Victoria Falls is a massive waterfall, which borders both Zambia and Zimbabwe.
April 15, 2017
April 15, 2017
By ED BROTAK
With the onset of warmer weather, pilots face the increased risk of encountering thunderstorms.
Although more common in the warmer months, thunderstorms can occur even in the winter, especially in the southern states. It’s estimated that 100,000 thunderstorms occur in the U.S. each year. Some locations in southwest Florida have 100 storms a year, but thunderstorms do occur in all 50 states.
Thunderstorms are most common in the late afternoon, but can occur at any time of the day.
Technically called convective cells, a thunderstorm can cover an area from 200 to 1,000 square miles. Storms can range in height from 10,000 feet to over 60,000 feet. Individual cells can last from less than a half hour to many hours.
THE DIFFERENT TYPES OF THUNDERSTORMS
There are different types of thunderstorms that develop under different conditions. “Air mass thunderstorms” typically develop in the late afternoon and evening due to the heat of the day. Development tends to be random, but they are more numerous over mountainous terrain. Although relatively weak, they can still pose problems and should be avoided. Fortunately, air mass thunderstorms tend to be slow moving.
A greater threat is posed by organized convection. These are stronger storms that often move quickly, up to 60 mph. They are often associated with fronts, especially ahead of cold fronts.
“Squall lines” form when convective cells develop in a line in response to prevailing atmospheric conditions. The line can extend for tens or even hundreds of miles. Although there are breaks between the cells, circumnavigation or remaining on the ground until the line passes is strongly recommended. Individual storms will die out only to be replaced by new cells, with the whole system lasting for hours.
MINIMIZING THE DANGER
It’s a good time to review the risks thunderstorms pose to aviators and what you can do to minimize the danger.
Many things are happening inside a thunderstorm cloud (cumulonimbus) that they pose a wide variety of threats to aircraft.
Lightning can certainly do some structural damage and affect electrical equipment inside a plane.
Hail, which can grow to the size of softballs, can damage windshields and the exterior of the aircraft. The occurrence of hail indicates sub-freezing temperatures at some height in the cloud.
Even with the warmth of summer, towering thunderstorm clouds easily reach and exceed the freezing level. This also means super-cooled water and the risk of icing is present.
One of the more subtle threats thunderstorms produce is erroneous aneroid altimeter readings due to the rapid pressure changes the storm induces. Readings may be off by 100 feet.
But by far the greatest risk is turbulence. Updrafts and downdrafts within the storm can easily reach 50 mph (73.3 feet per second) and can reach 100 mph (146.6 feet per second). Planes can literally be torn to pieces by the turbulence generated between the up drafts and down drafts.
Even if there is no structural damage to the aircraft, loss of control is a distinct possibility.
And obviously within the cloud, IMC exist and the risk of Controlled Flight into Terrain (CFIT), especially in uneven terrain, is great.
And keep in mind that convection can develop very quickly. What was VMC everywhere can quickly contain areas of IMC.
TROUBLE ALL AROUND
Dangerous weather conditions are not limited to within the storm cloud itself.
Turbulence above the cloud top can extend upwards for thousands of feet.
Interestingly, the massive core of the storm can actually act as a solid impediment to the prevailing winds, almost like a mountain. Clear Air Turbulence (CAT) can be produced in the air flow downwind of the storm and extend tens of miles.
Beneath the storm cloud base, conditions can also be treacherous. Blinding rain and even hail can extend to the ground. IMC conditions are common.
Extreme downdrafts, called downbursts or microbursts, can occur even without precipitation. Once these downdrafts hit the ground, they can spread out, sometimes for tens of miles, producing strong, shifting winds that can exceed 100 mph, and the dreaded wind shear.
Before you start your flight, your preflight weather check, including TAFs and FAs, should highlight any convective problems.
Particularly note any CONVECTIVE SIGMETS, forecasts that warn of dangerous flying conditions due to convection in the next two hours.
But keep in mind, it is impossible to predict exactly when and where thunderstorms will develop in advance. And convection can develop rapidly, sometimes in a matter of minutes.
Closer to takeoff, you can check the latest METARs and PIREPS to see if convection has been reported.
Weather radar is the best tool for locating and tracking thunderstorms. The heavy rainfall rates associated with convection are well depicted as areas of yellow, red, or even purple if hail is present.
Movement and changes in intensity can be determined by tracking storms over time.
Major terminals are well covered by land-based radar. Terminal Doppler Weather Radar can detect thunderstorms and even wind shear near an airport. Larger airports also have specialized wind shear monitoring equipment for the runways. Smaller GA airports are often not as well equipped.
IT’S UP TO YOU
It’s up to the pilot to determine thunderstorm risk. Fortunately with today’s technology, a variety of weather radar products are readily available over the Internet and there are even apps for smartphones.
Always check the time on any radar display you’re checking. Delays due to processing are common. The radar image you’re looking at could be up to 20 minutes old. In fast developing convective situations, that could be crucial.
If your aircraft is equipped with radar, it can be extremely helpful in convective situations. Current radar data is always available, allowing you to detect significant convection 300 nm away.
April 15, 2017
March 20, 2017
There can be, depending the ceiling, visibility, turbulence, ice, and how soon you want to get out of the clouds. But any time you choose a non-precision approach over a precision, you’re also taking on more workload, and opening yourself up to the possibility of a mistake while descending on the approach.
Let’s look at this example in Olympia, WA. Runway 17 is in use. The visibility is 10SM, and the ceilings are overcast at 700′.
Looking at available approaches, the ILS to 17 is your first pick. But like most ILS approaches, you can also shoot a localizer only approach to runway 17 using this chart.
What’s the difference? The ILS gets you down to 218′ above touchdown, and the LOC, which is a non-precision approach, gets you down to 433′ above touchdown.
Since the ceiling is 700′ overcast, both approaches with get you out of the clouds with no problem. But if you fly a localizer only approach, it can get you out of the clouds sooner, depending on your descent rate. Why would you want to do that? It can give you more time to visually orient yourself with the runway and surrounding area. And if you’re getting beat up by turbulence or picking up ice, it can give you, and your passengers, some added relief.
Let’s start with the ILS to 17. If you’re flying a 90 knot approach speed on a 3 degree glideslope, you’ll need to descend at roughly 450 feet-per-minute (FPM) to maintain the glideslope.There’s a pretty easy rule-of-thumb to figure that descent rate out. Divide your ground speed by 2, then add a 0 to the end. So if you take 90 knots / 2, you get 45. Add a zero to the end, and you get 450 FPM.
On this approach, glide slope intercept is at 2400′ MSL. Since TDZE is 207′ MSL, that means you’re roughly 2200′ above the touchdown zone when you intercept glideslope. And since the ceilings are 700′ overcast, you’ll need to descend roughly 1500′ before you break out of the clouds.
That means if you’re descending at 450 FPM on the ILS, it will take you roughly 3 minutes and 20 seconds before you break out of the clouds.
Now lets look at the LOC only approach. You know that the MDA of 640′ MSL (433′ above TDZE) is still easily going to get you out of the clouds. And if you increase your descent rate even slightly, it can get you out of the clouds sooner.When you cross the FAF, if you start a descent at 600 FPM, which is still a very reasonable descent rate, it will take you about 2 minutes and 30 seconds before you break out of the clouds. That’s 50 seconds sooner than shooting the ILS.
In almost all cases, using a precision approach is the best choice. That’s especially true in low visibility. Following the glideslope on a precision approach means you know you’re at the right place, at the right time, all the way to DA/DH.
But if you want to get yourself out of the clouds to get oriented with the runway and surrounding area a little early, or if you’re trying to get yourself out of the clouds when there’s turbulence or ice, using a non-precision can do that for you. Just make sure you’re flying a stable descent, you’re ready to level off at MDA, and you’re prepared to make a stable descent from MDA to touchdown.
March 20, 2017
A few miles south of my recent post and less recent one, yesterday morning’s location offered this nice view which several people have said reminds them of African plains. Having never been there, I can’t say, but I’ll take their word. That certainly wasn’t on my mind at the time. Many of the trees here are apple, so quite different in that respect…nary a baobab in sight.
March 20, 2017
March 18, 2017
These two Great horned owls flew between various perches among the strip of trees I found them in east of High River. I watched them for two hours as the morning’s overcast sky brightened. They were unsettled by ravens a couple of times but mostly seemed to be resting while keeping eyes on the fields they […]