RNAV Post-Implementation Review

The Civil Aviation Authority (CAA0 has finally published its Post-Implementation Review of the introduction of concentrated flight tracks (RNAV) at Luton Airport – four and a half years after the change was made.

RNAV was introduced in 2015, but technical issues meant the assessment did not start until 2017. By that time there had been a substantial rise in complaints – but also, due to financially incentivised growth at Luton, a substantial rise in aircraft movements as well. The CAA did not look into this and simply said “complaints were as expected”. Odd, for a change which was supposed to reduce noise…

The truth is that concentrated flight tracks may or may not deliver benefit. If it means aircraft can consistently avoid communities and fly instead over noise insensitive areas, they can be beneficial. But if they avoid (naturally noisier) towns and fly over (naturally quieter) rural villages, they are a curse for the few to benefit the many – even though the many mostly notice the benefit at night when the town is also quieter.

The worst of all worlds is when a concentrated track passes between two tightly spaced communities – say half a mile apart. At altitudes of around 5-6,000ft the concentrated tracks will sideways-radiate noise into each of those communities even if they are not directly overflown.

In Luton’s implementation of RNAV, the airport operators led communities to expect a noise reduction due to significant reduction in overflight: the 30,000 people overflown would, they said, reduce to 3,000. For people who were not across all the technicalities, this sounded like a reasonable deal. The Airport also said that the centre line would move further away from Harpenden. What they did not make clear was that a whole load of planes would move closer to Harpenden as they started to follow the new track.

And despite the Airport’s promises of less overflying, the NATS air traffic controllers still disperse a sizeable proportion of the flights directly over communities which the RNAV procedure had intended to relieve of overflight. Aircraft which do stay on the track then thread through a narrow 1km gap between Harpenden and St Albans. People on the fringes of both communities now perceive them all, due to sideways radiation of noise.

Finally, Sandridge is directly overflown by the new centre line. The Airport did baseline noise measurement pre-RNAV but botched its monitoring and ended up with a noise sample where the quieter half of the readings were entirely missed off. This led to average baseline noise levels per aircraft type being louder than was actually the case. When they measured again more competently in 2017, the noise averages per type magically came down.

The CAA did not spot this obvious error, and declared that noise in Sandridge had not increased. They ignored completely the effect of increased concentration – which does not say much for the CAA PIR team’s understanding of aircraft noise impact.

So overall the PIR looks like a CAA rubber-stamping exercise which avoids any of the hard questions we raised about whether the implementation had achieved its noise objectives. The key is “what actually were the objectives?” The CAA only considered the technical matters, and ignored the general claims about noise reduction made during consultation. And even though it mentions the continued dispersal over St Albans and Harpenden, it overlooks the fact that the Airport has had five years to resolve this but has failed to do so.

In 2017, LADACAN submitted a detailed formal report  to the CAA review process: click here >> Luton Runway 26 RNAV PIR submission from LADACAN.

The CAA report can be found at the end of this page: CAA Post Implementation Review