Month: December 2013

Windy Weather!

We’re having our fair share of windy weather here in UK & Ireland this month! This is thanks (or no thanks) to the deep low pressure systems that develop over the Atlantic and make their way over here with full strength. The pressure was incredibly low in Dublin – the lowest I’ve seen, 959Hpa. That is the equivalent pressure that would be at around 1,500ft in the air on a normal day!

The low pressure system currently over UK & Ireland, centred over Scotland.
The low pressure system currently over UK & Ireland, centred over Scotland.

I was on the early shift today, which meant waking up at 4am to get ready and get to the crew room in time to do my pre-flight preparation. The windy weather did wake me up a little earlier, and right away I knew that I’ll be having an interesting day today – with either diversions or delays, going by past experience.

The first flight was to Dublin, so we checked the weather for it and any suitable alternates. Dublin was already indicating that it was out of limits for landing.

EIDW 270630Z 22036G54KT 8000 -RA FEW017 SCT028 BKN038 07/04 Q0960

This is a METAR – a coded format to communicate weather to pilots. It says that at 6.30am, the wind was coming from 220º at 36 knots (around 41mph) gusting to 54 knots (around 62mph). It was raining, visibility down to 8km, few clouds at 1,700ft, scattered clouds at 2,800ft and a broken layer of cloud at 3,800ft. The temperature was 7ºC and the pressure at sea level 960HPa (this increased slightly from the very low 959HPa I saw reported earlier).

Boeing guidance says that the 737-800 can handle up to 40 knots of crosswind on a runway of standard width. However the company imposes specific limits that are more conservative – in the interest of safety. We uplifted extra fuel – to give us the flexibility to attempt some approaches, to keep our options open in terms of diverting to alternate airports and to give us some time to hold in the air to see if the weather was going to improve in order to allow us to make a successful landing.

We took off from a very blustery runway and as we approached Dublin the tower was still reporting wind vectors that were out of our limits, so we opted to hold in the air close to the airport to see if there was any trend in the weather towards improvement that would allow a successful landing.  After all, we do want to get our passengers to their destination with the least disruption possible! There was a passenger that had a flight to New York with a very short connection time, so we did get him to Dublin – I just hope he made it to New York as well! After a few circuits in the holding pattern we heard some good news – the wind was within limits! I advised the Captain “let’s have a go at an approach, we don’t know how long the wind is going to stay within limits!” We knew from the forecast that the wind could soon go out of limits again! The controller was informed of our intentions – that we would like to attempt an approach, and so we were cleared to exit the holding pattern and were given headings to fly to set us up for the approach. The wind was quite strong, nonetheless we managed to make the first successful landing into Dublin today.  It looked like a window of opportunity had opened up, and I saw a few more aircraft land behind us, and aircraft were beginning to queue up for departure again.

We managed to depart from Dublin after some delay – again due to the wind being out of limits for takeoff. Thankfully, though we did experience some strong winds at our other destinations, they were all within limits and didn’t cause further delay. It was a lot of hard work, but I enjoyed it, and it did get me excited. What made it even more worthwhile was how patient, appreciative and gracious the passengers were. One passenger even shook my hand as a gesture of thanks to the crew. How lovely!

Crosswind Landings

The recent low pressure system over UK & Ireland has certainly brought some interesting weather, and in particular the wind! I recently flew to Northern Ireland from UK, and the pressure difference between my departure and destination was in excess of 20 hectopascals (or around 0.6inHg), which says there’s a steep pressure gradient.  Consequently we experienced some gusty crosswinds which were approaching our operating limits.

I found a video that was uploaded recently which demonstrates what an approach and landing would look like in such weather:

The aircraft is “crabbed” into the wind on approach so that it can track the runway centreline.  This means that the aircraft is not pointed directly towards the runway, which would normally be the case if there was no wind, or only a wind coming directly towards the aircraft. However, the aircraft is pointed away from the runway and slightly more towards where the wind is coming from to mitigate the effects of the wind blowing the aircraft away from the runway.  The pilot maintains this angle all the way down to the flare or touchdown. On the 737, we have two options, we can either “de-crab” on touchdown or in the flare.  This means that we can either re-align the nose of the aircraft so it points in the runway direction once we are on the runway or just a few seconds before touchdown, in the flare (where the pilot slows the descent rate and sets the correct airplane attitude for touchdown).

The video title is a bit misleading – I wouldn’t say it’s terrifying, but it can get the adrenaline going.