Quick the virgin of the vapours,
Breathed a fog upon the waters,
Made it settle on the warship
Of the heroes of the Northland,
Held the minstrel, Väinämöinen,
Anchored in the fog and darkness.*
In my previous blogs, I covered the topics of instrument flying, locating an airport with radio waves, the autopilot and airport equipment. All these play a role when we make a landing at an airport in foggy weather. This is how we do it.
If we are flying to a destination airport commonly affected by fog, this is taken into account during flight planning. Generally, landing in thick fog, namely a CAT III procedure, slightly increases the time it takes for landing aircraft to clear the runway, so the full capacity of the airport in terms of traffic numbers is not available. Often this means waiting on the ground at the airport or in the air in a holding pattern close to the destination airport.
For this, “extra” fuel is carried along just in case. During the flight we monitor the development of the weather. Before descending from cruising level, information on the approach and the landing runway is loaded into the flight management systems. In addition, we go through the details and performance of the approach procedure.
Before starting the approach, we check that the aircraft and airport equipment are functioning perfectly and make sure that the airport is applying low visibility procedures and that the visibility is at least 75 metres. Seats are adjusted to ensure that the visual segment from the cockpit is optimal. With Finnair, when the RVR (runway visual range) is below 400 metres, the captain always does the flying while the first officer, on the captain’s right, focuses on monitoring the flight instruments.
On reaching the initial approach altitude, the aircraft’s speed is already low enough to ensure that we can gradually deploy the landing configuration (leading and trailing edge devices that improve lift). Generally through instructions given by the radar controller, sometimes by navigating independently, we reach the Instrument Landing System (ILS) localiser. The checklist for landing has already been performed and both autopilots can be armed by the captain for the approach.
The autopilot captures the localiser and turns onto the final track. A little later the glide slope is captured. Often the landing gear will be deployed at this point. Landing lights are often kept off in low visibility, because they are dazzling, rather like a car’s full headlights in fog.
By viewing the distance measuring equipment 10 km away from the runway, or with the aid of a radio beacon, we check at around 500 metres height that the glide slope is correct.
The first officer checks that the final approach checklist is ready, and the captain asks the cabin crew to be seated with seat belts fastened for landing.
At 300 metres height at the latest the aircraft should be stabilised: in addition to the flight path, the vertical speed, thrust setting and aircraft configuration must be such that the autopilot can remain on the ILS beam. In the event of a malfunction after this altitude, the approach will be abandoned for a new approach attempt. Let’s continue this Airbus automatic landing, however.
At around 100 metres height we check that the autopilot’s internal logic has detected no faults and that it has advanced to LAND mode. At 30 metres height – if all the preconditions still hold – the captain decides to land by stating: “Landing”. The runway is only a couple of hundred metres away – the captain gazes outwards, but the runway may not yet be visible.
The first officer continuously monitors the automatic systems and announces “FLARE” when the autopilot shifts from LAND mode to FLARE mode. The aircraft’s audio system fetches information from the radioaltimeter and reads aloud the radioaltimeter values. Without glancing at the altimeter, the pilots receive the distance of the main gear from the surface of the runway to within an accuracy of metres.
At around six metres height the audio system says “RETARD”, which urges the captain to pull back the thrust levers, thus ensuring that the engines are idling.
The aircraft typically touches the runway surface at a speed of around 240 km/h. The captain obtains the first visual signs of the runway at the same time as he engages the engine jet blast, namely thrust reversers, to slow the landing roll-out. The sound of the engines changes slightly at this point.
The first officer monitors the deployment of the spoilers, checks the thrust reversers and the operation of automatic braking: “SPOILERS, REVERSE GREEN, DECEL”.
The captain’s eyes are on the centreline of the runway, whose individual lights disappear under the radome one after another. When the first officer states that the speed has slowed to around 150 km/h, the captain disengages the thrust reversers and the engines are idling once again.
With a light touch of a pedal, the captain switches from automatic braking to manual. Moreover, with a press of a button on the left-hand control stick, the “AUTOPILOT COMES OFF”. The captain’s left hand moves to the tiller situated next to the control stick, which is used to turn the nose gear.
The green and yellow centreline lights guide the aircraft off the runway. The first officer reports to the tower that the aircraft has cleared the active runway. A routine landing has taken place. In fact it is only now that the most demanding stage of the flight begins: taxiing in thick fog following the airport signs to the aircraft parking place.
For the take-off back to Helsinki, visibility of at least 125 metres is required. The higher visibility requirement is due to the fact that take-off is always performed manually and, in the event of an engine failure before a certain decision speed, there must be enough visibility to allow the captain to abort the take-off on the remaining length of runway. But perhaps we’ll leave that subject for another time…
With his sword he smote the billows,
From his magic blade flowed honey;
Quick the vapour breaks, and rises,
Leaves the waters clear for rowing;
Far extend the sky and waters,
Large the ring of the horizon.*
Wishing you a clear and bright autumn,
Jussi Ekman
*Extracts from the Kalevala, Finland’s national epic. Translation by John Martin Crawford (1888).
Filed under: Safety | Tagged: autopilot, fog, landing, Safety
