Since the world population shows no signs of slowing down its rapid increase, it's pretty safe to say that means we can only expect more concurrent flights as time moves forward.

Is it physically possible (with current aircraft models) to further reduce RVSM to say 500ft?

Since automatic altitude maintenance is required when flying in this airspace, I can't see why not.

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    $\begingroup$ There really isn't a need for it. Runway availability is a more limiting factor than cruise airspace. The areas where excess traffic is really a problem is not in RVSM cruise level airspace, it's in the vicinity of airports. $\endgroup$ – TomMcW Sep 19 '18 at 17:34
  • $\begingroup$ @TomMcW - It may vary by region, see this comment by DL: chat.stackexchange.com/transcript/message/46727594#46727594 $\endgroup$ – ymb1 Sep 19 '18 at 19:54

Reducing further to 500 ft separation is not safe with current aircraft models.

One reason is because of wake turbulence, this is described by ymb1's answer. Another, perhaps more compelling reason, not to use 500 ft separation is that it inevitably will lead to mid-air collisions.

The actual metal-to-metal spacing between aircraft in RVSM is often below 1000 feet. This is because of a number of factors:

  • physical aircraft height: e.g. an A380 is approximately 24 meter high. That is 75 feet.
  • deviations from the assigned altitude. For example overshoot/undershoots of 150 ft are acceptable when levelling off; brief deviations of 75 ft in cruise are not uncommon.
  • altimeter system error: the altimeter does not always indicate the exact pressure altitude; errors of up to 245 feet are possible for RVSM compliance.

It is easy to see that the sum of these components easily can eat more than half of your proposed 500 ft separation. If two aircraft on adjacent 500 ft separated flight levels have a bias toward each other, a collision will be the result. 500 ft separation is just not safe with the current aircraft.

In order to keep 1000 ft separation safe, aircraft operating in RVSM airspace are frequently monitored by RVSM regional monitoring agencies. They verify aircraft altimetry systems are operating within safe tolerances based off their transponder signals.

For the RVSM safety case (see ICAO DOC 9574), a total vertical error (TVE) of up to 300 ft can be tolerated by the monitoring agency.

The TVE is the sum of the Assigned Altitude Deviation (AAD) and the Altimetry System Error (ASE). The ASE may never exceed 245 feet.

The AAD is the result of small vertical disturbances and the altitude control system reacting upon that. Autopilots typically keep the AAD within 50ft, but some situations may degrade the height keeping performance.

Ideally the ASE is 0ft, but in reality there is often a bias in the altimeter. ASE can be caused by:

  • Damage to static ports and pitot tubes
  • Pressure leaks in pitot/static pipes
  • Air Data Computers out of tolerance
  • Poor paint finish in static port sensitive areas
  • Inadequacy of RVSM inspection procedures
  • Component life span
  • Non-optimised Static Source Error Corrections
  • Pressure variation caused by skin waviness effects

picture of damaged static port

source: EUR RVSM RMA/Skybrary

If the RVSM Regional Monitoring Agency notices that an aircraft is trending towards the 245 ft limit (action is initiated at 180 ft error), they contact the aircraft operator so that corrective maintenance can take place. This way they ensure that RVSM with 1000 feet separation remains safe.

Below is picture of ASE measurements of a single aircraft that developed an altimetry error of 150ft within one year. The operator performed corrective action after being contacted by Eurocontrol's RVSM RMA. Thereafter the ASE was around the 0ft mark.

Picture of ASE measurements showing deteriorating altimeter error, and correction

source: EUR RVSM RMA/Skybrary

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    $\begingroup$ "physical aircraft height: e.g. an A380 is approximately 24 meter high. That is 75 feet." and that is assuming the aircraft is level, roll or bank could presumablly increase the effective height significantly. $\endgroup$ – Peter Green Sep 19 '18 at 22:42
  • $\begingroup$ Great information. So, how do the RMA monitor the ASE? How do they know the system that they use to check altitude is 100% accurate? $\endgroup$ – Cloud Sep 20 '18 at 6:15
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    $\begingroup$ @Cloud, that is a question on its own. In short, they measure geometric height using multilateration, they correct that using meteo data, they compare it to the altitude transmitted by the aircraft's transponder. The multilateration system monitors its own calibration by using looking at known reference transmitters that are at known locations. In addition there are many checks in the post processing of the raw data. $\endgroup$ – DeltaLima Sep 20 '18 at 15:02

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In cruise the wake turbulence levels off about 900 feet below the cruise level, which is just enough for another plane flying head-on 1,000 feet below to miss it or encounter the mild part of it. Encountered wake turbulence in RVSM should be reported to ATC, because the wind also affects it, and then the ATC will advise on a deviation for the remainder of your route and for the other aircraft behind you, for example.

Regarding the projected increase in travelers, new airways can be utilized, more use of free route airspace, using bigger (but not too big) planes, as well as improving the throughput of the runways using various technologies (NextGen and SESAR for example).

  • $\begingroup$ Why is the area above the wake turbulence marked "avoid", rather than the area within it? $\endgroup$ – Sean Mar 21 '20 at 0:51
  • $\begingroup$ Yes, but how does that make it unsafe for another aircraft to fly through the area where the wake was, as long as it doesn't fly through where it currently is? $\endgroup$ – Sean Mar 21 '20 at 15:18
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    $\begingroup$ @Sean: Wake doesn't dissipate instantly. Same with planes on approach in sequence, the "glide slope" isn't free of wake once a plane passes. hence the "caution wake turbulence". $\endgroup$ – ymb1 Mar 21 '20 at 15:20

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