Runway: the Finnair blog

One of the most basic items on the lesson list for all trainee pilots is the use of Flight instruments. When you’re in the clouds, you can lose track of your own – and your aircraft’s – attitude in a matter of seconds if you only trust your instincts.

Instrument flying is based on continuous practice and on the unshakeable belief that the instruments are showing the real figures, not ones that have been guessed at.

The various flight instruments show speed, artificial horizon, altitude, turn and slip, and compass and vertical speed data. In addition, of course, the cockpit contains engine instruments, as well as displays for the control of navigation and systems and all kinds of other apparatus.

In the most modern passenger aircraft, digital technology and displays have made it possible to combine many of these different functions, but the basic principles of instrument flying are the same as ever. It’s based on monitoring a variety of information and using it to keep desired attitude and navigate, which is why pilots require the ability to multi-task and put many skills into practice at the same time.

To mitigate for the unlikely event of instrument failure or error, several instruments of the same kind are required as well as a system that gathers data from different sources. The modern-day pilot also most likely receives automatic warnings if an instrument is faulty.

There are three separate and independent instrument systems on the latest passenger aircraft: one for the captain, one for the first officer, and one ‘emergency’ system.

So modern flying continues to be a combination of automation and human piloting skills, in which the instrument functions are confirmed three times over and continuously monitored by two pilots, not just one. Another example of how safety is ensured to the maximum possible degree.

Safe flying is based on the good training of the pilots and the technical condition of the aircraft. But the airport’s equipment is also vital.

Aircraft fly continuously in different weather conditions, which means that it is important to obtain continuously updated information on what the weather is like at the destination airport: wind, visibility, cloud base, precipitation, dew point, air pressure. In low visibility, an RVR device (Runway Visual Range) measures visibility to an accuracy of metres.

The most important instrument for approach and automatic landing is the ILS (Instrument Landing System).

A localiser antenna, which transmits radio signals to the right and left of the centreline of the runway, is located on the threshold at the opposite end of the runway. These signals ‘draw’ an extension of the runway into the sky.

At the side of the runway, at a distance of around 300 metres from the threshold, there is a glideslope antenna array, which like the localiser ‘draws’ the correct approach angle for the runway. This angle is rather gentle – only around three degrees.

In addition to the ILS, on the final track there is a radio beacon or distance display by which the aircraft can check that it is approaching at the correct altitude. In addition to navigation equipment, the runway also has painted markings, signboards and strong lights.

Lights on the approach line, the edge of the runway, the centreline and the touchdown zone help the pilot observe the aircraft’s true location when landing and also help the pilot exit the runway.

In addition to all this equipment, airports are also required to employ low visibility procedures in visibility of less than 400 metres. Such procedures ensure that the equipment will function in the event of incidents, i.e. for example that a back-up energy source is available and that the movement of aircraft and ground equipment is monitored and controlled.

Because the air-traffic controller may not be able to see out from the air-traffic control tower in bad weather, traffic movements in the airport area are best tracked using surface movement radar. Many air-traffic control and other airport employees therefore require training in low visibility procedures. In the next few months, after a few minor improvements, Helsinki-Vantaa will be equipped for automatic landings where a couple of hundred metres of visibility is sufficient for landing.

The aircraft, on the other hand, must have an ILS receiver and, for automatic landings, an automatic fail operational flight control system, which means two separate autopilots that can be switched on simultaneously, so that in the event of one malfunctioning the other can independently handle the entire approach and landing process without the intervention of the pilot.

Moreover, air speed control should be switched on when an automatic landing is being performed. For the autopilot to be able to perform an automatic landing, the aircraft must be in the correct orientation at the correct time, which means that the aircraft must have two sensitive pressure altimeters, which, with the aid of radio signals, calculate the aircraft’s true altitude from the ground surface with an accuracy down to a few ten of centimetres. In bad weather, automatic braking can also be used after landing.

In order to achieve the perfect automatic landing, the aircraft must be able after landing to remain on the centreline of the runway. This is done with the aid of the ILS localiser, but using to some extent different controls than in the air.

An aircraft can therefore land automatically, but this rather complex technical achievement is continuously monitored by the pilots.

Jussi Ekman