Weatherwatch with Irv Lee

Runway Meteorology

Wise pilots give due respect to area and route weather, but how many are lulled by familiarity into ignoring the meteorological effects in the very place they feel safest - the airfield? For hasty, unobservant pilots, the dangers from weather are often waiting to catch you unawares on the runway.

It's not only the route weather and local wind strength or 'gustiness' which should contribute to go/no-go decisions. There are many local meteorological factors to consider, as follows:

Crosswind component
Know the aircraft's and your own crosswind limits. To calculate crosswind component:

Take the angle between the ground wind and runway. If it is 60 degrees or more, the whole speed is 'crosswind', otherwise take this angle as a fraction of 60, and multiply by the speed to give the crosswind component.

Example: Runway in use is 25, and the wind is steady from 280 degrees at 20 knots. This means 30 degrees of wind angle, 30 divided by 60 is half, and half of 20kt gives 10kt as the crosswind component.

Wind v runway
A dangerous situation can develop with an initially 'acceptable' crosswind. Winds in the UK often veer during the day, and sometimes back again towards evening, occasionally making a runway change vital once, if not twice, in a few hours.

Even light tailwinds can be frightening and dangerous on a runway - don't try them, just believe it! A mere 5kt tailwind component means a more than 20 per cent increase in the distances required.

You, the pilot in command, must ultimately decide on the runway to use at the airfield and if needed, insist on a runway change. Inexperienced pilots arriving late at quiet airfields often accept the ground T-square direction, refusing to look around for windsock or smoke clues to actual conditions.

Local meteorological phenomena
Daytime variations from coastal effects, or temporary changes from passing CB clouds can alter very rapidly wind vectors, even during 'taxi' time. It's important to keep a close weather eye on the windsock.

Temperature
Aircraft take-off distances are quoted for 15 degrees celsius. The danger comes in the warm spells. Temperatures in the 20s or higher mean significantly more runway required, often a surprise to UK pilots accustomed to cooler weather. A warm September day with a temperature of 25 celsius means more than 10 per cent extra runway will be required on take-off.

Pressure
Aircraft runway performance figures are quoted for 1013 hectopascals. If the QFE is less than this due to cyclonic weather or airfield height then take-off runway required will increase - for example, over 10 per cent more for a 980hPa QFE.

Moisture
Wet grass can mean 30 per cent more take-off runway required, and even more for landings. High humidity means less dense air and a small increase in take-off distance.

Note that the latter three effects - higher temperature and humidity, lower pressure - reduce engine performance, which in turn affects take-off distance and climb rate.

Take a few minutes to revise aircraft performance and runway calculations, add a minimum 30 per cent safety margin and appreciate the effect of airfield meteorology.

What's the worst case? Warm, humid, crosswind days in low pressure at higher airfields, with shower CBs developing

This article first appeared in FLYER magazine's September 2000 edition