Meu Caro,
Na nossa última croniqueta abordámos diversa problemática relacionada com o
combustível. É um tema interessante e importante. Afinal o combustível é como o sangue
que flui nas nossas veias e alimenta o corpo. Sem um fluxo correcto e adequado correm-se
riscos, quantos deles desnecessários.
No primeiro artigo desta semana debate-se o tema do empobrecimento da mistura. Afinal
uma operação muitas vezes é esquecida. Afinal uma operação tão importante quando
voamos acima dos 3.000’.
GESTÃO DO COMBUSTÍVEL
Quando fazemos os nossos cursos somos instruídos nas mais diversas operações de voo.
Contudo, às vezes, falham certos pormenores. É natural. Afinal, são tantos.
Esta semana ao leccionar a cadeira de Performance tive a oportunidade de explicar aos
alunos algo que me haviam explicado a mim, há um bom par de anos. Todas as tabelas de
performance e consumo de combustível do avião são obtidos conduzindo o avião na
situação de mistura de combustível “LEAN”.
Nunca vi nenhuma referência aos consumos do motor conduzido com o “Ar Quente ao
Carburador” ligado. Aprendi-o à minha custa, felizmente sem consequências de maior,
devido à prudência que sempre apliquei a todas as coisas do ar.
Como é que aprendi? Num voo entre Santa Cruz e Muxamel (Alicante). Descolados de
LPSC subimos gradualmente para os 6.500’ de cruzeiro, empobrecendo a mistura a partir
dos 3.000’. Atingido o cruzeiro, verificámos que a temperatura do carburador fizera o
ponteiro do respectivo termómetro deslocar-se para a zona do arco amarelo – a zona de
perigo de gelo. Como piloto cumpridor vou puxando o botão do ar quente até ver o ponteiro
sair do arco para onde eu nunca queria que ele nunca tivesse entrado. Deste gesto resultou
uma viagem com o “ar quente” sempre metido.
Porque Muxamel ficava mesmo nos limites da autonomia do avião – o Cessna C172N –
havíamos resolvido fazer uma paragem intermédia em Córdoba. Em boa hora o decidimos.
Parqueado o avião na placa de Córdoba, salto para o chão e dou uma volta pela frente do
mesmo. Ao olhar para a zona do “capot” inferior reparo que a zona junto ao tubo de escape
do motor estava completamente enegrecida de negro de fumo. Fuligem saída pelo escape.
Sinal de mistura muito rica.
Este sinal, de imediato, sugeriu uma medição do combustível remanescente nos depósitos.
E, para espanto dos pilotos, o normal consumo de 30/32 litros de combustível tinha-se
transformado num gordo consumo horário de 40 litros. Sem a paragem intermédia tínhamos
tido acidente, certamente.
Por favor, tira as respectivas ilações do que é voar com ar quente ao carburador e a sua
influência no consumo horário de combustível. E tem cuidado porque podes não poder fazer
uma aterragem intermédia.
FUEL SCHOOL
By Mark Twombly's
A crack Army Reserve helicopter pilot got checked out in a Mooney. On his first trip, he
nearly ran out of gas long before reaching his intended destination. The pilot, who until then
had flown only turbine helicopters, had not been taught how to lean the mixture in a pistonpowered airplane. In cruise flight he left the mixture in the full-rich takeoff position, which
meant the engine burned 25 to 30 percent more gas than if he had leaned. It was an
unforgettable lesson in fuel management. (Afinal este piloto tinha uma razão: os vícios
adquiridos por voar motores de turbina.)
The term fuel management sounds pretty officious when applied to a light, single-engine
aircraft. What's to manage? Take off with the tanks full, and land before they are empty.
Sounds simple, but there's more to it than first meets the eye. In fact, when we sign up for
primary flight instruction we also enroll in Aircraft Fuel Management 101. And, like the
business school frosh destined for an MBA, we need to acquire more sophisticated fuel
management skills as we progress into more sophisticated aircraft.
Fuel school includes classes in three basic subjects: the fuel system, aircraft performance,
and weight. Systems class begins with the preflight inspection - physically looking in the
tanks to confirm that there is enough fuel for the mission, and draining the sumps to check
for water contamination and ensure that the tanks do indeed contain avgas and not jet fuel.
The systems class also covers the fuel selector, fuel quantity gauges, and the auxiliary fuel
pump if the airplane is so equipped. The simplest fuel selector has a Both position, meaning
that the engine draws fuel from both the left and right tanks at the same time. However, you
can correct an imbalance - when one tank has more fuel than the opposite tank - by moving
the selector to the fuller tank until the balance is restored.
If the selector does not have a Both position, basic management technique calls for taking off
and landing with the selector set to the fuller tank. In flight, use the selector to maintain an
approximate balance between the left and right tanks.
The lesson to learn about fuel quantity gauges is to treat them with skepticism. Experience in
the airplane will tell you if the gauges are anywhere close to being accurate. Experience also
teaches that the best fuel quantity gauge is a clock. Once you learn the fuel consumption
habits of a particular airplane in climb, cruise, and descent, you can use a clock to accurately
determine fuel used and fuel remaining - provided that you know how much fuel was in the
tanks when you started the engine.
Proper use of an auxiliary electric pump is another fuel management issue. A simple highwing single may employ gravity as an infallible alternative to an aux fuel pump. Its low-wing
counterpart, however, needs the help of an aux pump in the event that the engine-driven fuel
pump fails. Normally, an aux fuel pump is switched on for engine start, takeoff, initial climb,
when switching tanks in cruise flight, and during approach and landing, but there may be
other situations when it is needed. Check your management textbook - the pilot's operating
handbook - to find out what those may be for the airplane that you fly.
Performance-based fuel management begins with leaning the mixture. Leaning is done to
achieve a fuel/air ratio that results in optimum combustion, horsepower, and fuel economy for
a given density altitude and throttle setting. Leaning may not be much of an issue on local
training flights, but it's a basic fuel management task on cross-countries, especially when
cruising at altitudes above 3,000 feet and when taking off in high-density-altitude conditions.
The POH is the basic text for learning how to lean. A flight instructor experienced in the
airplane ought to be able to provide useful insight on various leaning techniques and their
effect on performance and fuel consumption.
Weight-based fuel management comes into play when the weight of people and bags - the
payload - combined with the weight of the fuel exceeds the allowable useful load of the
airplane. In that case, you have two choices: take off with less than full fuel, or reduce the
payload. Obviously, it pays to do the fuel weight and payload planning before going to the
airport. It's a hassle to have to offload fuel or, worse, pick someone or someone's bags to
leave behind on the airport ramp.
Fuel management gets more interesting as you work your way up the ladder in terms of
aircraft complexity, weight, and performance. For example, if the airplane has been modified
with optional auxiliary tanks, you'll need to carefully study the flight manual supplement to
learn how to manage the fuel. The aux tanks may deliver fuel to the main tanks or the
engine-or both at the same time. Obviously it's important to know which, and how and when
to select certain tanks.
One popular fuel system option in higher-performance airplanes is a fuel totalizer. When
teamed with a GPS, these electronic devices typically provide all kinds of useful fuel
management information - real-time fuel flow, fuel used, fuel remaining, gallons remaining at
destination, endurance, and so on.
The Achilles heel of most units, however, is beginning fuel quantity. It's up to the pilot to
program the device with the exact amount of fuel in the tanks. If you don't get that figure
correct it means that all quantity-based calculations performed by the totalizer will be
inaccurate. Garbage in, garbage out. Avoid managing garbage by studying the flight manual
supplement for the fuel totalizer and paying close attention when the airplane is fueled.
The worst that can happen to a failing business manager is to get the old heave-ho from the
boss. A business school student who doesn't cut it gets a failing grade. The potential
consequence for a pilot who mismanages the on-board fuel supply is far more serious than a
simple firing or failing grade. Do the homework, pay attention in class, and you'll pass with
flying colors every time.
Lembras-te do que eu disse mais acima? Lembras-te que eu fiz uma paragem por
precaução em Córdoba? Se eu não o tivesse feito, não tinha chegado de certeza a
Muxamel. Nota que se tratava dum voo VFR feito num daqueles dias em que dá prazer voar.
Se fosse um voo em condições IMC a situação seria muito mais complicada.
Como diz Thomas Horne, no artigo seguinte, a causa mais frequente para paragens de
motor é o fim do combustível. Uma vergonha para qualquer piloto. Repara que,
normalmente, não é um finale majestoso...
FUEL
By Thomas A. Horne
What's the major cause of engine stoppages? You got it? Running out of fuel. This makes
having plenty of fuel one of your best outs. But how much is enough? Regulations require
that you plan your flights so as to land with 30 minutes' fuel reserve on day VFR flights and
45 minutes' worth of fuel on night VFR and flights under IFR. Those are the legal minimums,
but personally it makes me nervous to land with that little fuel aboard.
Is this enough? That depends in large part on a risk factor that we'll discuss shortly?
Weather. It also depends on the length of your flight and the remoteness of the destination
airport.
If the weather is severe-clear, then those 30- and 45-minute fuel reserves might give you
enough of a margin for error. If a headwind becomes stronger than advertised, your fuel
reserve cushion is diminished; but if the weather is good you can always make an
intermediate stop, gas up, and continue the trip. Of course, this assumes you've been
keeping close track of your fuel consumption, groundspeed, and the winds aloft.
What if the weather is chancy, and you face a long route without any nearby airports? You'd
certainly want to take on an extra fuel reserve? One that would let you comfortably fly in
unanticipated headwinds, permit you to circumnavigate thunderstorms or other cumulus
buildups, or even let you make a 180-degree turn and return for a landing at the departure
airport.
IFR flying presents an additional challenge. Complicated routings, en route changes to your
clearance, weather avoidance, and holding can ruin your original flight plan and consume a
lot more fuel than you originally planned to burn. That's just a fact of IFR flying. Will a 45minute reserve be a comfortable enough cushion if you have to perform a missed approach
at your destination? How far away is your named alternate airport? Fifty miles? One hundred
miles? Two hundred miles? Don't laugh. Manufacturers and pilots of large jets have planning
information that accounts for alternates that far away from intended destinations. That's
because they realize that a combination of a suitable alternate airport and better weather
may be very far away. You should realize this, too, and plan your fuel reserves
conservatively. A one-hour fuel reserve is better for actual IFR trips; two hours' worth is even
better. On the transatlantic ferry flights I've flown, I always landed with at least three hours of
fuel still in my tanks (and sometimes five or more)? Because alternate airports were so far
away.
You get the idea: More fuel is better. Tankering a lot of fuel is a great out, and knowing you
have a lot of gas is a good feeling when you're struggling with other challenges.
Em matéria de combustível vale sempre a pena ser sempre muito conservador. Por favor,
nunca corras riscos. Afinal acabas por gastar sempre o mesmo dinheiro na bomba de
combustível... 4
CAIXAS NEGRAS PARA A AVIAÇÃO GERAL
Uma “caixa negra” não é um dispositivo de segurança activa. Podemos, sim, considerá-lo
um dispositivo de segurança passiva que permite a analisar, a posteriori, o que se passou
durante um voo. Afinal da análise dum voo, muitas vezes, podemos tirar lições para o futuro,
isto no que diz respeito à segurança do voo.
Eis que, agora, é anunciada aquilo que se pode designar por a “Caixa Negra da Aviação
Geral”. Um princípio incipiente, mas um princípio.
'BLACK BOXES' SOON AVAILABLE FOR CIRRUS AIRPLANES
A Massachusetts manufacturer, which was developing an engine trend monitoring device for
an air taxi operator now using Cirrus Design aircraft, has expanded the product to a $5,000
flight data recorder or "black box" for the Cirrus SR20 and SR22 models. The device, made
by Alakai Technologies of Hopkinton, Massachusetts, can help owners by providing easily
downloadable engine performance data, but will also aid insurance company investigations
of fault in aircraft accidents. It could save up to $1,000 a year in owner insurance, a company
official estimated. The 3.5-pound device should receive a supplemental type certificate from
the FAA as early as December. The air taxi operator will become the launch customer for the
always-on Digital Flight Data System. It can record information such as engine performance,
rate of climb or descent, true and indicated airspeed, heading, altitude, engine trends, miles
per gallon, and GPS navigation waypoints. There have been initial discussions between
Alakai and Cirrus Design about the device. It will be manufactured for Alakai by another
Massachusetts company, Accent Technologies, located in North Andover.
Todos aqueles que praticam Rally Aéreo ou Voo de Precisão utilizam, a bordo, um logger
GPS que não é mais do que uma “black box” dos pobres. Afinal regista a maior parte dos
parâmetros do que esta anunciada pela Alakai Technologies.
Naturalmente, não confundimos um “logger” com uma “black box”. Afinal, uma “black box” é
um “logger” que tem de estar preparado para suportar os esforços e as temperaturas
extremamente elevadas que ocorrem, muitas vezes, quando acontece um acidente
aeronáutico.
Mas, olha! Dá muito jeito ter um “logger” a bordo. Pelo menos sabes com uma precisão da
ordem dos 30 metros por onde andaste e, quando te quiserem acusar de certas iniquidades,
podes comprovar que fizeste um voo com a maior das seguranças. E se o voo não foi feito
nessas condições, ao menos sabes porque é que te chamaram à atenção... 4
Deixa-me terminar recomendando-te 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
Um abração do
Fernando