HEAVY, CLEAN, AND SLOW

Transcription

1 Meu Caro, Se a tua operação aeronáutica se faz exclusivamente em pequenos aeródromos em que o MTOW das aeronaves que ali operam não atinge 1 ton, podes não ler o artigo seguinte. Afinal ele não te diz respeito. Contudo, há um provérbio português que diz que o saber não ocupa lugar. E, sabes? Nunca se sabe quando é que um dia nestas histórias há sempre um dia esse saber não virá fazer a diferença entre o viver e o morrer. Por isso, vá lá, lê o artigo seguinte porque ele pode ser-te útil. Não te esqueças porém de ler a última parte desta croniqueta sobre segurança aeronáutica. Nela analisamos o sistema de vácuo e algumas das suas falhas. São falhas que podem ser fatais. TURBULÊNCIA DE RASTO Este perigoso fenómeno afinal não é exclusivo dos aeródromos. Ele pode acontecer, quer no solo, quer em voo. Em voo, com a agravante de não haver um pré-aviso do movimento de uma aeronave mais pesada. Lembro-me de, há algum tempo atrás, ter lido que um Embraer EMB-110 Bandeirante ficou em voo invertido sobre os céus de Paris. Felizmente teve altura para recuperar da incómoda atitude. Um simples problema the wake turbulence invisível provocada por qualquer outro avião que teria passado antes nas proximidades. Imagina-te, com o teu avião que nem pesa uma tonelada a passar no mesmo local. Provavelmente, terias feito todo o programa de Acrobacia Livre. A turbulência de rasto não se vê mas mata. Vejamos algumas recomendações de Dave Wilkerson e o que nos aconselha o FAA sobre tal fenómeno. HEAVY, CLEAN, AND SLOW By Dave Wilkerson "Every airplane produces wake turbulence." All examiners hear applicants say these words sooner or later. The student pilots who say this are correct, but sometimes they don't really understand what they're saying. When you present yourself for the private pilot practical test-the checkride-you may expect your examiner to ask questions about airplane-induced wake turbulence. Examiners know that wake turbulence lies in wait like a Hollywood villain, with ambush skills that are equally well-honed and results that are just as final. Your pilot privileges will allow you to fly yourself and your passengers into wake turbulence situations, so the FAA has addressed the issue in the "examiner responsibility" section of the practical test standards (PTS). The PTS says, "Examiners shall test, to the greatest extent practicable, the applicant's correlative abilities rather than mere rote enumeration of facts throughout the practical test." Using this statement as a guide, many examiners thoroughly test applicants on wake turbulence awareness. Even if your examiner doesn't emphasize this topic, the vicissitudes of chance and air traffic control (ATC) may thrust it into your flight. When you are faced with a real wake turbulence

2 situation on the checkride, your objective becomes not scaring the examiner. During the ground, or oral, phase of your testing, you might reasonably expect anything from brief to in-depth questioning regarding wake turbulence. Some examiners ask direct questions like "What is wake turbulence?" Others use stealthier strategies to sample your knowledge, insinuating wake turbulence considerations into questions that seem to focus on other issues. My personal favorite involves giving the applicant a scenario wherein a twoway-radio communications failure occurs after the tower has issued downwind instructions that place the applicant's airplane behind a C-130 or similar large aircraft on a long final approach. I expect the student to know how to avoid the rolling shakes of wake turbulence and keep the aircraft out of danger. Common misconceptions about wake turbulence include just when it begins. Occasionally applicants announce that danger first appears when the generating airplane begins the takeoff roll. (It actually begins the moment the airplane leaves the ground.) (NOTA: Meu Caro, aqui entro em desacordo com o autor do texto. Em meu entendimento ele comete um erro grave quando afirma que...it actually begins the moment the airplane leaves the ground.... Não! A turbulência de rasto começa quando o avião inicia a sua corrida para a descolagem. A partir deste momento ele começa a criar sustentação. Uma sustentação que só igualará o peso no momento da rotation (retirem-se os valores de segurança). Nesse momento (na altura da rotação), por aumento do ângulo de ataque quando o piloto puxa o manche, o Lift aumenta particularmente depressa, iguala o Peso, e o avião começa a voar. Devido à existência de vórtices de ponta de asa, tanto mais intensos quanto maior a diferença de pressão estática entre o intradorso e o extradorso da asa (isto é, vão-se intensificando à medida que o avião ganha velocidade na pista) já há wake turbulence (devido à resistência induzida pelo downwash, por causa dos vórtices de ponta de asa) durante a corrida de descolagem, mas relativamente pequena. Além disso o downwash é barrado ao bater na pista, limitando o aumento de resistência induzida, e de wake turbulence. Quando finalmente o piloto puxa o manche, tudo aumenta rapidamente de proporção. O que muitas pessoas pensam é que só quando o piloto roda o avião para descolar se geram estes fenómenos. Não é assim. Eles já existem durante a corrida de descolagem, se bem que fracos, cada vez mais fortes à medida que o avião acelera, mas é na realidade quando o ângulo de ataque é aumentado na rotação do avião que estes fenómenos aumentam espectacularmente.) Others state that wingtip vortices aloft somehow remain level for a significant time (they actually fall toward the ground and may linger just above the runway depending on wind conditions), and a few report that heavy, fast, and clean airplanes beget the strongest slipstream upheavals (it's the heavy, slow, and clean aircraft that are the most dangerous). Few examiners would fail an applicant for these mistakes alone. But if you make such an error, expect more questions. Applicants who don't realize that the primary hazard from wake turbulence is the loss of control that results from the roll that wingtip vortices produce raise doubts in examiners' minds. Examiners want your first goal to be that of avoiding wake turbulence. If your examiner sees that avoidance is your urgent desire, questioning will likely move to other subjects. Another common series of misconceptions includes the idea that a light crosswind weakens and rapidly dissipates wingtip vortices on the runway behind a departing airplane. The fact is that a light wind from the side can trap the upwind vortex in the takeoff zone for longer than one might imagine. You can read about this if you have the FAA advisory circular Aircraft Wake Turbulence (AC 90-23E) (Ver texto desta Advisory Circular no fim deste artigo). It's available free on the Internet ( safety-products/wakeac.html). This booklet can help you to avoid another misconception: Wake turbulence is always the

3 harbinger of disaster. Most pilots have experienced wake turbulence at some point. My first encounter with it was as a student pilot in the 1960s. I knew I had made a good 360-degree steep turn by the way my little airplane skipped timidly through its own propwash. (At the time, I quietly wondered about invisible potholes of my own making and never once connected that experience with the thought of trailing behind a bigger airplane.) But wingtip vortices can create a similar jolting effect of varying severity, depending on how close you are to the larger airplane and a variety of other factors. The fact that these wingtip vortices are invisible multiplies their danger to pilots. Asked about avoiding wake turbulence behind a jet, some applicants say that they would notice the jet's rotation point and plan to lift off before that point, climbing above the jet's flight path. That's the correct book response, but a pilot who tries to do this could still find himself in trouble. The logical follow-up question involves the airplane's ability to climb as steeply as the jet. Some jets can climb as if they forgot something on Mars. Taking off behind one in a typical training airplane and hoping to climb more steeply than a jet shows a certain lack of awareness and judgment. Too many good pilots have challenged the horizontal tornado of wingtip vortices and lost. Some examiners pose their question in this way: "If you wanted to refresh your knowledge on wake turbulence, in which FAA publication would you look?" The answer is the Aeronautical Information Manual (AIM). Such a query is an open invitation to open the book and look. The AIM now includes accounts of an actual wake turbulence incident in which a DC-9 flew less than two miles in trail of a DC-10, rolled, and struck the ground with a wingtip. You may know that, under certain circumstances, ATC advises pilots of potential wake turbulence. You should also know that acknowledging a wake turbulence advisory means that you, the pilot in command (PIC), accept the responsibility for providing wake turbulence separation. As PIC, you have the authority to request additional separation, for example, two minutes instead of four or five miles, depending on the type of airplane that you must trail. Incidentally, this is precisely why ATC specifies the use of the word "heavy" in certain radio transmissions. Of course, some airplanes that weigh less than a "heavy" can produce wingtip vortices equivalent to a larger plane. The AIM notes that the Boeing 757 is such an airplane and lists separation criteria for wake turbulence avoidance. It further states that controllers may not reduce or waive this interval. Your examiner may not ask, but it's a good idea to know where to find this information. Be sure you know the terms wake turbulence, vortices, thrust stream turbulence, jet blast, jet wash, propeller wash, and rotor wash and how they relate to your operations both on the ground and in the air. You must understand that wake turbulence avoidance is every bit as much a part of safe ground operations as it is a part of safe flying. FAA ADVISORY CIRCULAR, AC-90-23E, CAUTION WAKE TURBULENCE The following information on Wake Turbulence AVOIDANCE is reproduced from FAA Advisory Circular, AC-90-23E, Caution Wake Turbulence. The section on Wake Turbulence RECOVERY (at the end of the article) is from FCI Emergency Maneuver Training s Upset Recovery Training Program. Wake Turbulence - The Problem All Pilots are taught to be aware of wake turbulence. However, recent incidents indicate that pilots need to keep in mind how severe wake turbulence can be. In any event, wake turbulence is still out there and it can put a pilot and the aircraft at risk. This page was prepared as a reminder to pilots, to make them aware of wake turbulence and how to best avoid it. Remember, the best defense against wake turbulence is to know and avoid areas

4 where it occurs. What is Wake Turbulence? All Aircraft produce wake turbulence. Wake vortices are formed any time an airfoil is producing lift. Lift is generated by the creation of a pressure differential over the wing surfaces. The lowest pressure occurs over the upper surface and the highest pressure under the wing. This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. Viewed from behind the generating aircraft, the left vortex rotates clockwise and the right vortex rotates counterclockwise. The intensity or strength of the vortex is primarily a function of aircraft weight and configuration (flap setting etc.). Heavy aircraft, flying slowly, in a clean configuration, produce the strongest vortices. For example, a large or heavy aircraft that must reduce its speed to 250 knots below 10,000 feet and is flying in a clean configuration while descending, produces very strong wake. Extra caution is needed when flying below and behind such aircraft. Induced Roll - The Greatest Hazard While instances where wake turbulence caused structural damage have been rare, the greatest hazard is induced roll and yaw. This is especially dangerous during takeoff and landing when there is little altitude for recovery. Short wing span aircraft are most susceptible to wake turbulence. The wake turbulenceinduced roll rates can be extreme. Countering roll rates may be difficult or impossible even in a high performance aircraft with excellent roll control authority. Parallel or crossing Runways - Stay Heads Up for the Wake During takeoff and landing, the vortices sink toward the ground and move laterally away from the runway when the wind is calm. A 3 to 5 knot crosswind will tend to keep the upwind vortex in the runway area and may cause the downwind vortex to drift toward another runway. At altitude, vortices sink at a rate of 300 to 500 feet per minute and stabilize about 500 to 900 feet below the flight level of the generating aircraft. Helicopter Wake Helicopters also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds (20-50 knots). Some mid-size or

5 executive class helicopters produce wake as strong as that of heavier helicopters. This is because two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades. Stay On or Above Leader's Glide Path Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach. Use extreme caution to intercept final above or well behind the heavier aircraft. When a visual approach is issued and accepted to visually follow a preceding aircraft, the pilot is required to establish a safe landing interval behind the aircraft s/he was instructed to follow. The pilot is responsible for wake turbulence separation. Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft. (Keeping the preceding aircraft stationary in the over-run prior to it passing the threshold will ensure you are above its glide path.) Warning Signs Any uncommanded aircraft movements (i.e., wing rocking) may be caused by wake. This is why maintaining situational awareness is so critical. Ordinary turbulence is not unusual, particularly in the approach phase. A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter. The onset of wake can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake vortices. Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is a MUST! How to Avoid Wake Turbulence 1. Takeoff If you think wake turbulence from the preceding aircraft may be a factor, wait at least 2 or 3 minutes before taking off. (See AIM para b & c). Before taking the runway, tell the tower that you want to wait. Plan your takeoff to liftoff before the rotation point of the preceding aircraft.

6 2. Climb If you can, climb above the preceding aircraft's flight path. If you can't out climb it, deviate slightly upwind, and climb parallel to the preceding aircraft's course. Avoid headings that cause you to cross behind and below the preceding aircraft. 3. Crossing If you must cross behind the preceding aircraft, try to cross above its flight path or (terrain permitting) at least 1,000 feet below. 4. Following Stay either on or above the preceding aircraft's flight path, upwind, or at least 1,000 feet below. 5. Approach Maintain a position on or above the preceding aircraft's flight path with adequate lateral separation. 6. Landing Ensure that your touchdown point is beyond the preceding aircraft's touchdown point, or land well before a departing aircraft's rotation point. 7. Crossing Approaches When landing behind another aircraft on crossing approaches, cross above the other aircraft's flight path. 8. Crosswinds Remember crosswinds may affect the position of the vortices. Adjust takeoff and landing points accordingly. 9. Helicopters Helicopter wake vortices may be of significantly greater strength than fixed wing aircraft of the same weight. Avoid flying beneath the flight paths of helicopters. BUT IF YOU FIND YOURSELF IN WAKE TURBULENCE: POWER PUSH ROLL GO AROUND POWER Whenever you are low and slow, add the power, you ll need it. PUSH Unload the wings or push on the yoke until you are slightly light in the seat. This reduces the angle attack of the wings which gives you better roll control with the ailerons, reduces the drag on the aircraft for better acceleration, and if you are rolling over, slows your decent towards the ground. ROLL We often get asked, which way do I roll, with or against the roll? That is a tough one to answer and is why pilots get all that extra pilot pay! Of course, if you have the choice, you d always like to roll (unloaded) to the nearest horizon. If there isn t a nearest horizon, or if you have rolling momentum, continue to roll (unloaded) in that direction to the horizon. GO AROUND Never try to salvage a landing after a traumatic event like that. Take it around the pattern, wind your watch and take a deep breath and get back to the task at hand landing the aircraft. Once safely in the hangar, then think about what happened, how you could prevent it in the future, and let other folks know what happened so they can also learn from the experience. Tem cuidado com este fenómeno e nunca te deixes influenciar por um controlador mais nervoso. Mais vale dizer Unnable to comply due to wake turbulence danger do que experimentar a realidade deste fenómeno. SISTEMA DE VÁCUO Quando um piloto para sobreviver depende dum objecto destes é preciso pensar duas vezes e duplicar os cuidados.

7 Foto obtida no Aero Clube de Torres Vedras O objecto da fotografia é uma bomba de vácuo que alimenta os instrumentos giroscópicos, permitindo uma orientação espacial ao piloto. O que nos diz a Air Safety Foundation sobre o tema? While accidents due to pneumatic system failures are rare, they are almost always fatal. Pneumatic systems, commonly known as vacuum or pressure systems, power the heading and attitude indicators in most general aviation (GA) aircraft, and in some aircraft, also power the autopilot and de-ice systems. For pilots who regularly fly at night or in instrument meteorological conditions (IMC) these systems are essential. This ASF Safety Brief explains how the pneumatic system works, how to recognize a system failure, and system redundancy options. Basic Operation Pneumatic systems in GA aircraft are pretty straightforward. The heart of these systems is a pressure or vacuum-creating engine driven air pump. The air pump draws air into the system through a filter. The fast-moving stream of air passes over the vanes within the heading and attitude indicator gyros, causing the gyroscopes to rotate at about 10,000 RPM. In many aircraft, the same air pump powers the autopilot and de-ice systems. There are two basic types of air pumps: wet and dry. Wet air pumps use engine oil to lubricate the inside of the pump. The more common dry air pumps have graphite vanes inside the casing which self-lubricate as they rotate. Early Recognition of System Failure Recognizing a pneumatic system failure early is important during any operation, but when flying IMC or night VFR it could be the difference between life and death. To accurately and quickly recognize a pneumatic system failure, you must first understand which flight instruments are pneumatically powered. In most aircraft, these would be the heading and attitude indicators, although in some newer aircraft these flight instruments are electrically powered. Check the aircraft s pilot operating handbook (POH) for specifics.

8 The heading and attitude indicators in many GA aircraft are powered by the pneumatic system. If the autopilot is also powered by the pneumatic system, the consequences of a system failure are magnified; just when the autopilot is needed the most, it s no longer reliable. Signs of Failure Early recognition of pneumatic system failure is complicated because the first warning signs can be subtle. Vacuum or pressure powered flight instruments will slowly begin to give conflicting and inaccurate information, so proficiency in instrument scanning is vital. It s important to include the suction or pressure gauge as part of your scan pattern, because a low reading will often signal a failure before the gyros start giving inaccurate indications. Pilots should consider installing easily visible annunciator warning lights, inoperative flags on the gyros, or flow indicators for early warning of a pneumatic systems failure. Early recognition of a pneumatic system failure can significantly decrease the chances of spatial disorientation. Annunciators and flags provide an early indication of a pneumatic system failure. While pneumatic system failures alone do not cause accidents, spatial disorientation does, and tragically these accidents are almost always fatal. (See figure below.)

9 To help avoid spatial disorientation: Install a backup power supply to the pneumatic system (see the Redundancy section below) Keep the suction gauge in your instrument scan Become and stay proficient at partial panel flying Cover up inoperative instruments during a failure Make timed turns instead of using the heading indicator Notify ATC of the situation and declare an emergency If in IMC, consider flying toward the closest VMC Check the weather at the nearest airport with a precision instrument approach Ask ATC for a no gyro approach Pneumatic system failures can occur at any time, regardless of the age of the system. Causes include: Contamination by solid particles from within the pneumatic system that can damage the pump and plug valve openings. Liquid contamination from oil, water, or engine cleaning solvents. A loose fitting or damaged hose allowing contaminants into the system past the filter. Worn out, misused, or incorrectly routed hoses. Abrupt engine deceleration (which can be caused by the propeller hitting water or tall grass). Sudden engine stoppage, such as that caused by a prop strike against a solid object. Whether you re an aircraft owner, renter, or operator defense against pneumatic system failure begins with a review of the maintenance logs and a talk with the mechanic who most recently worked on the aircraft. Study and adhere to the aircraft and component part manufacturer s recommendations regarding inspection and replacement intervals of pneumatic system component parts. Redundancy Redundancy in a pneumatic system can take a load of worry off your plate. While many newer aircraft come with redundant systems, older aircraft usually do not. Pilots who frequently fly in IMC or night VMC should install pneumatic system redundancy. Redundancy comes in several forms. Options include: Electrically-powered backup attitude and heading indicators Air pump redundancy with an electric or engine driven pump Standby vacuum system that utilizes the pressure differential from the engine s intake manifold Points to Remember Here are the key points to remember about pneumatic system failures: Pneumatic systems fail. Expect it and be prepared. You can lessen the likelihood of a failure by making sure the pneumatic system has been properly maintained. Consider installing a backup system and a prominently placed annunciator. Stay current on instrument scanning techniques and partial panel flying. With these points in mind, you can feel more at ease the next time you need to rely on your pneumatic powered flight instruments and systems. Vejamos agora a análise de um acidente que vitimou um companheiro americano já com bastante experiência acumulada.

10 WHEN THE VACUUM SYSTEM FAILS, PROFICIENCY PREVAILS Accidents resulting from vacuum pump failures are rare. Unfortunately, vacuum failures can be hard to detect, which can lead to spatial disorientation, unusual attitudes, and death. On November 25, 2003, the pilot of a Beechcraft Bonanza and his three passengers were killed when the Bonanza broke up in flight near Warren, Oregon, after a vacuum failure and subsequent loss of control. The IFR cross-country flight had departed Arlington, Washington, with a destination of Medford, Oregon. The pilot got a weather briefing two hours before the flight, which included advisories for mountain obscuration, occasional moderate turbulence and moderate rime or mixed icing in the clouds. Ceilings in the area were forecast to be broken at 2,000 feet and overcast at 4,000 feet with tops at 20,000 feet. The flight departed Arlington at about 5:35 a.m. Pacific time. At 6:35 a.m., level at 11,000 feet, the pilot contacted Seattle Center and asked for a higher altitude. He was cleared to 13,000 feet. At 6:39 a.m., the flight was level at 13,000 and the pilot reported that he was clear of rime ice. Some time later, the pilot radioed, "Ah, we just lost our suction gauge." The controller responded "climb and maintain 15,000? You requesting a higher altitude? Is that what you said?" The pilot replied, "Mayday, mayday, mayday." There were no further transmissions from the pilot. Between 6:50 and 6:52 a.m., the Bonanza made numerous turns to the right and left. At 6:51 a.m., the plane descended from 13,100 feet to 10,700 feet in 24 seconds -- a descent rate of 6,000 fpm. The descent steepened to more than 18,000 fpm, and thirteen seconds later, the Bonanza was at 6,800 feet. The last radar return showed the plane at 6,400 feet. Wreckage of the Bonanza was located on the east side of Scappoose Bay, 3 nautical miles northeast of the Scappoose Industrial Airpark. Aircraft records show that a vacuum pump, overhauled on April 22, 1992, was installed on November 21, The Bonanza had been flown hours since the pump's installation. Scoring consistent with an overstress fracture at the coupling's designed shear point was found during disassembly, and representatives from the company that overhauled the unit noted that "it appeared that sometime in the recent past the pump's rotor, vanes, and coupling were replaced by an unknown party with parts from an unknown source." The pilot was an instrument-rated commercial pilot in both single and multiengine land airplanes. He also held an aircraft airframe and powerplant certificate. He had accumulated 3,263 hours of flight time, with 80 hours in the Bonanza. The NTSB determined the probable cause of this accident was the failure of the vacuum pump, and the pilot's subsequent failure to maintain control of the airplane. Early recognition of a vacuum pump failure is complicated because the first warning signs can be subtle. Vacuum- or pressure-powered flight instruments will slowly begin to give conflicting and inaccurate information. Staying proficient with partial panel operation will help mitigate the problems experienced when your vacuum pump fails. Meu caro, como se diz no artigo não é um assunto fácil de detectar à primeira. Em voo faz um varrimento frequente dos instrumentos inclusive o manovacuómetro e mantém-te treinado nas técnicas de voo com painel parcial. 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: robin.andrade@aopa.pt. Gostaria de contar com a tua presença na nossa AOPA.

11 Como sempre, um abração do Fernando