1 Meu Caro, Antes de mais uma notícia de ultíssima hora. Fruto dos esforços da AOPA Portugal junto do Instituto de Meteorologia, este passou a disponibilizar na internet, para toda a comunidade aeronáutica, informação específica para nós pilotos. Consulta o site do IM em: O IM fez um excelente trabalho. Outros passos se seguirão. Mais uma semana se passou. Esperemos que tenha sido uma semana de bons voos. Acima de tudo, seguros. Já la vai o tempo em que reinava o espírito do Major José Eduardo de Cook e Alvega. O célebre Major Alvega que praticava as mais alucinantes fantasias aeronáuticas, a maior parte das quais ofendendo os mais elementares princípios da Segurança Aeronáutica. Na fantasia da nossa infância víamo-las como símbolo de heroicidade. Hoje vemo-las como símbolo de asneiras a nunca cometer. Mas deixemos o nosso Major Alvega em paz e respeitemo-lo como responsável de muitos momentos de felicidade da nossa juventude. Hoje, sejamos pilotos responsáveis e conscientes. É a melhor homenagem póstuma que lhe podemos fazer. Em Portugal raros são os pilotos com qualificação de PPA que realizam voos de viagem por necessidade de deslocação. A maior parte faz voos para manter as qualificações, voos de passeio ou voos para mostrar a paisagem aos amigos. Estes últimos podem, ainda, servir para outra coisa: para exibirmos o nosso ego de piloto. Para mostrarmos que somos superiores aos amigos por estamos dentro do reduzido número daqueles que é capaz de pilotar um avião. Quantas vezes dei comigo a pensar sobre outras pessoas: Tens um grande sucesso na vida. Porém, eu sou capaz de aterrar um avião e tu não és!. Diz-me lá se também não pensaste o mesmo. Sê franco! Todos nós um dia ou outro pensámos isto. Todas estas manifestações de exuberância do nosso ego fazem bem às nossas tensões internas. Libertam o stress. Porém, desde que libertadas em doses moderadas. Se, de repente, nos esquecemos das mais simples regras da prudência, então pomo-nos em perigo, a nós e aos nossos passageiros. A última asneira da nossa vida, eventualmente. Se não acreditares que isto pode acontecer lê a seguinte notícia. DON'T LET AERIAL SIGHTSEEING TURN TRAGIC Spring is a wonderful season for aerial sightseeing. However, a passenger's desire to "get a closer look" needs to be managed properly. The tragic accident that follows illustrates this point all too well. In May 2003, the pilot of a Rockwell Commander and his three passengers were killed when they crashed in the Sierra Nevada. The Commander left South Lake Tahoe at about 1:40 p.m. local time, destined for Santa Ana, California. The weather was reported as clear with more than 10 miles of visibility and a temperature of 91 degrees Fahrenheit. Before departure, the airplane was fueled with 35.2 gallons of fuel. The estimated weight of
2 the Commander before departure was 3,175 pounds 35 pounds over the certified gross weight. A digital camera belonging to one of the passengers was found intact in the debris. The images documented the flight path departing South Lake Tahoe, flying over the Half Dome area of Yosemite Valley, and then proceeding to the southeast at an altitude well below the surrounding mountains. The flight was headed in the direction of a lake that the pilot's wife had camped at as a child. The wreckage was found on a plateau in the Sierra Nevada at an elevation of 12,660 feet. Surrounding terrain ranged in elevation from 4,900 feet to 12,795 feet. The density altitude at the accident site was calculated to be 15,000 feet. The pilot held a private pilot certificate, had more than 1,500 hours of experience, and had flown at least 40 hours in the previous 90 days. The NTSB determined the cause of the accident was the pilot's failure to maintain an adequate airspeed while maneuvering close to the ground over mountainous terrain in a high-density altitude environment, near the upper performance capabilities of the airplane. All these factors lead to an inadvertent stall and crash. Taking friends and family on sightseeing trips is a wonderful way to share your hobby and expose them to general aviation. Unfortunately, sometimes the pressure to please can yield bad results. This flight had the pilot departing over gross weight in high terrain, with even higher density altitudes over the route of flight. The rising terrain exceeded the capabilities of the Commander, and four people died as a result. Este nosso companheiro cometeu vários erros. O primeiro foi descolar com um peso superior ao MTOW (Maximum Take-off Weight). Dir-me-ás: mas afinal foi só 1% de excesso de peso... Por 1%, nenhum avião cai. É verdade meu caro. Contudo as regras são para se respeitar pois, de outra maneira, de 1% em 1%, podemos chegar aos 100%... Depois, provavelmente, o piloto esqueceu-se de tomar atenção à velocidade para se preocupar com a paisagem. Erro de palmatória. Afinal, os aviões deixam de voar na velocidade de perda. A esta velocidade a Sustentação passa a ser inferior ao Peso e o avião cai, atraído pela força da gravidade, como qualquer outro grave. Ao mesmo tempo, provavelmente, o piloto esqueceu-se da influência da Altitude de Densidade no aparecimento do fenómeno da perda. Ao mesmo tempo, provavelmente, o piloto esqueceu-se nas cargas que se aplicam na aeronave em voltas apertadas. Ao mesmo tempo... Provavelmente a lista de ao mesmo tempo poder-se-ia prolongar por muito mais linhas. Por isso meu Caro, não transformes um agradável passeio numa coisa muito desagradável. Nunca te esqueças daquilo que te foi ensinado nas aulas teóricas do teu curso de pilotagem. Todas estas matérias, mais dia, menos dia, vão ser-te úteis e podem fazer a diferença entre o viver e o morrer. O mês de Maio, em Portugal, é um dos meses de instabilidade meteorológica. É o mês que, do ponto de vista estatístico, apresenta o mais alto nível isocráunico (maior número de dias com trovoadas). Por isso, podemos sair para um voo em condições de CAVOK e, de repente, ficarmos no meio de condições meteo muito desagradáveis. Por isto, proponho-te uma pequena revisão dos teus conhecimentos meteorológicos. Vamos a isto? Aceitas o meu desafio? Olha que t epode ser útil! AMONG THE CLOUDS
3 What's it like in there? BY THOMAS A. HORNE (From AOPA Pilot, March 1997.) We should all be weather-watchers, and not just on the days we plan to fly. Knowing the basic cloud types, their movements, their shapes, and a few other basic weather variables can give us rudimentary forecasting skills. On long cross-country flights, unforecast clouds are apt to appear, forecast ones may not show up, and you may have to fly in or close to clouds for extended periods of time (assuming you're instrument-rated and IFR-current, that is). Knowing your clouds can obviously help you to formulate critical in-flight decisions. Press on or turn around? Land now or divert to an alternate? Sometimes it depends on the clouds. The World Meteorological Organization (WMO) the people who gave us METARs and TAFs have come up with hundreds of different cloud types, each one with an erudite Latin name. The National Weather Service's Cloud Code Chart lists 27, arranged according to altitude. Here we'll check out the most common. Cumulus. We all know these. If you're with the WMO, there are dozens of Latinate cumulus clouds, but I think of them as coming in one of three varieties: good, uh-oh, and bad. Good cumulus clouds are found in high pressure and are scattered. Those with flat bases usually occur after a cold frontal passage and indicate a rough ride below the cloud base. It'll be turbulent in the clouds, too, with a good chance of clear or mixed icing when temperatures are between 0 and minus 20 degrees Celsius. Above, the air is usually smooth. The bases are usually around 3,000 to 4,000 feet agl or so, and tops seldom go higher than 8,000 feet agl. Uh-oh cumulus (cumulus congestus, calvus, and capillatus, in order of development) are clouds that are building in height. These typically appear on the back side of a high pressure system or ahead of a cold front. Bases may be lower (especially east of the Mississippi); tops are higher and can rise well into the flight levels during the heat of the day. And don't try a VFR climb or descent around them. If a scattered condition exists, it may not stay that way long. Broken to overcast skies are the rule. Expect moderate turbulence and, when temperatures are right, icing in these clouds. VFR flight beneath these clouds may be possible but uncomfortably turbulent. The problem with uh-oh cumulus is that they can turn into Bad cumulus, which implies stronger convective activity and thunderstorms. It may be difficult to maintain VFR; the sky ahead will become darker and the ride bumpier, and precipitation may start. If in the clouds, turbulence and icing will quickly become issues. Best to turn around and land. These clouds can move quickly and may be part of a complex of violent frontal weather. Stay on the ground until they pass, then try launching in the morning, when convective activity is less intense. Pretend that you had to run a search on these clouds in some computer database and tried the following keywords: west winds; turbulence; clear ice; smooth on top; was VFR on top, now have to descend on instruments; why is it so dark?; wish I hadn't gone in this cloud; what's VA for this airplane?; when will this be over?; is my insurance paid up? Stratus. Stratus clouds usually are precursors to a nearby warm front; post-cold frontal instability clouds; or marine- or lake-influenced onshore layers. You often hear that stratus clouds consist of shallow layers no more than 2,000 feet thick. But I've tried to work my way through a "thin," ice-laden stratus layer that went from 25,000 feet right down to 2,000 feet agl. The ride in stratus is usually smooth. If there's any ice, it ought to be of the rime variety. Precipitation can be continuous and widespread. Ceilings can go right down to the ground when the temperature is right on top of the dew point. VFR-only pilots can fly underneath a high stratus layer with no problems as long as the layer stays high. Just remember: you may be flying toward a warm front in the next hour or two, with lower stratus and precipitation.
4 Keyword search: southerly winds aloft; smooth air; onshore flow of air; warm front to the west or south. Neat feeling breaking out on top of a shallow layer after an instrument departure or seeing clouds part on an instrument approach, needles centered, and stratus fractus (scud) sliding by as the runway appears directly ahead (instrument-rated pilots). Wish I had an instrument rating because it's so smooth and I know I'd be on top in 5 minutes (noninstrument-rated pilots). Don't need sunglasses when you fly in direction of sun and below cloudbase; cloud shield can stretch for hundreds of miles; this must be coastal California in the summertime (all pilots). Cirrus. By definition, these clouds are found between 16,500 and 45,000 feet. But even if you're flying turbine-powered airplanes, they pose few problems. Cirrus are composed of ice crystals, and temperatures are usually so cold that ice doesn't accrete. These clouds are the earliest harbingers of an approaching warm front; they can appear 500 miles or more ahead of the front. Good VFR conditions prevail, and turbulence is rarely a problem. Winds aloft can run from a variety of directions but the westerly and southwesterly quadrants are favored directions below 12,000 feet or so. At higher altitudes, expect westerly winds. Beware of a cirrus shield that gets progressively denser. A high overcast may form, followed by lower layers that signal the front's approach. Keywords: Learjet country; contrails; trouble in 2 days. Finally, a serious word about flying in any type of cloud or visible precipitation. Please remember to turn on your pitot system heat before entering these conditions. Also, please, please be on the lookout for any signs of a drop in manifold pressure or any other indications of a loss of power. These are subtle warning signs that sneak up on pilots. It's a big clue that carburetor or some other type of induction system icing is forming. It's absolutely imperative to activate your carburetor heat (carbureted engines) or alternate source of induction air (fuel-injected engines) if these signs crop up. The alternate air doors fitted to fuel-injected engines are usually designed with a spring release, sometimes backed up with a means of manually opening the doors. When the normal air source becomes restricted, the force of the induction air automatically pulls open the spring-loaded alternate air door(s). This can all happen without any pilot action, which can be an important safety feature. However, it's important to emphasize that some airplanes don't use the springed-door arrangement, relying instead on a manually-activated door system. Also, springed doors may fail to work properly. If they don't open and the pilot fails to open the doors manually (assuming there is a manual method), the engine(s) may quit because of induction air starvation. So if you fly fuel-injected airplanes, don't forget the alternate air, as so many do. You do know how your alternate air system works and where any alternate air controls are, don't you not the alternate static source for pitot-static instrument air, mind you, but the alternate engine air source? Meu Caro, nunca te esqueças que a formação de gelo no carburador pode ser-te fatal. Utiliza o Ar Quente ao carburador sempre que tiveres dúvidas. É preferível aumentar um pouco o consumo do que ficares sem consumo... Existe, entre muitos pilotos da nossa comunidade nacional, a ideia de que os aviões equipados com motor de injecção são imunes ao gelo. Não é verdade. Espero que o final do artigo te tenha convencido definitivamente disto. Por tudo isto, evita, sempre que possível, voar em condições em que exista o risco da formação de gelo. Por isso, muita atenção às nuvens. Se quiseres fazer um curso rápido de refrescamento vai a este endereço e, humildemente, relembra aqueles conhecimentos que estão um pouco esquecidos: É, simplesmente, excelente! Meu Caro, nunca tiveste a dúvida sobre o porquê da velocidade de melhor angulo de subida
5 Tiveste de certeza, tal como eu. Contudo, a Mecânica de Fluidos não é uma ciência hermética. Basta estudar os seus princípios para que certas dúvidas se tornem claras. Às vezes, porém, é mais interessante fazer um estudo direccionado para não andarmos perdidos num universo vasto. Assim, um nosso companheiro americano que se identifica pelas iniciais K & M, resolveu dirigir-se a esse guru da AOPA (USA) que dá pelo nome de Rod Machado (será um luso descendente?). Vejamos a pergunta posta por K & M a Rod Machado: Dear Rod:...During flight training we have all been taught that Vx increases and Vy decreases with altitude, converging at either the absolute ceiling of the aircraft. However, in twelve years of flying I have never received a clear explanation of why this is the case. One CFI, to his credit, did some research and then passed on to me a quite technical explanation. However, since I lacked what appeared to be the requisite Ph.D. in fluid mechanics, my understanding of the subject was, shall we say, only modestly enhanced. Can you offer any help. Thanks, K.M. WHY V X AND V Y CHANGE WITH ALTITUDE Your question about Vx and Vy is a good one. In fact, I think this is one of the more challenging aviation topics to explain. Any explanation requires discussing several ideas to provide an adequate answer to the question. So here's my best shot at attempting to explain the concept. A Detailed Explanation As you know, Vx is the best angle of climb speed. At this speed the airplane gains the most altitude (vertical movement) for a given distance (horizontal movement) over the ground. Take a look at Figure 1 which depicts an airplane's rate of climb curve for a given true airspeed. (Note: in this article I'll assume that indicated airspeed is the same as calibrated airspeed.)
6 The blue line is the rate of climb curve. Let's say I placed you in the far left hand corner of the reference box at the 0-FPM and 0-airspeed corner. If I asked you to look up from that position and point to the highest part of the ROC curve that you could see, your arm would point up at the same angle of the tangent line shown in Figure 1. Your arm would point to directly to the tangent point on the graph (identified by the yellow dot). Mathematically speaking, the upward angle of this tangent line represents the maximum amount of vertical gain for a given amount of horizontal movement to the right. But isn't the maximum amount of vertical gain (or altitude gain) for a given distance over the ground also called the best angle of climb? Indeed, it is. So if we drop straight down from the tangent point (yellow dot), we see that the best angle of climb is achieved at a true airspeed of 59 knots at sea level. So Vx is 59 knots true airspeed (TAS) at sea level. What about the speed for the best rate of climb? Isn't this where you gain the greatest amount of altitude for a given amount of time? Isn't this just another way of saying that, at the best rate of climb speed, the VSI needle has the greatest upward deflection? Yes, it is. Therefore, the greatest rate of climb occurs at the very top of the ROC curve (the green dot). If you drop straight down from this point you'll find the speed for Vy (the best rate of climb). For this airplane Vy is 76 knots (Figure 1). ROC Curve Changes Shape With Altitude Now, ask yourself what happens to the ROC curve when altitude increases. Since the maximum power (thrust) the engine is capable of producing decreases with an increase in altitude, the ROC curve will change shape and grow shorter as shown in Figure 2. The ROC curve for three different altitudes are shown: sea level, 5,000 feet MSL, 10,000 feet MSL. (A nonturbocharged airplane is assumed here.)
7 It stands to reason that a shorter ROC curve causes the same tangent line to make less of an upward angle with the horizontal. Figuratively speaking, you can think of lowering tangent line as the path of an airplane climbing at less of an angle as a result of an increase in altitude. Therefore, dropping straight down from the tangent point (yellow dot), we find that the best angle of climb speed has increased to 69 knots (TAS) as shown in Figure 3. And this is the basic reason why Vx increases with altitude. (These angles are only symbolic representations. The airplane's climb angle isn't the same upward angle made by the tangent line. This is, nevertheless, a reasonable way to compare ROC curves for the same airplane at two different altitudes.) In Figure 3, it's also apparent that Vy has increased to 80 knots (drop straight down from the
8 green dot). How can this be? Weren't you told that Vy decreases with an increase in altitude? Yes, but that's only if you're talking indicated airspeed, not true airspeed (I'll explain this shortly). Right now, as far as true airspeed is concerned, Vy increases with an increase in altitude. It increases for several reasons. To visualize why it increases, look at how the ROC curve (as measured by test pilots) changes shape with altitude. The curve shrinks vertically and its top moves to the right slightly (the shape of the ROC curve changes because of changes in drag and in propeller efficiency with altitude). Figure 4 shows Vx and Vy for the altitudes of sea level, 5,000 feet MSL and 10,000 feet MSL. At 10,000 feet MSL, Vx is 77 knots (TAS) and Vy is 83 knots (TAS). Here's the most important thing to notice. Vx and Vy (as true airspeeds) both increase with an increase in altitude, but Vx increases FASTER than Vy as shown in Figure 5. Eventually, as altitude continues to increase, Vx will catch up with Vy at the point where the airplane has a "zero" rate of climb. This point is known at the airplane's absolute ceiling.
9 Why Vy is Said to Decrease With Altitude So far we've spoken only about true airspeeds. When we talk about indicated airspeed, things change a bit. Here's why. In order to understand how Vy (as an indicated airspeed) decreases with altitude, we need to convert the true airspeeds from Figure 5 to indicated airspeeds. As a rule of thumb, we know that our true airspeed increases approximately 2% per thousand feet over our indicated airspeed. In other words, if we're indicating 100 knots at 10,000 feet, then our true airspeed is 20% more than what we indicate, or 120 knots. But what if I want to know what indicated airspeed is needed to produce a certain true airspeed? You can determine this by using a simple mathematical formula like the one shown in Figure 6. Let's use it to convert the true airspeeds in Figure 5 to indicated airspeeds. Find the IAS required to produce a specific TAS by using this formula (Figure 6). Now let's put those values on a chart as shown in Figure 7.
10 When you convert the true airspeeds to indicated airspeeds for the altitudes mentioned above (S.L., 5,000' & 10,000'), you find that Vy, as an indicated airspeed actually decreases with altitude. Vx, on the other hand, increases with altitude as an indicated airspeed (Figure 8). Here's another way of looking at these results. Let's assume for a second that Vy, as a true airspeed, stays the same with altitude. If that was so then the indicated airspeed to maintain a single (specific) true airspeed would decrease with altitude. That should make sense since the air becomes less dense, therefore, we move faster through the air but show less of an indicated airspeed reading while doing so. In reality Vy as a true airspeed actually increases with altitude. But the increase in TAS readings is slow enough that each specific IAS required to produce a specific TAS decreases with altitude. You can see this clearly from Figure 9.
11 Looking at the Vy (bright red) line, notice that both TAS and IAS are assumed to be equal at S.L. At 5,000 feet, it takes an IAS of 73 knots to produce a TAS of 80 knots. At 10,000 feet, it takes an IAS of 69 knots to produce a TAS of 83 knots. Thus, you can see how Vy as an IAS decreases with altitude. On the other hand, Vx (as a TAS) increases faster than Vy (as a TAS) with altitude. Therefore, the IAS required to produce a specific Vx (as a TAS) will also increase faster that that for Vy. While this is hard to explain verbally, you can see this clearly from Figure 9. The Vx (purple) line shows that both TAS and IAS are assumed equal at S.L. At 5,000 feet, it takes an IAS of 63 knots to produce a TAS of 69 knots. At 10,000 feet, it takes an IAS of 65 knots to produce a TAS of 77 knots. Thus, you can see how Vx as an IAS increases with altitude. Remember, differences between IAS and TAS occur because TAS increases with altitude. I hope this helps. Best, Rod Meu caro percebeste, agora, porque é que os nossos aviões têm um tecto aerodinâmico? 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 e manda as tuas perguntas para o Presidente da AOPA Portugal através do seguinte address: Gostaria de contar com a tua presença na nossa AOPA.