Gelo nos motores

INTRODUO: Este trabalho tem como objetivo proporcionar um "refreshment" sobre a operao do sistema de "Engine TAI (Thermo Anti Ice)", tirar certas dvidas e prover aos leitores, informaes decisivas sobre um macete freqentemente utilizado para o acionamento do sistema e suas possveis conseqncias. Para isso segue-se um texto sumrio em tpicos, correspondncia eletrnica trocada com entidades aeronuticas internacionais e transcries de passagens importantes selecionadas dos manuais e publicaes do 737.

SUMRIO DA PROBLEMTICA: O exato processo de formao de gelo nos motores  reao, apesar de muito comum,  algo complexo do ponto de vista explicativo, envolvendo uma srie de fatores contribuintes e grandezas fsicas cuja combinao acarretar desde a completa inexistncia de formao at os acmulos perigosos. Entretanto, podemos considerar que a presena de gua em um motor desprotegido, em temperatura propcia ao congelamento, define, basicamente, a formao de gelo.

COMO SE PROTEGER: Salvo sob condies meteorolgicas extremas de engelamento severo quando manobras evasivas e procedimentos especiais sero requeridos, a soluo  simples e sempre a mesma: Ativao do "Engine Anti Ice" antes de entrar na regio de formao.

PROTEGER-SE EXATAMENTE DO QUE? O gelo, uma vez aderido ao motor, poder ocasionar, entre outras coisas: - Leituras imprecisas de parmetros, especialmente EPR.
- Vibraes
- "Engine Stall"
- Aumento excessivo de EGT
- FOD ("Foreign Object Damage"). Esse ,em grande ou nfima escala, sempre se suceder ao no acionamento dos "Engine TAI" no momento apropriado. Caso os "Engine TAI" sejam acionados tardiamente, o gelo acumulado, ao desprender-se, ser ingerido pelo motor, acarretando FOD. Se os Engine TAI no forem acionados em momento algum, o gelo acumulado desprender-se- durante a descida, ao atingir-se temperaturas mais altas, possivelmente at muitas milhas depois de j encontrar-se em condies visuais. Isso  particularmente perigoso pois a combinao de gelo desprendido e sendo ingerido pelo motor em "IDLE", com "Ignitions OFF" pode culminar em um "Flame Out" inesperado, voando VMC.

QUANDO SE PROTEGER: Os manuais fornecem um envelope muito simples e definitivamente seguro para a operao do "Engine Anti-Ice" transcrito mais adiante. Basicamente a presena de mistura visvel sob qualquer forma, dentro do envelope de temperatura de +10C TAT at -40C SAT definem a necessidade de proteo.

QUANTO SE GASTA POR USAR O ANTI ICE: O uso do "Engine TAI" consome, em mdia 50 Kgh de querosene por motor (dependendo da ACFT). O no uso ou o uso incorreto pode, no mnimo, diminuir gradativamente a vida til do motor, causando prejuzo maior.

O MACETE DO GELO NO PRA-BRISAS Ao contrrio do gelo que possa vir a se formar nas asas, o gelo que se forma nos motores no pode e no deve ser associado  presena de gelo nos pra-brisas ou hastes dos limpadores, com a finalidade de ativao do "Engine TAI". A formao de gelo nos motores comea bem antes da formao de gelo nesses locais. O sistema de "Wing Anti Ice" pode ser utilizado como "De-ice" enquanto o "Engine TAI" no pode, devido as conseqncias da ingesto de pedaos de gelo pelos motores. Os principais motivos para a no utilizao desse "macete" so os seguintes: Os pra-brisas so eletricamente aquecidos, estando os mesmos em uma temperatura maior do que a temperatura externa real. Alm disso a temperatura do ar que entra nos motores  drasticamente reduzida pela acelerao do fluxo nos "Inlets". Aguardar sinais de gelo nesses locais para ativar o "Engine TAI" implica no acionamento tardio do sistema e conseqente FOD.

O QUE DIZ A BOEING SOBRE GELO NO PRA-BRISAS (E-MAIL) "Our studies and operational experience have indicated that use of visual cues, such as ice appearing on the windshield wiper post or around the windshield edges is not a reliable indicator of engine inlet conditions. Using only visual airframe icing cues can result in engine ice accretion and subsequent engine damage due to ice shedding into the engine. In the Operation Manual Supplemental procedures section, we specifically advise not to rely on airframe visual icing cues before activating Engine Anti-Ice." Regards Lawrence A. Van Kirk

PERGUNTAS  PRATT & WHITNEY (E-MAIL) "Below you will find the questions you recently sent to P&W, along with a reply. Please contact me if you have further questions." Best Regards, Dave

Why we shall not rely in the ice that forms in the windshield, wipers and frames as an indication of ice buildup in the engines? "Because it is possible to accumulate ice on the engine nacelle/inlet with no accumulation on the aircraft. This is possible because the air accelerates into the engine nacelle/inlet which reduces the static air temperature. This can result in air temperature below freezing in the inlet but not below freezing around the aircraft."

When flying in temperatures below -40 deg C, with visible moisture, we don't need or we cannot activate Engine Anti Ice? Why? "Engine anti-ice can be activated with SAT less than -40 deg C, but Boeing states that moisture below that temperature will be in the form of ice crystals which do not adhere to the airplane or nacelle."

Why we must activate Engine TAI prior to descent even in temperatures below -40 deg C? "Boeing's philosophy is that anti-ice should be used for all takeoffs, descents and landings, when icing conditions exist. Boeing suggests keeping anti-ice off in climb and cruise, when SAT is less than -40deg C, to save fuel. In descent anti-ice should be on, in icing conditions even with SAT less than -40 deg C, probably in anticipation of increasing outside air temperatures, and to simplify the procedure."

If flying into temperature & moisture envelope without protection for short periods, how long the engine could run without experience any problem? E.G.: Flying through scattered clouds. Even without engine parameters change this situation could shorten the engine life? "Defining how long an engine will operate in icing without anti-ice protection is impossible because the answer depends on so many variables, for example, the intensity of the icing in the clouds, the frequency of the clouds, the flight conditions, and the thrust setting, etc. The engine nacelle, inlet, fan, etc. can be accumulating enough ice to do damage with no apparent change in engine parameters. The damage could be caused when the ice breaks off and enters the engine." David M. Levanto Pratt & Whitney Flight Operations

O QUE DIZ O FCTM "Engine Icing may form when not expected and may occur when there is no evidence of icing on the windshield or others parts of the airplane. Once ice starts to form, accumulation can build very rapidly. Although one bank of clouds may not cause icing, another bank, which is similar, may cause icing. Therefore, the engine anti-icing system should be turned on whenever icing conditions exist or are antecipated". Note: Failure to follow the recommended anti-ice procedures can result in engine stall, over temperature or engine damage." (FCTM 737 3.2)

ALGUMAS PASSAGENS IMPORTANTES DOS MANUAIS BOEING: "For information regarding the mandatory use of the engine anti-icing system in icing conditions, refer to Supplementary Procedures, section 3A-7 Ice and Rain Protection, Engine Anti-Ice Operation." (Vol 3, 23.30.37) "Crews should be alert when operating in areas of possible icing to activate engine anti-icing system prior to entering icing conditions. Late selection of engine anti-icing may allow inlet ice buildup and ice shedding into the engine" (Vol 1, 02.04.19) "Engine anti-ice must be ON during all ground and flight operations when Icing Conditions exist or are antecipated, except during climb and cruise when the temperature is below -40 degrees Centigrade SAT. Engine anti-ice must be ON prior to, and during, descent in all Icing Conditions, including temperatures below -40 degrees Centigrade SAT." (Vol 1, 02.17.01) "During ground operation and takeoff, engine and wing icing conditions exist when the OAT is 10 degrees Centigrade or below and inflight TAT of 10 degrees Centigrade or below and visible moisture in any form is present, such as clouds or fog with visibility of less than one mile, rain, snow, sleet, and ice crystals." (Vol 1, 02.17.01) "WARNING: DO NOT RELY ON AIRFRAME VISUAL CUES TO ACTIVATE ENGINE ANTI-ICE. USE THE TEMPERATURE AND VISIBLE MOISTURE CRITERIA SPECIFIED IN THIS PROCEDURE." (Vol 1, 02.17.01) O que a Boeing quer dizer com "WARNING": "An operating procedure, technique, etc., which may result in personal injury or loss of life if not carefully followed." (Vol 1, 00.01.04) "CAUTION: DO NOT OPERATE ENGINE OR WING ANTI-ICE WHEN THE TOTAL AIR TEMPERATURE (TAT) IS ABOVE 10 C." (Vol 1, 02.17.01) O que a Boeing quer dizer com "CAUTION": "An operating procedure, technique, etc., which may result in damage to equipment if not carefully folowed." (Vol 1, 00.01.04) "Engine TAI must be on when icing conditions exist or are antecipated, except during climb and cruise below -40 C SAT." (Vol 1, 01.10.03)

UMA FRASE DE EFEITO: "When ice is encountered, immediately start working to get out of it... Unless the condition is freezing rain, it rarely requires fast action and certanly never panic action; but it does call for positive action." Capt. Robert Buck - AOPA Freezing Rain: "Supercooled raindrops freezing on impact" 
