SEGURANCA AERONAUTICA
Nº 114
2007-01-18
Meu Caro,
Há voos em que a mãe Terra parece atrair com uma força brutal os aviões mas, há outros,
em que parece acontecer o contrário. Dir-me-ás que se a força da gravidade fosse mais
fraquinha todas as nossas aterragens seriam suaves e isso seria óptimo para as aeronaves
e... para nós, pilotos. Feliz ou infelizmente nós, na aceleração da gravidade, não podemos
actuar. Ela é que actua sobre nós. Porém, nos fenómenos aerodinâmicos, já temos alguma
intervenção. Às vezes, não com a autoridade que esperamos.
O EFEITO DE SOLO
Operei muitas vezes no aeroporto de Córdoba (LEBA), em Espanha. Conheço bem a sua
pista e as suas aproximações. Conheço bem as temperaturas elevadíssimas que se fazem
sentir em certas alturas do ano nesta região. Andaluzia no seu melhor.
Para os que não conhecem o aeroporto de Córdoba posso dizer que a sua pista asfaltada
tem 1380 x 45 m. Uma maravilha para um Cessna C172. Merece uma visita seguida por
uma deslocação à cidade para uma visita aos seus inúmeros monumentos, a começar pela
mesquita.
Há uns anos, numa das minhas deslocações desportivas a Córdoba, passei por um episódio
que poderia ter consequências desagradáveis se eu não estivesse consciente da situação
que se passou. Um piloto menos experiente e uma pista mais curta formariam um binómio
que poderia levar ao acidente.
Mas, antes de mais, deixa-me contar o que se passou. Vinha eu aos comandos do meu
companheiro de tantas horas de gozo aeronáutico – o CS-AQX, um C172N – e entrava, de
Norte, para o circuito da pista 03 de LEBA. Eu, dois passageiro e respectivas bagagens.
Final fácil sem obstáculos e, como sempre fiz toda a minha vida de piloto (maldita prática de
fazer aterragens de precisão em Rally Aéreo) defino o “touch down point” como sendo os
“números” da pista. Faço a final com 40º de flaps e arredondo para o “flare” junto ao 03
pintado na pista. Velocidade a morder os 50knots e throttle todo atrás. Vou puxando o
manche para entrar numa suave perda que me leve a tocar suavemente com as rodas do
trem principal no chão. A interessantíssima operação de trocar energia potencial por energia
cinética até que a primeira se esgote.
Para meu espanto, a certa altura, dou comigo de manche todo à barriga, throttle todo atrás,
buzina de perda a reclamar e o avião a flutuar junto à pista sem nunca pôr as rodas no chão.
Passa-se meia pista e nada. Uma perfeita situação de “efeito de solo” agravado pela
“térmica” que estava formada em cima da pista devido à elevada temperatura desta. Como
sou daqueles pilotos que pensa que uma manobra de “borrego” não faz cair os parentes na
lama, começo a definir o ponto onde iniciaria um “go-around”. Finalmente, quando passo em
frente aos “aprons” as rodas tocam no chão, numa daquelas aterragens que quase não se
sente. Tinha “gasto” cerca de 70% de pista a flutuar.
E se a pista não fosse tão comprida? Teria feito um “go-around” muito mais cedo e tentaria
uma aterragem com menos 20º de flaps ou, talvez, mesmo sem flaps já que tinha suficiente
pista para desacelerar a velocidade.
Muitas vezes recordo este episódio em que se misturou o “efeito de solo” com uma térmica
fortíssima sobre o asfalto.
Meu caro, ainda te recordas do que é o “ground effect”? Certamente. Contudo, deixa-me
recordá-lo para aqueles outros pilotos que já estão mais esquecidos.
FLYING IN THE REALM OF ALTERED AIR FLOW
By David A. Borrows
Ground effect is caused by the interference of the ground surface with the airflow pattern
about the airplane in flight. When the wing is under the influence of ground effect, there is a
reduction in upwash, downwash, and wingtip vortices. It can be detected and measured up to
an altitude equal to one wingspan above the surface and is most significant when the
airplane is maintaining a constant attitude at low airspeed close to the ground. A decrease in
induced drag makes the airplane seem to float on a cushion of air beneath it, so if a power
approach is being made, the power setting should be reduced as the airplane descends into
ground effect to avoid overshooting your desired touchdown point.
Ground effect is a wonderful aerodynamic phenomenon that has enabled me to win many
bets - and save my neck. I discovered it when I was about 14 and armed with a pellet gun.
My discovery was not the result of my being an aeronautical genius; my knowledge then
didn't extended beyond that required to make a glider out of carefully shaped balsa wood
bought with hoarded pocket money.
Doing what many kids of that age do, my friends and I would often toss a can or some other
floating object into the local pond and use it as a target. When the wind would blow a can out
of range, we would have to angle our barrels up so the pellets would describe a parabolic arc
and hopefully land in the vicinity of the target. The nearest to the target won the game (and
the pocket money wagered on it).
After a while, the range would be so great that the target could not be reached even with the
barrel angled up at 45 degrees. But I discovered that if the gun's barrel was parallel to and
just an inch or so above the water's surface, a pellet would travel clean across the pond and
strike the bank on the far side, way in excess of the gun's theoretical maximum range.
This discovery enhanced my aviation career because the target-shooting wagers I won
allowed me to purchase far more balsa wood from the local toy store. Some time passed
before I found a better use for the discovery of ground effect than betting my friends that I
could shoot the pellet gun farther than they could.
Pellet Principle
What was happening to my pellets explains the principle of ground effect well. As a pellet
sped through the air, its blunt nose forced air away in a circular pattern. Being circular, the
pattern was symmetrical and the aerodynamic forces around the pellet were equal. The only
unbalancing forces were drag, which slowed the pellet, and gravity, which pulled the pellet
toward the earth.
When fired parallel to the surface of the water, however, the airflow around the pellet was no
longer symmetrical. The air passing above the pellet was forced out of the way in the usual
manner. But a different phenomenon was occurring to the air passing below the pellet.
Because the pellet was near the surface, the air was not free to be deflected as it was above
the pellet and the downward flow was slowed. This created a difference in airflow around the
pellet itself since the air passing around the pellet moved faster above it than below it.
This difference in the speed of the flow above and below the pellet created a pressure
differential. Because of its greater speed of the air flow, the pressure above the pellet was
lower than the pressure below. In effect, this created lift that was sufficient to counteract the
downward pull of gravity.
As the pellet slowed, lift decreased. This allowed gravity to begin to draw the pellet toward
the water, which further slowed the air flow below the pellet and intensified the ground effect.
In other words, the lower it went, the greater the pressure differential, and the greater the lift.
In this way, ranges in excess of any that could have been gained by simple ballistics were
achieved.
Swiss scientist Daniel Bernoulli's Law (often incorrectly called Bernoulli's Theorem) explains
this effect by stating that pressure from the wall of a closed tube containing fluid, or, in this
case, the high pressure air below the pellet (or airfoil), provides the energy to accelerate the
air in the constriction above. Lift is the outcome. As an aside, Bernoulli's Theorem was
postulated by Daniel's uncle, mathematician Jacques Bernoulli, and refers to the laws of
probability.
Fixed-Wing Effect
Ground effect is found to be at a maximum when within one wingspan or rotor diameter of
the ground and will not be noticeable when higher than two wingspans or rotor diameters
above the surface. Because they normally fly at altitudes above this distance, pilots of fixedwing craft must only consider ground effect when taking off or landing.
When properly used, such as when making a soft field takeoff, ground effect allows an
airplane to become airborne at a speed lower than normal. Lifting off before attaining the
normal takeoff speed eliminates the drag caused by the wheels rolling on a soft surface.
Flight at this slower-than-normal speed is made possible by the additional lift produced by
ground effect. But pilots should not attempt to fly out of ground effect before achieving the
proper climb speed, either best angle (VX) or best rate (VY), depending on environmental
needs. Without the needed speed, which, in effect, replaces the extra lift provided by ground
effect, the airplane may quickly return to the surface with an embarrassing thud.
Ground effect produces the opposite results on landing. If the landing speed is higher than
normal, the airplane will float seemingly forever until the airspeed decays to the point where
the wings will no longer produce lift. If you want to know how far your total landing distance
will be increased, divide your actual touchdown speed by the normal touchdown speed, and
then square the answer (multiply it by itself).
For example: If an airplane that should normally land at 50 knots (kts) touches down at 55
kts, its landing distance will be 21 percent greater. If it touches down at 70 kts, the airplane
will need 96 percent more landing distance. If there was ever a perfect reason for precise
speed control on landing, this is it.
While precise speed control on landing is important to all aircraft, it is especially so for lowwing aircraft, especially those with short landing gear. Ground effect increases the closer one
flys to the surface, and low-wing aircraft get a lot closer to the ground than high wingers do.
This is why the pilots of low-wing aircraft tend to have more floaters than those flying high-
wing aircraft.
Meu Caro, se te sentires numa situação semelhante àquela pela qual eu passei não leves a
tua tomada de decisão para além do ponto de não retorno. É preferível “borregares” e
preparar-te para uma aterragem com uma nova configuração do avião. Um conselho do
Fernando! 4
PONTO DE ORVALHO OU DEW POINT
Ontem, estive todo o dia no aeródromo de Évora. À medida que o Sol se ia “apagando” e o
frio aumentava, os aviões estacionados na placa, começaram a cobrir-se de uma fina
camada de orvalho, sinal de uma atmosfera carregada de humidade e de uma descida de
temperatura. Tu sabes bem o que é que acontece quanto o “ponto de orvalho” ou “dew
point” e a temperatura do ar se encontram. Temos precipitação numa das suas diversas
formas.
Por isso, durante a noite, quando regressei a Lisboa encontrei uma auto-estrada coberta por
extensos bancos de nevoeiro. E se eu estivesse a voar? O que aconteceria? Tinha ficado
dentro do “caldo” como se costume dizer no jargão aeronáutico. De repente, tinha ficado
dentro de condições IMC, uma situação das mais perigosas para quem faz VFR.
Porque todos os anos se dão acidentes derivado ao facto dos METAR’s ficarem, de repente,
como aquele que se faz sentir, neste momento, em Lisboa:
LPPT 181200Z 06004KT 020V090 0400 R03/1200VP1500 R21/0350V0500N PRFG
VV001 11/11 Q1032 NOSIG
em que 11/11, é importante não ser apanhado em rota por uma situação destas.
Alinhas numa revisão dos conceitos relacionados com o dew point? Olha que o saber não
ocupa lugar...
DEW POINT REVIEW
By THOMAS A. HORNE
The inside story behind saturated air and fog formation
Here's an important pilot weather factoid we all learn: When the temperature-dew point
spread is less than 5 degrees Fahrenheit (3 degrees Celsius), expect fog. This rule of thumb
is important to remember, but there are other ways of looking at how dew point temperature
(better known simply as dew point) and other moisture measurements influence aviation
weather.
First things first. Warm air can hold more water vapor in suspension than cold air. Why?
Think of warmer air as having faster-moving, higher-energy molecules. Now imagine water
droplets, like those that make up fog. Increase the air temperature and, all other variables
being equal, the droplets evaporate in the high-energy air. That is, water molecules leave the
droplets.
Now take that higher-temperature air and cool it. Thanks to the cooler, lower-energy state of
the air parcel, any moisture condenses back into a water droplet. The moisture in the air has
gone from a liquid state (fog) to a gaseous state (water vapor) and back again. Again, you
may ask, why?
There's a limit to how much water vapor can exist in a gaseous, invisible state. Reach that
limit, and the air is said to be saturated. Add any more water vapor and the result is
supersaturation, then condensation, which in large amounts means precipitation — rain,
snow, or sleet (ice pellets, IP in METAR code). Now get this: This limit, as you must have
guessed by now, is a function of air temperature. You can also think of this limit as the
maximum amount of pressure exerted by water vapor; any more pressure from more water
vapor and — presto! — condensation, droplets, fog, and clouds. This limit is called saturation
vapor pressure.
I've plotted the saturation vapor
pressures for various temperatures
(see the chart below) and just one
look tells you why lower temperatures
can mean fog trouble. The lower the
temperature, the less water vapor is
required to saturate the air.
Saturation relates to relative humidity.
Most people think relative humidity is
a measure of the actual moisture in
the air. Not so. Relative humidity is the
ratio between the moisture level in the
air and the maximum possible
moisture capacity (the saturation
vapor pressure) of the air at a
particular temperature. In other words,
it's the ratio between what is and what
could be, in moisture-speak. Relative
humidity 50 percent? Then the air is only halfway to the saturation point.
Where does dew point figure in? Unlike relative humidity, dew point is a measure of the
actual water vapor in the air. This is the temperature to which air must be cooled in order to
become saturated, and therefore reach 100-percent relative humidity. At this point, fog or
clouds are almost certain to form.
As we've seen from the chart, this can be any temperature. But for any given temperature the
amount of water vapor needed to saturate the air varies. Low air temperature? Then it won't
take much water vapor to cause saturation. High air temperature? Then lots more moisture
will be needed to create fog or clouds.
These facts lead us to some significant truths, all of them worth bearing in mind as you plan
your flights:
Temperatures rise during the day, then fall at night. If nighttime temperatures fall to the
dew point, don't be surprised if fog occurs. Once the sun comes up, the temperature-dew
point spread widens, relative humidity decreases, fog droplets evaporate, and visibility
improves. This process can take several hours.
The higher the dew point, the more moisture in the air. As we move into the warmer
months of the year, keep an eye on dew points. Dew points above, say, 60 degrees F (15.5
degrees C) mean that not much cooling is needed to create fog or clouds. Increasing dew
point temperatures also can confirm that frontal passages are imminent, as fronts push
moisture-laden air masses ahead of them. Conversely, low dew points indicate dry air
masses.
High dew points correlate well
with the probability and severity
of thunderstorms. Once thermals,
fronts, and other lifting forces go to
work on air masses with high dew
points,
towering
cumulus
or
cumulonimbus clouds can soon
follow. Cooling takes place as
moisture-laden air is lifted higher and
higher in unstable air masses. Most
severe thunderstorms (those with
50-plus-knot surface winds, threequarter-inch hail, or tornadoes)
happen in air masses with dew
points above 70 degrees F (21
degrees C).
The
temperature/water
vapor
relationship constantly changes.
Saturation of an air mass can occur
via two mechanisms. Cooling to the
dew point is one. Addition of
moisture — through frontal activity or flows of humid air — is the other. As temperatures and
moisture levels rise and fall, these variables influence each other and cause dew points to
fluctuate.
Dew points below 32 degrees F (zero degrees C) indicate a chance of frost. If
temperatures drop to these dew point values, you may be in for a lengthy preflight frostremoval exercise.
Dew point values can forecast nighttime low temperatures. Forecasters often use dew
point as a quick and dirty indicator of the evening's lowest temperature. That's because
temperature cannot be lower than dew point. This rule of thumb works best when there is no
surface wind, and works especially well in the warmer months of the year. Another way to
anticipate fog is to look at the difference between the air temperature and the soil
temperature (soil temperatures are approximately 5 degrees F [3 degrees C] colder than
surface temperatures). If the air temperature is 30 degrees F (17 degrees C) warmer than
the soil temperature, then fog is likely to occur.
Close temperature-dew point spreads don't always mean fog. There are times when
close temperature-dew point spreads do not presage a fog-, dew-, or frost-shrouded airport
in the morning. Plunging nighttime temperatures often are the result of radiational cooling,
the kind associated with high pressure and clear skies. But what if there are cloud layers that
prevent low-level heat from escaping? Cloud layers can trap heat near the surface — and
maybe even cause temperatures to warm up. This widens the temperature-dew point spread,
and prevents the cooling necessary for fog formation. Fog also tends to rely on temperature
inversions. That is, situations where the lowest layers of the atmosphere increase in
temperature with altitude. When no inversion exists, and temperatures decrease with
altitude, then low-level air and moisture is free to mix with drier, higher-altitude air. Surface
winds higher than five knots or so can do the same thing. A fast check of surface reports
and, if you're really interested, atmospheric soundings can give you an idea if the overnight
weather is conducive to this kind of false-positive setup. Absent these conditions, however,
you can pretty much count on lowered visibilities in fog or mist.
Por tudo isto, quando fores fazer uma viagem, faz sempre um briefing meteorológico
cuidadoso. Utiliza as ferramentas que o Instituto de Meteorologia pôs ao dispor de todos os
pilotos. Visita o site http://brief.meteo.pt/ e obtém um briefing certificado Anexo III. Um
conselho do Fernando.4
Deixa-me terminar recomendando-te mais uma vez que te associes à AOPA Portugal.
Perguntarás, de imediato, como o poderás fazer. Visita o site da AOPA Portugal em
www.aopa.pt e manda as tuas perguntas para o Presidente da AOPA Portugal através do
seguinte e-mail address: [email protected]. Gostaria de contar com a tua presença na
nossa AOPA. 4
Como sempre, um abração do
Fernando