser to decision.

However, more rules of thumb come in to play. If you are flying at 60kts G/S then every knot of crosswind is 1 degree of correction. So a 60 kt approach with an 8 kt crosswind would suggest flying 8 degrees off the centreline into wind. A 120kt approach uses half the crosswind (i.e 4 degrees) and a 180kt approach uses a 1/3rd of the crosswind and so on. So right at the top of the drop you can crank on the appropriate wind corrected heading and fly the ILS.

Commercially, most operators require their aircraft to be flown very accurately indeed. Witha Go Around being mandatory when either the Glide slope or the localiser goes out by more than half scale deflection i.e either needle being more than halfway off.

That's it in a basic nutshell. However it can get complicated. Say for example you are flying an aircraft that has a crosswind limit of 30kts onto a Northerly facing rwy. the wind is 090/29 gusting 37kts. So its the full crosswind component of 29, which is okay, but what about the gusts? or the worst case 090/18 with occasional gusts upto 32. Well generally you only take the basic wind into account, the gusts are for consideration only. So you'd make the approach, but be anticipating it possibly getting a little hairy at the bottom and maybe a very late G/A having to be flown.

So in the briefing stage you'd take all the above into account. It only takes a few second to calculate, and brief your approach. if the runway is wet and it's gusty then you may elect to take full reverse and go for a rapid de-acceleration to avoid any wing lift as you run along the runway. But that may not be required as the Ground Lift Dumping devices on the wings can normally assist in stopping that happening.

Hopefully the above is a small help. It's not really all that hard, and accurate calculations are impossible. this is because the wind backs off and gets less as you approach. It would not be unusual to have a 30kt crosswind at 2500', a 15 kt crosswind at 1500' and a calm day on the ground. So it's more a fly it and see what is happening. maintaining an accurate scan of your instruments to prevent excursions out of your Glideslope and localiser limits.

Cheers.

[Semaphore Sam]

To summarize; with high cross-winds, get F/O to have book out to calculate cross-wind at 100' (250' in marginal wx) to figure legality; based on result either go-around, or (if legal) apply x-wind techniques and land (and hope for the best). If CAT II or III, go by reported wind; if not within limits, break off approach early.