November 5, 2019

The complexities of planning

I’ve been waiting for a weather window to fly 24ZN, my ‘English’ X-air, over from England to France since the first week of September during which period the southern parts of the UK and the northern parts of France have been ravaged by high winds, lashing rain and thunderstorms. So all I’ve got to do is wait until there’s a weather forecast for two or three days of suitable weather and just fly it over, right?

Well, yes and no. For starters, what does a ‘suitable’ weather forecast look like? You’d think that that would be an easy question to answer – light winds, fair weather, no rain – but it isn’t and I’ll explain why.

When you have a route set up for a flight on a specific day at a specific time there are weather services from whom you can get hold of a detailed weather forecast for the whole of the route, such as the UK Meteorological Office. However, such forecasts require considerable resources and when you’re just looking ahead for a weather window at some undefined time in the future, you can’t keep requesting such services ‘on the fly’ in the hope that a suitable window will be identified.

So you have to resort to using the ordinary ‘day-to-day’ services that are available to everyone on the internet and trying to spot when a suitable window might pop up, and then when one has, there are other services that pilots such as myself can use that provide ‘short term’ eg next day, data that can be used to plan a route in detail.

One I use is an internet app called Metam which provides a world map showing current weather reports (METARs) and forecasts (TAFs) for all major and participating airports, data that can be used to prepare a forecast by pilots like myself for the whole of a typical route flown by a small aircraft.

After getting 24ZN all ready to go, I waited for three weeks in the UK for a weather window to fly it over before giving up and returning to my home in France. During the time I was waiting I could see for myself on a daily basis how the flight I had planned would be impossible due to the weather outside the door and since my return I’ve been doing the same thing ie watching for a suitable opportunity to arise, but from a distance.

The only information that I have to go on are the published weather forecasts and it has become increasingly clear that not only are they far from accurate but they also rarely agree with each other. Many times in recent days I know from talking with my family in the UK that they are enjoying a fine day with light winds when the forecasts that I’ve been viewing have been for almost the complete opposite, so it begs the question, ‘what can one believe?’

Here’s what I’m talking about. It appears that there could be a suitable weather window for my flight during the first half of next week. I use two forecasting systems, XCweather, a well-know UK system that gives a forecast for a week ahead, and Tameteo, a French system that gives one for the next 14 days. It’s reasonable firstly to ask how these forecasts compare for the same locations at the same times and the results do not inspire confidence.

Let’s look at a take off from Headcorn at 9.00am local time on Monday 11 November. XC says that the wind will be a light breeze of 4 kmh from the north-east while Tameteo says that it will be 4 gusting 25 kmh from the south-east. 25 kmh is just about the cross-wind limit for an X-air and flying for any longish period with such a gusting cross-wind would be tiring to say the least and definitely not to be recommended for a long flight. So would it be wise to take off on a long flight in such a wind?

Here’s another. Calais at around 11.00 am on Tuesday 12 November. XC forecasts a wind of 4 gusting 7 kmh from the north ie perfectly safe and manageable but Tameteo forecasts a wind of 15 gusting 29 kmh from the east which would be beyond a reasonably safe limit and surely it would be unwise to even start the flight from Headcorn knowing what was lying in wait on the other side of the Channel.

So then you have to ask another couple of questions. How good are the forecasts and which of the two is the more reliable? Tricky ones to answer without keeping a fairly long log of ‘forecasts’ v ‘actuals’ for each system. Qualitatively, my own feeling is that both of them err on the pessimistic side as far as wind velocities are concerned because as I mentioned previously, there have been several occasions in recent weeks when unacceptably high winds have been forecast and outside it has been a perfectly good flying day. However, that’s only a feeling and I have no quantitative data to back it up.

But let me give some final examples for today. The local forecasts for today were as follows:

XCweather…. 1500hrs…. wind from the south-west 13 gusting 26 kmh
Tameteo…… 1500hrs…. wind from the south-west 19 gusting 50 kmh
Actual……. 1500hrs…. wind from the south-west 5 kts (9 kmh)

XCweather…. 1700hrs…. wind from the south-west 13 gusting 33 kmh
Tameteo…… 1700hrs…. wind from the south-west 18 gusting 46 kmh
TAF for Brive 1700hrs…. temp 12/18 260/15G25kt ie west-south-west 28 gusting 46 kmh
Actual……. 1700hrs…. wind from the south-west 5 kts (9 kmh)

So who the heck can you believe? Tameteo looks to be very pessimistic re wind velocity but on the other hand, seems to reflect the ‘official’ aviation weather forecast more closely than does XCweather. However, the ‘official’ forecast also doesn’t appear to be that reliable and on today’s evidence, although still some way out, the XCweather forecast seems closer to the actual than the ‘official’ one was.

So what to do now? There is a little bit of good news. Even if both XCweather and Tameteo are forecasting higher winds than there actually will be on the days in question, both are for now suggesting that I will be able to do my 2-day flight down to the Dordogne either on 11/12 November or 12/13. The bad news, though, is that experience shows that between now and then everything can change and if I jump in my car to drive back to the UK next Friday say, I could well find myself staring outside after I arrive at yet more high winds and rain 🙁