GENERAL PRINCIPLES - LANDING Flashcards
(10 cards)
Define Landing
–The landing stage of flight is defined as being that stage of flight commencing from 50ft above
the landing threshold and terminating when the aeroplane comes to a complete stop.
– The 50 ft point is sometimes referred to as the landing screen height. The landing screen height is fixed at 50 ft for all classes of aeroplane unlike the take-off screen height which is 35 ft for class A aeroplanes and 50 ft for class B aeroplanes.
Landing reference speed, known as VREF. Why is it important ?
–VREF for Class A aeroplanes must be no less than ________________________
–VREF for Class A aeroplanes must be no less than the greater of 1.23 times the stall reference speed in the landing configuration (1.23 VSRO) and the velocity of minimum control in the landing configuration (VMCL).
– V REF for all other classes of aeroplane must be no less than 1.3 times the stall speed (1.3 VSO) in the landing configuration.
–VREF is a very important speed to attain since the landing distances in the aeroplane flight manual
are based on aeroplanes flying at VREF. Therefore if a landing aeroplane is not at VREF the landing
distance given by the manual will not be achieved by the pilot.
What is a landing flare ?
This procedure of reducing thrust and increasing pitch is known as the landing flare, although other terms like “roundout” are commonly used. The landing flare will allow the aeroplane to touch down onto the runway using the main wheels first.
Define Landing Distance Required
The combined length of the ‘airborne section’ (from screen height to the point where the main wheels touch the RW )and the ‘ground run or ‘landing roll’ is known as the “landing distance required”. Pilots need to make sure that the landing distance required does not exceed the Landing Distance Available.
LANDING DISTANCE AVAILABLE (LDA)
–The landing distance available is the length of runway from one threshold to another.
–These are not always at the edge of the runway, sometimes there are displaced thresholds which
are some way in from the edge of the paved surface.
–You will recall that the landing distance starts at 50 ft (screen height). This point must be directly above the threshold. Landing on the threshold is not the aim of the landing. So if you are maintaining on a 3 deg glideslope, if you cross the threshold at 50 ft then you will touchdown after 1000 ft from the threshold. You can calculate that from one in sixty rule.
How is lift detrimental during the landing roll ?
–Producing lift will reduce the load placed on wheels and therefore decrease the braking effect. In large commercial aeroplanes once the main wheels have settled on the runway the lift spoilers or lift dumpers are deployed which act very quickly to disrupt the airflow over the wing and destroy lift.
– Normal reaction on the main wheels has to be maximised. This will ensure that Maximum braking effort can be applied quickly without risking wheel skid. Therefore, maximum weight has to be put on the main wheels after touchdown. Remember the graph for increase in Co-efficient of friction as the brakes are applied. F= mu x N
Explain why the reverse thrust cannot be used down to zero speed ?
It is important to note too that jet engine reverse thrust cannot be maintained right up to the
point when the aeroplane comes to a full stop. Reverse thrust must de-activated before the
forward speed reduces to a minimum value. As the aeroplane slows down the redirected
airflow may start to be re-ingested into the compressor. This means that the jet engine recycles
its own gas flow which significantly increases the engine temperatures but it also means that
debris on the runway can be sucked into the engine, potentially causing major damage. As a
result of this danger to the engines at low forward speeds reverse thrust be must deactivated
below about 50 kt.
–A propeller aeroplane can also maintain reverse thrust until the aeroplane comes to a full stop.
This capability gives the propeller aeroplane a greater braking advantage over the jet aeroplane
during landing.
– But some ac can use reverse thrust to reverse taxi. For ex C-17, MD-11, Boeing 727. These ac have engines mounted high up with no threat of debris ingestion.
What is Wheel Drag ?
Wheel drag is the friction force between the wheel and the runway and with the wheel bearings, whereas
brake drag is the friction force between the brake discs and the brake pads. Wheel drag will
come into play as soon as the aeroplane touches down on the runway. However, as friction
is a function of the force pushing two surfaces together, because there is still a lot of lift being
generated, wheel load is small during the initial part of the landing run, and therefore wheel
drag is also small. As speed reduces and as lift is destroyed by the spoilers, the wheel load
increases which in turn increases the wheel drag. Therefore wheel drag increases throughout
the landing roll and will reach a maximum value just before the aeroplane comes to rest.
What is Brake Drag and how does it depend on the Wheel Drag ?
However, brake drag is only effective if there is also sufficient wheel drag or wheel friction between the tyres and the runway. If wheel drag is low, brake drag will also be low. Consequently, the brakes are effective only if there is sufficient friction between the tyres and the runway. During the early part of the landing run when there is not much load on the wheels and therefore not much wheel friction. Brake drag is consequently ineffective in slowing down the aeroplane. However, as the lift reduces and
more weight is placed on the wheels brake drag does become more effective in slowing down
the aeroplane. Therefore, brake drag increases as the landing roll progresses. This concept
explains why pilots need to destroy lift as soon as possible after touchdown so that the braking
action can be at its peak effectiveness early on in the landing.
–Notice that during the early part of the landing roll, aerodynamic drag provides the majority of the drag, whereas once the speed has dropped below 70% of the landing speed, brake drag provides the majority of the drag.
