Thursday, 21 June 2018
Acknowledgements: AIR FACTS (Richard Collins)
(Ed. Note: Taken from an article written by Dick in 2012)
“…… I am going to use two events to give you some items to ponder about summertime flying…..
Event 1: When I got up on the morning of June 29th, as usual I looked at the weather, including the radar. Even though there had been zero mention of thunderstorms or even rain the day before, there was a sizable cluster of storms moving by to our north. It was headed southeast and was going to miss us. The annual drought would continue. Little did I know what would come later in the day, but that is getting ahead of the story which, at this point is about nocturnal thunderstorms. I have read meteorological theories about what causes them until my eyes crossed and my brain shorted out.
Nocturnal storms are hard to predict, and occur when it is warm at the surface with a high dewpoint and a bit of cold air aloft wanders overhead and provides the instability required for the things to fire up. Night storms can be every bit as mean as daytime storms but they are pretty easy to see. I used to love to sleep on my grandparents’ sleeping porch in Arkansas when a nocturnal storm would light up. The show was great ……..
The first few times this happened, I had to fly east the next morning, and thought I would have to deal with the storms. That was no problem because they usually dissipated around sunrise. If there were going to be more storms during the day, they would start building by noon. You could look at the cumulus at noon, and if they were congested it suggested an active afternoon.
June 29thwas a really hot and sticky day. I was aware of some thunderstorm activity out in Ohio but nothing suggested that the area from Ohio to the middle-Atlantic would be ripped apart by straight line winds from a line of strong thunderstorms that afternoon and evening. It was actually a fairly narrow line of storms and it passed through in 20 or 30 minutes, but hurricane-force winds don’t have to last long to do serious damage, (which on this occasion they certainly did!)
The storms we most often deal with in our airplanes in the Summer are air mass storms, but there are also fronts and low pressure systems that spawn storms. Mostly, we deal with individual storms or clusters of storms:
· En-route we deal with them by avoiding them like the plague.
· In terminal areas we deal with them by developing a good knowledge of the circulation in and around thunderstorms and avoiding bad places. Changing winds and downdrafts can take a terrible toll on aircraft performance.
I am going to use an airline event to illustrate this because this particular accident related to the two big hazards of summer flying: thunderstorms and density altitude.
Event 2: July 9, 1982. New Orleans. A typical summer day, hot and humid with scattered thunderstorms in the area but none reported as severe. As the crew taxied a heavily-loaded 727 for take-off they discussed, among other things, the heavy load, the fact that theybarely had balanced field length, the procedures to maximize take-off performance, possible fuel dumping in case of any power loss, and the fact that the wind was swirling around the airport as recorded and reported by the low level wind shear alert system.
There was a thunderstorm on the north-east corner of the airport, visible both to the eye and to the radar in the nose of the 727. The thing that made it more difficult to assess was the fact that it was moving toward the southwest. That is the opposite of what we usually deal with. This meant that the front of the storm was where we usually find the back of the storm. The strongest winds out of a storm generally come out of the side toward which the storm is moving. Whether this crew had ever seen a westerly moving storm before was not addressed in the accident report. When they started to roll on runway 10 the last thing they heard about wind was that it was from 070 at 17 and that a heavy Boeing had reported a 10-knot wind shear on final to that runway.
The airplane climbed to about 100 feet where it encountered a 400 fpm downdraft and a decrease of 38 knots in the relative wind (shearing from a headwind to a tailwind) over a distance of about a half a mile. The microburst that caused all this was located just north of the runway and a little more than halfway down the runway. The airplane hit 50-foot trees first, near the tops, and then crashed in a residential area.
There were lessons here. The margins were pretty thin on that take-off and everything combined to make what would usually be a manageable wind shear encounter unmanageable.
The crew was aware before take-off of what was going on, and had thorough discussions about most of the factors influencing the take-off. They did not, however, openly discuss the possibility of a downdraft and the resulting wind shear. If they had been taking off toward a storm that was moving away from them, the sequence of events would have been different. But this storm was going the “wrong way.”
A thunderstorm is, by nature, unstable. That relates both to the atmosphere that creates and supports it, and to the capricious nature of the storm. They are constantly changing, literally from moment to moment, and where one flight might pass through with a bit of turbulence, one a minute later might encounter a severe wind shear.And while they almost always move in an Easterly direction (in the Northern Hemisphere), there are exceptions.
I have in my life only seen one Westerly moving storm, so they are rare. But it’s those rare things that will bite you if you don’t think them through. Have YOU ever seen a westerly moving storm in the Northern Hemisphere”.
(Ed. Note: Hindsight is a wonderful thing, but would YOU have made a decision to depart in those conditions, or would you have elected to wait for the storm to pass and lived to tell the tale?)