Wednesday, December 3, 2014

Fundamentals of airliner performance, Part 5; Approach and landing

By Bjorn Fehrm

The time has now come to cover descent and landing in our articles around airliner performance. As many aspects of descent are similar to climb we will repeat a bit what we learned in Part 4:

  • For high speed operation the pilots fly on Mach as this gives him maximum information around possible effects on the aircraft when he is close to the high speed limit, the maximum Mach number. Beyond this the aircraft gets into supersonic effects like high speed buffeting or unsteady flight.
  • For operations under the cross over altitude for Mach 0.78 to 300 kts IAS the pilot flies on Indicated Air Speed (IAS) which gives him maximum information how the aircraft reacts should he go close to the aircraft’s lower speed limits.

Lets now start to go through the steps that our 737 MAX 8 performs after leaving its cruise altitude.

Descent from cruise altitude

The descent rate like the climb rate depends on the amount of excess thrust over the aircraft’s drag, only in descent we don’t talk about positive excess but negative. This means the pilot has to reduce thrust under the drag of the aircraft to make a descent at constant Mach or lower down at a certain IAS. The normal engine setting for descent from cruise is flight idle which gives a very low thrust from the engines.

When a descent from cruise can be started is very much dictated by requirements from Air Traffic Control (ATC) and this clearance was often given so close to the destination airport that speed brakes (normally the part way deployment of the wing spoilers) had to be used for keeping the Mach and later IAS to the desired level. Use of speed brakes is a waste of hard earned potential energy and therefore the industry is today trying to maximize economy and total fuel consumption by a descent that can be started so early that a continuous optimal speed descent can be flown all the way to the final approach for landing. This maximizes the potential energy that the aircraft gained when climbing to cruise altitude. The target for modern ATC is therefore to have flexible enough descent and approach procedures to let that happen.

Speed wise the descent is very much the inverse of the climb. Initial descent is normally at cruise Mach, 0.78 in our case. This is kept until the altitude where M 0.78 is meeting the gradually increasing IAS of 300 kts (see part 4 for an explanation of this cross over between Mach and IAS). The 300 kts IAS is then kept until entry into the Terminal Maneuvering Area (TMA) where we are asked to keep 250 kts until final approach is commenced. The 250 kts is once again to keep aircraft from overtaking each other during approach and landing. One of the requirements on ACT is that they arrange for proper separation of aircraft during the approach and landing procedure. The separation varies depending on local air traffic rules and quality of radar coverage but is normally 3 nm close to the airport but can be up to 10 nm if the radar coverage has low resolution.

Should the weather conditions allow, the ATC can ask the pilot if he would accept making the final approach under Visual Flying Rules (VFR). If the pilot OK this, the separation responsibility passes to the pilot and separation can normally be reduced or a more efficient final landing procedure can be used. The normal procedure is to route the aircraft to a point 10nm straight out from the landing runway, where the aircraft have an altitude of 2000ft over the runway. From this point a standard ILS glidepath is flown, either under Instrument Flight Rules (IFR) or VFR rules. An ILS glidepath has an angle of 3° (5.2%) and is normally followed also for straight in visual approaches as one can then check the aircraft’s vertical position by glancing on the IFR glideslope indicator.

Final approach and landing

The most important thing when landing is to get the speed low enough so that aircraft can be brought to a stop on the runway available (which might be wet or even slippery of snow) but also in order to give the pilot more time to react and correct any miss alignments there might be during his landing procedure. Normal speeds on the last 10 miles for modern airliners are 120-150 kts, our 737 MAX 8 would be close to the predecessors 142 kts.

To get our 737 MAX 8 to fly at the desired speed of 143 kts at maximum landing weight of 69 tonnes or 153,000 lb we need to create lift from the wing of the same magnitude. If we divide the lift of 153,000lb with the wing area we get the lift per unit wing area at 535kg/m2 or 360 lb/ft2. An even more used measurement is the lift coefficient, Cl, which also takes the air density and aircraft speed into account. Using this universal measurement we need a Cl of 1.7 to fly the approach at 143 kts at our maximum landing weight. Figure 1 shows how we achieve this lift at the low speed, it shows the high lift devices which are used during landing.

Double slotted flap, slat and spoiler

Figure 1. Typical landing configuration for a modern airliner with slats and flaps deployed. In this case also the spoiler is deployed to slow down the aricraft. Sourc: Leeham Co.

On the leading edge there is a guiding vane called a slat. It serves to increase the canting angle the wing can use relative to the approaching air, the so called angle of attack or Alfa angle. Figure 2 has plotted the lift coefficient against the Alfa angle for a wing without and with slat on the first curve from the bottom. This figure is a generic diagram and not specially drawn for our 737 MAX but serves to show what we want to explain.

Cl alfa curve landing

Figure 2. Landing configuration diagram showing lift force coefficient, Cl versus angle of attack for the wing. Source: NASA

With a clean wing the aircraft has zero lift when the wing has no angle of attack (Alfa is zero), it then increases linearly until 13° angle where the wing goes into stall and the lift force reduces with increased nose up attitude of the aircraft. The reason for the wing stalling is that the air stream can not follow the leading edge, it curves to abruptly. If a guide vane in form of a slat is deployed the wing works all the way up to 20° angle of attack.

On the other end of the wing flaps are deployed (Figure 1). They serve two purposes:

  • The increase the wings lift capability by curving our air downwash stronger downwards, thereby giving us the needed lift at 143 kts.
  • To lower the aircraft’s attitude. As can be seen in the diagram we could have reached Cl 1.7 (the violet line) by almost slats only (c) or by a simpler single slotted flap (b) which would be lighter and easier to maintain then our double slotted flap (a). But this lift would have been reached at a very high Alfa angle and thereby nose up attitude. The pilot would have had difficulties seeing the runway and the tail would risk hitting the runway when touching down.

Assuming that 5° angle of attack is the max which would be acceptable given these reasons we can see that a single slotted flap system would not work, our needed 1.7 Cl would be reached at 8° Alfa (b). Although the diagram does not fully describe the situation for our 737 MAX 8 it shows that we need a double slotted flap to reach Cl 1.7 below 5° Alfa angle. In practice our MAX 8 has a double slotted flap on the inner part and a single slotted on the other part of the wing. With this combination it reaches the necessary lift at an acceptable Alfa angle.

Drag during landing

It is important that the final glideslope of 3° angle can be flown with enough engine thrust so that there is margin to slow the down the aircraft or descend faster should this be needed. This can only be done by reducing thrust further or deploying the spoilers partly to act as speed brakes (Figure 1). The speed brakes increase the drag, drag is therefore not a negative during landing, there are aircraft types (Fokker F28 and Avro RJ / BA146) which are landed with deployed air brakes for this reason.

Our landing configuration is quite draggy so we do not need the air brakes, the excess thrust over flight idle is there to allow the pilot to fly the final approach and landing with enough thrust margin to give him a finer adjustment capability then the air brakes could give him. Higher drag is a positive for fine adjustment on the glideslope but also a positive from another aspect, the go around procedure. Should one not see the landing lights at the minimum height (often around 200 ft for ILS approach) an abort of the landing needs to be done (an abort is also the prescribed action for many other types of disturbances during landing). When performing such an abort, or Go Around as it is also called, the engines needs to responds fast with max continuous thrust. This response time is helped if they are not spooled down to a very low thrust during approach.

A landing configuration is therefore quite draggy, this is achieved with flaps extended to the landing configurations (full or close to full angle) and below 10nm with the landing gear extended. At go around full continuous thrust is selected and gear up is commanded, thereby we remove drag and get more thrust quickly, we get a positive acceleration of the aircraft and can gain speed and height. Soon after gaining speed and height flaps and slats are reduced to takeoff settings and then retracted fully like described in our climb clinic in part 4.

Touchdown

Just before passing the end of the runway the pilot reduces the speed to the so called reference speed Vref, for the 737 MAX about 5 kts lower then our final approach speed. Vref is defined as a speed which is 30% higher then the stall speed in the landing configuration and works as the guiding speed for all other speeds during the landing. Our final approach speed is therefore often given as Vref + XX kts (5 in our case) in aircraft manuals.

Once passed the end of the runway, or threshold as it is also called, the pilots starts the flare where he gradually raises the nose while reducing thrust to touch down with minimum speed. When both main landing gears are loaded the spoilers deploy in full to take away any remaining lift (they are also called lift-dumpers), this in order for the aircraft to stay safely down with good weight on its wheels. After having reduced the speed to taxing speed we turn off the runway and taxi to our gate.

Summary

We have described the descent from cruise and our approach and landing of our airliner. The descent is very much an inverse of the climb with the same speeds being used for the same reasons.

For the landing we use an extensive high lift configuration to get the landing speed down, this is quite draggy but we don’t mind. We need a certain amount of drag to give the pilot margin to regulate his glidepath by changing his thrust but also to reduce spool-up time for the engines should he need to execute a go-around.

We have now covered all parts of a normal flight, in the next part we will start looking at some of these phases a bit more in detail.

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