HOW WE FLY

Flying blind is easy. Fog on the ground is another matter, says PHIL HEARD Phil Heard認為，在伸手不見五指的濃霧中飛行並非難事，但降落卻是另一回事

PHIL HEARD explains how planes land safely in thick fog

NOT BEING ABLE

to see where you’re flying is – believe it or not – perfectly safe. Aircraft have radars to ‘see’, while movements are digitally monitored by air traffic controllers to keep aircraft at a safe height and distance from each other. But that’s at altitude.

‘On the ground it’s less straightforward,’ says Captain James Toye, Line Operations Manager at Cathay Pacific and Boeing 777 pilot. ‘A lot of the ground control from the tower is visual – and it’s hard to direct aircraft taxiing around if you can’t see them.’

Fog slows everything down, including the arrival (or ‘flow’) rate of aircraft. ‘At Hong Kong on a clear day – or night – we can turn off the runway at 50 knots (90 kilometres an hour),’ adds Toye.

‘In low visibility it’s not just about leaving the runway quickly, it’s also about finding the turns on the taxiways. I’ve been in conditions where you can only see a few lights ahead of you, and these are spaced at 20-30 metre intervals. Looking for one reference point can be disorientating, and it has to be done slowly. We normally taxi at up to 30 knots (55 kilometres an hour), in low visibility that’s reduced to 10 knots or slower (20 kilometres an hour).’

Company policy dictates that low visibility landings must use the aircraft’s automated systems that interact with the airport’s instrument landing system. This is the array of metal poles at the end of a runway that generate a radio beam for aircraft to follow. The airport will also transmit readings from transmissometers, which measure light every 15 seconds, to help pilots decide if they have enough visual references (ie, they can see the runway and runway lights) to land.

While most airports are equipped to deal with fog safely – some are more efficient than others. Taipei’s Taoyuan International Airport has dedicated lowvisibility taxiways with fewer turnings. But even for an airport like London Heathrow, which is well used to fog, the volume of landings means it doesn’t take long for a reduced arrival rate to cause disruption.

Hong Kong International Airport is not often affected by fog, but can witness thick, localised patches causing flights to be diverted to Macao, 40 kilometres away. But it could be worse. ‘ There’s no low visibility support at Adelaide, for instance,’ says Toye. ‘Get fog there, and you could be diverting to Melbourne…’

信不信由你，在什麼也看不到的情況下飛行，是一件十分安全的事。飛機裝置了雷達，可以「視」物，而飛機飛行的情況，則由航空交通控制員以數碼方式監控，令飛機維持在安全的高度，並且與其他飛機保持安全的距離。不過這一切都是在空中時的情況。

國泰航空航線運作經理兼空中巴士波音777機長James Toye說：「不過在地面就不是這麼簡單了。在控制塔內進行的地面控制都要靠視覺來進行，如果你看不見飛機，就很難指示它們如何滑行。」

濃霧出現，就會將一切事情的步伐拖慢，括影響飛機降落的效率（或稱為「流量」）。Toye補充道：「在香港，若是天朗氣清的白天或晚上，飛機可以50節（時速90公里）的速度在降落後迅速離開跑道。」

「在能見度低的時候，我們關心的並非如何迅速離開跑道，而是找出轉往滑行跑道的位置。我曾經遇過能見度低到只可見到前面數盞燈的情況，而每盞燈的距離是20至30米。尋找參考點時很容易迷失方向，因此必需慢慢進行。我通常以最多達30節（時速55公里）的速度滑行，在能見度低時就會減至10節（時速20公里）或更慢。」

公司明文規定，飛機於能見度低時降落，必須使用可與機場的儀表著陸系統互相交流的自動系統操作。儀表著陸系統是安裝於跑道盡頭的一組金屬管，可發出無線電訊號讓飛機跟隨。另一方面，跑道視程儀每15秒就會測量光線的強度一次，機場會將這些數據傳送出去，讓飛機師獲得充足的視覺參考資料，例如是否見到跑道及跑道上的燈光，以便進行降落。

雖然大部分機場均配備了應付濃霧的安全設施，但有些設施的功效比其他設施更勝一籌。台北桃園國際機場設有低能見度的專用滑行跑道，它們的轉彎位比較少。不過即使如倫敦希斯路機場，已經對濃霧習以為常，但是由於飛機降落的數量非常大，一旦碰上大霧的情況，飛機降落的效率很快就會下跌，對機場運作造成干擾。

香港國際機場並非經常會受濃霧影響，但有時會出現局部地區性的濃厚雲層，飛機就要轉飛40公里外的澳門。不過情況可以更壞， Toye說：「例如阿德萊德就沒有應付低能見度的設施，如果出現大霧，就可能要轉飛墨爾本。」 DISCOVER MORE Visit cathaypacific.com/discovery for more on the airline’s operations

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