or low, look out below?
this isn't about altimeter errors.
It's a question that continues to be as
ubiquitous as the
"dive and drive" versus "stabilized approach" debate amongst instrument
pilots. In other words, when working your way between/around
area of convection or when convective potential is in the making along
your route, is it
better to be higher up or down low, perhaps picking your way around the
rain shafts and cloud bases? Keep in
mind that what makes sense during daytime flight conditions may not be
an advantage when it is dark. Night flights within a
environment can be very challenging in their own right, deserving a
separate protracted discussion.
The playing field will be different for every
flight you may take. So, there's not always a definitive
every case and general "rules of thumb" may not always apply.
First of all, being somewhere in between high and low is fine
assuming there are no clouds present. Moreover, you may be
right at the
top of the haze layer making forward visibility challenging.
when convection is anticipated during the afternoon, you may also find
a broken field of cumulus clouds develop in the early afternoon similar
the clouds shown on the left (click here
to view a larger image). There's little doubt that choosing
altitude that keeps you popping in and out of the clouds in
cumulus deck is not the sane choice. Sure, most of these clouds
may be harmless, but who wants to be tossed around for an hour or more
in and out of instrument meteorological conditions (IMC). Therefore,
take your pick; climb up above this field of clouds or descend down
the cloud bases. Is one choice better than the other?
Before we take this a single step further, the prime directive in
convective flying is that you should endeavor to
stay visual at all
times. Staying in visual
meteorological conditions (VMC) will go a long way to keeping you out
the nastiest part of the storm, that is, in-cloud turbulence.
There's a lot of buzz these days about "flying IFR
the pros." In a convective environment this is one suggestion
may want to completely toss, especially if you want to take the low
Many pilots don't have the experience or
to handle IFR when there are convective SIGMETs also proposed for your
route. Depending on the local terrain and obstructions, frequent
deviations around convective cells while negotiating the airways near
the minimum en route altitudes (MEAs) is going to be a nightmare from
One possible alternative for flights
below FL180 is
to fly visual flight rules
The flight restrictions for VFR force you to remain well
of the physical cloud boundary and it keeps you separated vertically
from any clouds.
Flying VFR also allows you the maximum flexibility if you
maneuver around the weather. Remember that pilots flying
above 10,000 feet MSL under VFR need to be horizontally
separated from clouds by one mile.
Altitude and forward visibility
are your friends
Having altitude on your side is often a good
choice in many aspects of flying. In a convective environment
higher altitude means better much better forward
visibility and no
thermal turbulence (smooth conditions) assuming you stick to the prime
above - stay visual. You need to climb high enough to be
the haze layer that is frequently
present during the late spring,
summer and early fall. This
can range from 8,000 feet to 17,000 feet depending on the
meteorological environment and location within the U.S.
on the right (click here
to view a larger image) is a good example. In a highly capped
environment (small positive or negative lapse rate above the cloud
tops), haze can linger between the
cumulus cells and below the
bases making forward visibility extremely difficult. Often
climbing above this haze layer will also keep you above most of the
A higher altitude keeps you separated from terrain
obstacles. The greater the separation from terrain the
better. When you are near the surface, it is easy to become
distracted in higher terrain and miss the presence of the mountain peak
ahead or tall radio antenna. Having an obstacle
board or terrain warning system is a wonderful asset, but be sure to
keep those databases updated. New cell towers are erected
frequently across the U.S.
Forward visibility is a must if you want to see the buildups
know which ones to challenge and which ones to avoid. When
down low, it is similar to having a short stature while in the back of an audience watching a
parade. If you can't accurately identify the danger ahead you
easily stumble into something uncomfortable or require a
hasty maneuver to avoid something nasty.
The goal is to be above the clouds most
en route. However, flying higher doesn't guarantee that all
clouds will be beneath you since there
will be instances where the cumulus may tower well above your flight
level. That's okay. Given the unlimited visibility
higher altitude, it is easy to identify these congested clouds in the
distance and plan an easy deviation around them. You
easily assess the height of those
taller clouds and know which clouds you need to keep at a safe distance
and those you can literally "brush by" without any
concern. Keep in mind, that all of these clouds are moving
the prevailing wind and may be growing or dissipating in time.
Icing is not your friend
One of the greatest threats of flying
high is the risk of structural icing. During the
summer, the melting level (better known to pilots as the
freezing level) ranges from 8,000 feet in the
parts of the U.S. to 17,000 feet in the southern tier of states. Of
course, this will
vary greatly from day to day and will usually be lower during the
spring and autumn months. A good rule of thumb in flying is
to never depart unless you know the
melting level regardless what the calendar says. Keep in
that AIRMETs (now G-AIRMETs) are not issued to warn the pilot
about this kind of convective icing. If you are
penetrate a cumulus cloud at a subfreezing temperature this will almost
certainly send you through a shower of supercooled liquid
water with drop sizes that could easily exceed 50 microns in diameter.
encounters with supercooled large drops (SLD) such as this can be
devastating to many airframes. Again, the prime directive
strictly followed here - remain in VMC.
Whether or not you
are a low or high flier will depend on several limiting factors.
factor is the length of your flight. On a short flight of 30
minutes or less, it may not make a lot of sense to climb to a high
cruise altitude just to be able to see the mess that's ahead.
you opt to fly low on a short flight, you must be absolutely prepared
to map out your route before
you depart. This may not be easy if you are flying IFR within
busy airspace. Your ATC clearance may compromise a solution
minimize your exposure to those convective buildups. Demanding frequent
deviations only complicates the controller's job and create a domino
effect forcing the controller vector other aircraft.
The second factor is the performance
your airplane. In order to fly on top you need at least two
capabilities. The first is oxygen (or pressurization) and the
second is service ceiling and rate of climb. If you don't
carry oxygen you won't
have the ability to climb above the taller buildups. The last
thing you want to do is get to 12,500 feet and
discover that higher
buildups won't allow you to stay on top defeating the number one prime
An aircraft that is not certified to fly well into
teens or has limited climb capability at higher altitudes will not have
as much success flying on top. A turbocharged or
aircraft will provide the most favorable opportunity to remain above
most of the cumulus field of clouds.
Stuck on top
One of the worries that a few pilots have with
is being "stuck" on top with no where to go. Certainly if you
flying under VFR this can be an issue if the cumulus cloud deck below
is broken and you are not instrument rated and current.
Maintaining the proper cloud separation on descent may
be impossible. If you are
instrument rated, a pop-up IFR clearance
may assist you to drop down below the deck as you descend down toward
your destination. It is rare, however, to have such a continuous cloud
deck without sufficient holes to descend through. Being
before your final descent allows you to see over a greater region to
assess where those holes might be. In some cases it may mean
overshooting your destination by a few miles before you can safely
descend. If descending under IFR, normally 10 and 20 degree deviations
are all that is necessary to maneuver around the big stuff as shown on
the left with an IFR descent from 15,000 feet (click here to view a larger
As a general rule, you don't want to attempt to
a building cumulus cloud. In situations with high instability
(high convective available potential energy), developing cumulus can
grow at rates as high as 6,000 feet per minute where 1,500 feet per
is very common. You'll likely find yourself climbing into
serious icing conditions if you do not have the excess performance to
climb on top in clear air.
Multi-cellular convection and
When flying high, avoid flight between
convection including towering cumulus. It is not uncommon for
cumulus cloud in the distance between two larger cells to look pretty
benign from the distance as can be seen from the cumuliform growth
between two cells in the image on the right (click here
to view a larger image). These "in between" buildups can
explode into a nasty cell in just five to ten minutes. This
applies to buildups that are at the "book-ends" of a single
multi-cellular line of storms. In this scenario, these
congested cumulus can grow into thunderstorms in just a short period of
time, ordinarily 10 to 15 minutes.
It is also
hard to judge just how fast it is growing and if you will be able to
"top it" when you reach that location. If you are at an
altitude above the melting level, you
don't want to enter that kind of cloud. There's copious
supercooled large drops (SLD) waiting for you...guaranteed.
Be careful flying too close to a
thunderstorm or towering cumulus when flying at an altitude near the
melting level. You might just find
your aircraft can induce a lightning strike out of the blue.
is especially true of precipitating towering cumulus as described in
this e-Tip discussing
aircraft-induced lightning strikes.
side of low
Most accidents attributed to thunderstorms happen
departing or approaching the airport. Generally, we don't see
aircraft break apart in flight at higher altitudes. This is
because flying down below or near the convective cloud bases means the
potential of being exposed to the most dangerous aspects of a
thunderstorm. Convective wind shear is likely the worst
you will face and causes the most thunderstorm-related accidents.
All thunderstorms produce some kind of outflow boundary
their life cycle. According to thunderstorm expert,
Charles Doswell, III, "cold, stable air is the 'exhaust' of deep, moist
convection descending in downdrafts and then spreading outward like
pancake batter poured on a griddle." After spreading outward,
leading edge of the outflow boundary is essentially a gust
Some of these outflow boundaries
are benign and some are extremely hazardous and often hard to detect
Another serious threat when ducking under the
stumbling into a convective downburst or microburst. Pilots
trained to keep their distance from giant supercell-type thunderstorms.
If you think this will also keep you from encountering a
microburst, think again. As described in this e-Tip,
microbursts are frequently generated from relatively benign-looking
cells and not necessarily a giant supercell-type storm.
High-based thunderstorms with an extremely dry environment
the bases doesn't seem all that threatening and may actually look very
inviting to any pilot trying to fly under the higher cloud deck.
This is a perfect environment for a microburst encounter.
As rain falls into the dryer air below the bases, it begins
evaporate which causes a cooling effect. Such cooling makes
air denser and negatively buoyant which can eventually unleash a very
intense downburst with winds that exceed hurricane force over a
distance the size of a long runway. Of course, other hazards
as reduced visibility, low ceilings, hail, lightning and strong
straight line winds can be found when flying at or below the bases in
and around most organized convection.
So you've got
Satellite-delivered weather is extremely useful
comes to avoiding thunderstorm hazards. But don't let that be
panacea for staying in visual conditions. While not every
dangerous situation will appear on your satellite-delivered weather
display, many do. When you see an active cell with reflectivity values
of 50 dBZ or greater enveloped in lightning, that's certainly a cell to
work around. Don't be fooled by the radar mosaic, however.
It is about three to eight minutes latent from reality
you didn't miss an update. This seems like a fairly short
but what looks like a harmless cell with very light precipitation on
the radar mosaic can turn into a dangerous thunderstorm with
reflectivity values exceeding 60 dBZ in 10 minutes as described in this
In the end, identifying what cells to avoid is much easier to
ascertain while flying in visual conditions above the
haze layer. Being
low makes it difficult to characterize the cells visually.
Keep a bigger fuel
When battling through a convective environment,
want to maintain a close eye on your fuel consumption and plan a larger
fuel reserve. Without fuel, you greatly limit your options.
or 45 minutes to your one hour fuel reserve will allow you to keep a
safe distance while deviating around a nasty area or line of
convection. Remember a climb to a higher altitude will also
into your fuel reserves as will ATC delays. If you start to
approach your original
one-hour fuel reserve, plan an early diversion to another airport to
refuel. Do not be tempted to think you can still make it to
original destination. You may put yourself into a position of
deciding to descend through a dangerous cell to avoid running out of
fuel during a large deviation around that storm near your destination.
|Did you know?
that the FAA imposes a hard character limit on text products such as
convective SIGMETs and the area forecasts (FA)? The Aviation
Weather Center (AWC) has a check program for the area forecast that
counts the number of characters. On busy weather days when
forecasters have a lot to mention in the forecast, they often bust the
character count. This forces them to cut from either the text
synopsis in order to make the size fit through the NADIN hubs.
Often the first casualty is the ellipsis (...).
use two periods instead of three. Another casualty is the
synopsis. Of course, the last resort is to cut from the text.
what you might see in the synopsis section with a very busy FA:
This has been a long standing problem that you
discover in the AIM or FAA handbooks. The AWC has asked the
many times to increase the character limit size. In fact,
according to NWS meteorologist Jim Roets at the AWC, "As late as last
week we had an issue with the Convective SIGMET. The
count was a little over 3,000, so the forecaster sent it anyway.
The product was kicked out of the FAA system." He
to combine outlook areas and cut some of the areal coverage on his
convective SIGMETs a bit which allowed the forecast to go out.
In general, the NWS Telecommunications Gateway in
Spring, Maryland allows them 15K per product. But when that
product goes from the Gateway to the NADIN hubs it has to be 3K or
less. Unfortunately, the character count issue goes back to
teletype days when data storage and transmission speed were small.
With the transmission speeds and data storage capacity that
exists today, it is very difficult to understand why the FAA won't
increase the size.