Why is RVSM necessary?

[FlyBieWire]

Can anyone explain why RVSM is necessary above FL 290. What is so special about those higher altitudes?

 

[srjorion]

Honestly, I'm not 100% sure on the scientific answer, but I'll give it a shot.

The higher you go, the less accurate barometric altimeters are. That's why there used to be a 2,000' split above FL290. Now, with the improved accuracy of Air Data Computers and Auto Pilots with altitude hold capabilities we can reduce the error and thus allow aircraft to fly those unused flight levels.

That's my answer. If anyone has a better answer fire away.

 

[3BCat]

The advent of small jets into the national airspace system has shifted commuter traffic from the upper teens to the upper twenties and higher flight levels. It pays to have more altitudes that are usable because more aircraft are in use. Transatlantic/International traffic is also up, so RVSM makes good sense for these operations, too.

 

[some_dude]

Why is it necessary? So Boeing and Airbus will sell more new airplanes, and so a bunch of avionics manufacturers and shops will make money!

 

[Midnight Flyer]

The advent of small jets into the national airspace system has shifted commuter traffic from the upper teens to the upper twenties and higher flight levels. It pays to have more altitudes that are usable because more aircraft are in use. Transatlantic/International traffic is also up, so RVSM makes good sense for these operations, too.

I agree. Back in the day, commuters didn't even have jets, so all their traffic was in the 20's. Now, some commuters (regionals, whatever) are 100% jet, and they're clogging the system.

 

[2000flyer]

Why is it necessary? So Boeing and Airbus will sell more new airplanes, and so a bunch of avionics manufacturers and shops will make money!

RVSM has nothing to do with who sells what. Its a capacity issue. Allowing reduced separation both vertically and horizontally means more aircraft in a given piece of airspace, plain and simple.

 

[FlyBieWire]

The higher you go, the less accurate barometric altimeters are. That's why there used to be a 2,000' split above FL290.

My question is: Why is this true.

 

[340drvr]

Less air density means the difference between, say, FL350 and FL 360 is not as accurately sensed by regular altimeter equipment, thus the need for higher resolution RVSM stuff.

 

[UndauntedFlyer]

I don’t have a reference for this but I believe the need for RVSM separation standards has to do with the non-linear vertical pressure gradient. As we go up in the atmosphere the pressure lowers at a non-linear rate, starting first with about 1” Hg. Per 1000 feet and then gradually getting less with each 1000 feet of increase in altitude. The altimeter must know about this and convert that change into a readable dial showing 20 foot increments of altitude. At some point in the upper atmosphere as we approach outer space you can see that the amount of pressure change per 1000 feet must be very small and almost impossible to measure on even the most sensitive of pressure instruments. Therefore at some point as we go up in the atmosphere there must be a point where an increasingly accurate pressure sensing system must be used if we are going to be able to provide a true vertical separation of 1000 feet as a standard.

In the past such a system was not available at a reasonable cost so the solution was to simply increase the vertical separation standards to 2000 feet for safety when flying above FL290. But now as there is a need for more available altitudes because of increased air traffic, it was decided to develop better altimeter systems and then go ahead and certify those systems that proved to be more accurate by being more pressure sensitive. Thus we now have RVSM and each aircraft that requests to fly above FL290 must be FAA-certified to do so. Also the crew must be FAA-approved to follow certain procedures when above FL290. The crew procedures help maintain the assigned altitude with greater accuracy. Some of those procedures include such things as use of the autopilot and recording a cross reference to a third altimeter for accuracy.

 

[Lrjtcaptain]

RVSM has nothing to do with who sells what. Its a capacity issue. Allowing reduced separation both vertically and horizontally means more aircraft in a given piece of airspace, plain and simple.

Vertically, absolutly, the NAS was wasting so much sky by the 2000ft rule, especially with the increase of RJ's out there.

As for horizontal....well, find me a center that is running you inside of 5 miles at the same alititude. Negative!

hence. REDUCED VERTICAL