This is a guest post from Tony Cooper
There’s going to be a new jet thrill ride in Queenstown – flying through tight mountainous terrain at night to the airport. Using new award-winning technology introduced by Airways New Zealand pilots will be able to dodge and weave among the mountains in the dark and even in cloud.
The technology is called Required Navigation Performance Authorisation Required (RNP AR) and allows computers to use satellite GPS technology to steer jets along very precise paths in a range of weather conditions without a human hand. When implemented in 2016 it will allow the airport to operate jets at night – current jet flights are limited to daylight hours.
Airways New Zealand, who provides air traffic control for our airports, won an international award for the Queenstown project in February 2013 – taking out the prestigious Jane’s ATC Award for Operational Efficiency ahead of 70 entries from aviation companies around the world.
Now the same technology is being rolled out nationwide. Even without mountains the precision of the flight paths brings many advantages. In particular, for Auckland it is expected that RNP AR will reduce distances flown over the city, fuel consumption, carbon emissions and noise for people living close to the airport.
This has worked well in other countries. For example, Brisbane Airport has implemented RNP AR – the precision of the flight paths has allowed jets to follow the curve of the river to minimise the noise footprint. In their first trial in 2008 they reported that over 1612 flights a total of 200,000kg of fuel was saved, 4200 minutes flight time saved, noise was reduced, and flight delays for non RNP AR flights reduced.
Hoping for similar success Auckland Airport trialled the technology during 2012 and 2013 calling the new approach the “SMART approach.” We are awaiting the results of that trial – the report will be released this week.
The purpose of this article is to provide background understanding for the trial and to show the issues involved. We will compare the new SMART approach with the standard approach. We will look at fuel savings and noise differences. We will only look at relative differences between the two approaches and not absolute noise levels over the city.
Also we hope to clear up some misconceptions that have appeared in the media. For example, the Herald reported on May 6 that a new regime was keeping aircraft in holding patterns above residential areas. This isn’t true because Auckland airport does not use holding patterns.
In fact, Auckland approaches are pretty efficient and that’s one of the problems with the SMART trial – it’s difficult to improve on the existing standard approach.
The quietest possible approach would involve a jet switching off its engines over the Tasman Sea and gliding in to the airport. Perhaps surprisingly, jets make good gliders and can easily glide the last 150km of the flight. Naturally, flying with the engines off isn’t safe so pilots do the next best thing – they descend with the engines at idle. This is as quiet as it is possible to get but is still noisy because the engines still make some noise and also the airframe makes as much noise as the engines.
When the plane gets down to about 2000ft both noises increase as wheels and flaps are lowered which creates more airframe noise and the pilot puts on more power to compensate for extra drag. This occurs in Auckland for Tasman flights around about Ormiston so it is Manukau City residents who bear this noise.
The gliding descent (also called continuous descent) or something close to it has been used since 2007 for most approaches into Auckland. And much of the portion of the flight between 5000ft and 2000ft is over the sea or farmland. So we are lucky compared to other cities where descents have powered level flight stages or holding patterns and are fully over residential areas. That’s what we mean when we say that Auckland approaches are pretty efficient.
Most busy airports around the world still use stepped approaches. This is the blue line in the chart below. The continuous approach is the red line. Since the blue line spends more time at lower altitudes where jet engines don’t work efficiently the blue approach uses more fuel and exposes the ground to more noise. In particular, during the level portions of the flight the aircraft produces considerable noise compared to idle gliding.
Continuous descents are tricky to instigate. The engines have to be idled at the right time. Too soon and extra power will have to be applied to reach the airport. Too late and the aircraft will overshoot the airport. Overshooting is a serious problem because it is difficult to slow down a plane that is already running on idle engines. Speed-brakes don’t work very well at approach speeds and when deployed add extra noise. An overshooting aircraft has to fly extra distance so messes up the careful spacing required by Air Traffic Control.
All these problems are exacerbated by the wind. To time the descent the wind speed has to be predicted at all levels of the descent and all locations on the flight path. This is impossible. So to compensate for the variability pilots like to fly the flight path a little low. Then closer to the airport they apply extra power to get back to the right height. Both flying low and extra power add to the noise effects on residents.
This is where the precision of the RNP AR approach can help. The computer controlling the flight can react to wind changes faster than a human and keep the aircraft at a precise height. This precision also allows Air Traffic Control to better space out arriving flights to further reduce noise and fuel.
Although we are lucky that current approaches are mostly on idle power we could be luckier if flights didn’t overfly the city at all. With two harbours why can’t jets fly over one of them? But Auckland airspace is busy – there are Whenuapai and Dairy Flat airfields to the north and Ardmore airport to the south to be avoided. Auckland airport itself occupies Manukau Harbour so the only reliable gap for Tasman arrivals is over the city.
The idea of the SMART approaches which were trialled from November 2012 to the end of October 2013 is to move the flight path west so that aircraft from the Tasman don’t have to fly out to Beachlands. This is shown in the chart below. The red line shows the standard approach and the gold line shows the SMART approach.
The distance saved by the shorter approach is 27km. An Airbus A380 would save about 400 litres of fuel with the shorter flight path. Smaller jets would save upwards of 100 litres so the savings can add up over thousands of flights. The savings could be higher because we didn’t take into account the extra time spent flying at lower altitudes and efficiency changes to air traffic control procedures.
The SMART approach has been described in the media as a “shortcut” and it looks like that in the chart but this is misleading. At the point in the upper left where the two paths diverge there is a height difference of about 2000ft – the SMART approach is the lower one. This is an important point to bear in mind when comparing the two flight paths.
The above chart is an idealised plot. The actual flight paths taken by jets over Auckland for a week during the trial are shown in the next chart. Red lines are arrivals and green departures. The precision of the SMART approach compared to the standard is evident. What also is evident is that almost the whole of the city has flights over it with the only exception being, ironically, the Mangere area adjacent and to the north of the airport.
What these charts don’t show is the altitudes of the flights over the city. Because the SMART approach is shorter than the standard approach and because the glide slope angle into the airport is the same for both then the height of the SMART approach is lower over the city.
The trade-off with the SMART approach is that the flight path is shorter so less fuel (and so less noise) is burnt over the city but the average height is lower (so more noise). We need to examine this trade-off very carefully to see the effect on residents.
The next chart shows for each approach the portions below 2000ft (cyan), the portion between 2000ft and 4000ft (yellow), and part of the portion above 4000ft (purple).
These heights of 2000 and 4000 feet are somewhat arbitrary and we chose round numbers. We chose 2000ft because this is about the height at which wheels and flaps are lowered and power increased. It also gives the boundary at which Manukau City houses need to have a note added to their LIM records that show they are affected by airport noise. However, it is above the height where houses qualify for free sound insulation which occurs closer to the airport.
The two flight paths converge at about the SH1 motorway. So to compare them we only had to look at houses to the east of the motorway.
Above 2000ft the next important height is 3000ft. This number (sometimes extended to 4000ft at night) seems to be used frequently worldwide as the height above which noise abatement procedures no longer apply. Presumably noise is not an issue for departures once the jet is above 3000ft. We also included 4000ft to include more of the flight path.
For this study we counted houses below each flight path within a 1 km ground distance of the path. We then used population density estimates provided by Critchlow Ltd to estimate the number of people resident in each house and this gave us the number of people living under each flight path. The 1km distance from the path is fairly arbitrary but if you think the distance should be doubled you can roughly double the people counts that we found.
This methodology of counting people for each altitude level is quite crude – presumably the SMART report will use decibel sound readings and report the number of people affected at each decibel level. Also different people have different sensitivities to sound (this can be measured on the Weinstein Noise Sensitivity Scale) so different suburbs will have different sensitivity to noise. The precision of the RNP AR approach allows precise noise contours to be calculated which we cannot do. But we believe that this crude approach will still give an idea of the differences between the approaches.
We only counted residential houses – the precision of the SMART flight path allows it to be placed over industrial areas where possible. The industrial areas show up as grey in the charts above. Farmland is green.
Our counts are:
Because the two flight paths are so close together below 2000ft the people counts are the same for both. Because the standard approach is over farmland when between 2000ft and 4000ft the counts are zero (apart from 3,400 people living in Beachlands at the 4000ft mark).
We didn’t count houses for parts of the flight paths above 4000ft but measured the lengths of the paths and used density estimates to get rough numbers.
We note the following points
- the shift of the flight path to the west has shifted the noise to the west
- under 2000ft equal numbers of people are affected
- from 2000 to 4000 feet about 20,000 more people are affected by the SMART approach
- above 4000ft about 60,000 more people are exposed to noise on the standard flight path than the SMART (due to the extra distance flown over the eastern suburbs)
- west of Royal Oak above 4000ft and all the way to west Auckland the SMART flight path averages about 2000ft lower than the standard flight path (this isn’t shown on any of the maps)
- although we mark the power settings for both approaches as “close to idle” the noise of the descent may be less in the SMART descent because of the precision of the computer control
Points (1), (4) and (5) illustrate the process called “noise sharing.” The total amount of noise hasn’t changed (assuming efficient gliding is used), it has just moved west. Not all flights in the future will be SMART flights so not all noise has shifted. People in eastern suburbs have less noise (SMART doesn’t go there) and people in the west have more noise (lower SMART flights) so it is said that the noise is being shared between suburbs. This is what is meant by the phrase “SMART allows us to equitably distribute flight paths across Auckland.”
Another example of this is that at Beachlands the standard flight path is at 4000ft which is the same height as the SMART path at Royal Oak. So when standard flights switch to SMART flights Beachlands will “share” some of its noise with Royal Oak.
People at 2000ft to 4000ft on the SMART path will be sharing their noise with cows at Whitford. So the cows win on that one.
Some noise has gone west out of the city boundaries and into the Waitakeres but the benefits to residents of that move is small because it is mainly high altitude flying out there.
- The SMART flight path does not produce any more noise but moves it west
- No extra people will experience noise below 2000ft
- Some extra people will experience noise in the 3000ft to 4000ft range
- The precision of the SMART approaches may actually reduce some noise but standard approaches are pretty quiet already
- Fuel savings will range from 100 to 400 litres, possibly more
- Our analysis ignores many factors such as the improved efficiency of Air Traffic Control
Misconceptions in the Media
There have been a number of disingenuous or misleading statements made in the media and the Internet in the context of the SMART trials and we elucidate them here. We are concerned that SMART noise is being confused with the standard noise.
- “Airways creates more noise by keeping aircraft in holding patterns above residential areas” – not true. Holding patterns generally aren’t used at Auckland
- “During the trials … so low … you could read Emirates on the side of the planes” – may be true but misleading because Emirates weren’t part of the SMART trial.
- “With this new flight path we are now woken every night” – may be true but misleading because the trials were not conducted after 10 pm.
- “Every airline is now using this shortcut route” – not true, only Air NZ, Qantas, and Jetstar were part of the trial.
- “Emirates EK435, aircraft type A380, recorded a horrendous 83.2 decibels during the Smart approaches trial” – loud but, again, not part of the trial.
- “continuous ascents and descents. This means they are travelling at slower speeds in near-level flight” – incorrect – continuous means that it is not level or even near-level. Continuous is the opposite of level. Continuous descents are the quietest descents possible – they are good. This comment misleadingly makes them look bad. Ascents were not part of the SMART trial.
- “terrible continuous ascents” – ascents were not part of the SMART trial. Complaining about non-SMART noise and including it in the same breath as complaints about SMART noise seems a little disingenuous to the SMART trial.
- “a low curved shortcut to the airport” – this is misleading. It’s not a shortcut and SMART approaches are no lower than standard approaches and may be higher.
- “mandatory 3000ft above Royal Oak” – The operating procedures for the RNP AR approach mandate 4000ft over Royal Oak. So this statement is not true.
This report was completed with the help of people from Airways New Zealand, Air New Zealand, Critchlow Ltd, and The Plane Truth and we thank those people. We solicited data from Auckland Airport but it was not forthcoming. All calculations, errors, and over-simplifications are ours.