How pilots minimise noise and carbon emissions on descent
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What goes up must come down, and the coming down portion of the fight is the part that takes the most thought and concentration from pilots. Contrary to popular belief, it’s not just a question of engaging the autopilot and enjoying the ride. Like any computer, the autopilot is only as good as the instructions that we, the pilots, give it.
Flying a descent and approach into airports such as London Heathrow, Los Angeles and the airports in the New York area are all done quite differently. For each approach that we make, we must ensure that we are flying the aircraft in not only the safest way possible but also in the most efficient way possible. Welcome to the world of Continuous Descent Approaches.
What is a Continuous Descent Approach?
In its most simple form, a Continuous Descent Approach, or CDA, is one where we are able to descend the aircraft using minimum engine power from the cruising level, all the way down to the point where we must start configuring the aircraft for landing. However, due to the constraints of other aircraft, both arriving and departing, flying a CDA can often be difficult. In busy sectors, ATC will often give us what is known as a “stepped descent,” clearing us to descend to a lower altitude where we fly level for some time before stepping down again.
Even though they can be difficult to achieve, flying a CDA benefits a number of people, from those living near the airport to those on board the aircraft. It also has wide-ranging benefits for the air quality around the airport and the global climate as a whole. As a result, airlines, airports, ATC and the authorities who design and regulate airspace make a concerted effort to maximise the use of CDAs.
The most efficient way to fly a descent is to bring the engine power back to idle and, in effect, glide the aircraft down. A 2017 study found that if CDAs were optimised at airports around the world, 350,000 tons of fuel could be saved, reducing CO2 emissions by 1 million tons.
When forced to fly a stepped descent, the time when we are flying with the power at idle is reduced. Each time we level off, the engine power must be increased significantly, using more fuel, increasing carbon emissions and also increasing noise for both those on the ground and in the aircraft cabin.
The safety benefits of CDAs
The aim of a well-flown descent and approach is to bring the aircraft to the point 1,000 feet above the ground whilst satisfying a number of criteria. The aircraft should be on the correct vertical profile for the runway, at the correct speed and in the landing configuration that is with the gear down and the flaps in the desired position for landing. This is known as a stabilised approach. If the aircraft does not satisfy any one of these criteria at the 1,000 feet point, the pilots must go around and make another approach.
The aim of the stabilised approach philosophy is to avoid the aircraft landing with an excess of energy and going off the end of the runway, known as a runway excursion. Sadly, still today, runway excursions occur at a rate far higher than they ought to and, in most cases, are ultimately attributed to the aircraft not meeting the criteria of the stable approach.
Related: How pilots avoid runway overruns
Managing the energy of the aircraft is key to achieving a stable approach and flying a CDA helps us manage that energy. When we talk about the energy of an aircraft, we are referring to its speed and vertical position in relation to the ideal target of a 3-degree descent path at our final approach speed.
If we are above this desired profile and, most likely, flying faster than the final approach speed, we are in a high energy situation. If we are below the profile and possibly flying slower than the final approach speed, we are in a low energy situation. Whilst it is marginally better to be in a low energy situation, both scenarios can pose a threat to the safety of the aircraft.
For every 1,000 feet of height we need to lose, we need around three miles along our route, flying at a defined speed. As a result, when in the cruise at 39,000 feet, we will look to start our descent around 120 miles from our destination. This, of course, assumes that we will fly a constant descent along our programmed flight path at that defined speed. The reality of this is normally quite different.
In practice, ATC will often start our descent earlier than we would like or, which makes life more difficult for us, start the descent later.
High Energy Approaches
Starting the descent late immediately puts us into a high energy situation. In order to regain the ideal 3-degree profile, we must increase our rate of descent and there are three ways of doing this. The first is to reduce the engine power to idle, however, as we fly most descents at idle power, chances are we will already be doing this.
The next method is to increase the speed. With the power at idle, the only way the aircraft can increase speed is by lowering the nose and it is this action that increases the rate of descent. The problem with this method is that, particularly when flying into busy airports, ATC has instructed us to maintain a set speed to keep us safely separated from other aircraft. If we suddenly increase our speed, we could get too close to the aircraft in front.
The final method is to use the speedbrakes. These are the large panels on the wing surface that you see extend on landing. In flight, these disrupt the flow of air over the wing, reducing the lift and enabling us to descend at a quicker rate, whilst being able to fly at the same speed. It’s the disruption of airflow over the wing that gives the strong airframe vibration which you are able to feel in the cabin.
If a high energy situation occurs at the start of the descent, it is fairly easy to deal with. If ATC keeps us high during the later stages of the approach, we could find ourselves in a high energy situation close to the ground and this is where countless accidents have occurred. In these situations, we can find ourselves in a race to lose the excess height in order to be able to land, putting us at risk of not being stable at 1,000 feet.
Low Energy Approaches
If ATC starts our descent early, it puts us into a low energy situation. Whilst not as dangerous as higher energy situations, low energy approaches are less efficient as they result in us flying at lower altitudes for longer, increasing fuel burn and, as a result, carbon emissions. As the engine power decreases and increases as we fly these stepped descents, the noise in the cabin, and for those on the ground is also increased.
Where low energy approaches tend to catch pilots out is when they lull them into a false sense of security. It’s a nice relaxed feeling knowing that you’re nicely below the ideal descent profile, however, a low energy situation can very quickly turn into a high energy situation, particularly when getting closer to the airport. If the crew do not keep a close eye on their vertical profile, all of a sudden they can find themselves high and fast, struggling to meet the stable approach criteria.
How Pilots Fly CDAs
How we fly a CDA very much depends on the airport into which we are making the approach. Some airports, such as Los Angeles, have designed their arrival procedures to enable us to fly as close to a CDA from the top of descent as possible. All we then have to do is manage our speed to comply with ATC instructions.
Before we start our descent, to ensure that the Flight Management Computer (FMC) is programmed correctly to comply with the vertical restrictions along the route, for example, to be between FL240 and FL190 at ANGLL. We also download the latest wind information for the descent and approach. With these all correctly loaded into the FMC, we then instruct the autopilot to fly the optimum vertical profile in a mode called VNAV — Vertical NAVigation.
For the most part, this arrival works pretty well as a CDA, enabling us to fly with the engines at or near idle power for almost all of the descent. ATC tend to clear us to “descend via the ANGLL 4,” which means we can descend down to the lowest altitude given on the chart (12,000 feet at CRCUS) so long as we comply with the various altitude restrictions on the way down.
Approaching CRCUS, ATC will clear us for the approach to one of the landing runways. At this point, once again, we descend to the lowest altitude show on the chart, before picking up the final approach guidance. As we get closer to touchdown, ATC will ask us to reduce our speed to ensure a safe separation is maintained between the aircraft in front and behind us.
The New York area
Hop across to the East Coast and things are not always quite so straight forward. The New York area is exceptionally busy for air traffic with a number of airports in a very small geographic area. As a result, ATC must juggle a variety of aircraft, all trying to go different places. This results in some quite inefficient flight paths and the potential for a low energy situation to very quickly turn into a high energy situation. This is particularly true when landing to the southwest at Newark.
The early part of the arrival normally has us descending to FL240 (24,000 feet) over 200 miles out from Newark — resulting in extra fuel burn and increased carbon emissions. To fit in around other traffic, we are then given a number of stepped descents, resulting in the engine power reducing and then increasing multiple times as we get closer to landing.
The approach into Newark from the north takes us directly over Teterboro, one of the busiest airports in the country for private aviation. As a result, we sometimes have to maintain our cleared altitude for longer than we would like, putting us into a high energy situation. If not expecting this, crews can find themselves with a suddenly increased workload as they struggle to get the aircraft stable at 1,000 feet. The key here is to “dirty up” the aircraft early by lowering the gear and flaps so when we are cleared to descend, we can keep the energy under control.
Somewhere between the two is the approach into London Heathrow. Once again, ATC is hugely constrained by traffic from other airports in the London area but are still able to give us clearances that enable us to complete a CDA — especially important for noise reduction, as most approaches are made over central London.
Unlike at LAX, the controllers at Heathrow must vector us around the sky to ensure that each aircraft is perfectly spaced from the aircraft in front on the final approach. As a result, when starting the approach, the controller will tell us the number of track miles we can expect to touch down. Using the philosophy that we require around three miles to lose 1,000 feet, with some quick mental arithmetic, we can work out whether we are high or low on the ideal approach path.
For example, if we are told that we have 21 miles to touch down, we would ideally be at 7,000 feet (3×7=21). If at that point we were actually at 9,000 feet, we would know that we are in a high energy situation and must work quickly to lose the excess height. Conversely, if we were at 5,000 feet, we would know that we are in a low energy situation, so can shallow off our rate of descent to ensure that we don’t end up flying level with high engine power.
Flying an approach is ultimately about safety, but we also have a responsibility to fly the approach as efficiently as possible. By flying a CDA, we are able to minimise noise for those living and working under the approach path and also reduce fuel burn and carbon emissions. The ability to fly a CDA very much depends on the airport, with some being better than others.
As an industry, we have a responsibility to reduce our impact on the environment, so expect to see CDAs becoming more common at airports around the world.
Featured photo by Nicolas Economou/NurPhoto via Getty Images.
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