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Not quite how it works. These are the instruments we are referring to not the ability of pilots. In fact in RVSM airspace the autopilot must be used. Instrume

Not quite how it works. These are the instruments we are referring to not the ability of pilots. In fact in RVSM airspace the autopilot must be used. Instruments must be very accurate given the reduced separation in RVSM airspace. Often on modern aircraft multiple altimeters are compared and voted to provide a single output provided to the displays and autopilot.

ceejayoz 3y ago
That's missing the point. If a human can manage to keep it within 100 feet of a desired altitude, an autopilot most certainly can; it didn't require new technology in the 2000s. Autopilots in the 1960s/1970s weren't seesawing all over the skies.
- sokoloff 3y ago
  RVSM is overwhelmingly about instrumentation accuracy and precision, not pilot capability. [0] The pressure difference between 5K MSL and 10K MSL at standard conditions is 14.6 kPa. The pressure difference between 30K MSL and 35K MSL at ISA is 6.3 kPa. For a given amount of aircraft-to-aircraft variability in their precision altitude sensing equipment, the resulting difference in actual altitude is more than double in RVSM airspace than in the lower altitude range above. That's the reason for RVSM: there is less change in pressure with change in altitude, coupled with a very busy altitude range (such that controllers would have an operational need to pass traffic overhead with only vertical separation rather than being able to use vectoring to achieve lateral separation between aircraft). It's not a linear relationship, but if I take an airplane with a 0.75 kPa absolute error in one direction and pass traffic with a 0.75 kPa absolute error in the other direction 1000' indicated above them, at low altitude, that 1.5 kPa total error is a little over 500 feet while IFR-IFR separation is 1000 feet minimum outside of RVSM. (These aircraft would likely be right on the border of passing a non-RVSM static system check.) If I take those same two aircraft into the mid flight levels and pass one over the other at 30K and 31K feet, the total error is around 1200 feet, which is why non-RVSM aircraft cannot be separated by 1000 feet in RVSM airspace, because you don't know that they'll miss each other. Improve the accuracy and precision of the static system and improve the examination criteria, making the airplane RVSM-capable, and now you can pass that traffic over each other at 1000' of indicated separation and be sure they'll miss. [0] - There is a pilot training requirement, which is focused on knowing the rules for RVSM and does not involve a checkride.
  - ceejayoz 3y ago
    Still missing the point. You’re talking about getting different aircraft to agree between each other. The post upthread expressed surprise at an aircraft maintaining a steady altitude to within tens of feet. That’s been a thing for many decades.
    - sokoloff 3y ago
      I think you're at least partially missing the point. For autopilots servo'd to pressure altitude, holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa (which is roughly the private pilot checkride standard as-tested). Modern autopilots are actually better at holding altitude to a very tight tolerance than ancient, analog autopilots. Both can hold standards well within the PPL ACS.
      - Dylan16807 3y ago
        > holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa "more" difficult is obviously true, but the difficulty of holding an altitude is only a small part of the overall difficulty of RSVM. In other words, RSVM is much more about accuracy than precision, and the claim was that planes were "probably fairly precise already". The reason they needed upgrades was to improve the accuracy, not so much to improve the precision.

ceejayoz 3y ago

That's missing the point. If a human can manage to keep it within 100 feet of a desired altitude, an autopilot most certainly can; it didn't require new technology in the 2000s. Autopilots in the 1960s/1970s weren't seesawing all over the skies.

sokoloff 3y ago
RVSM is overwhelmingly about instrumentation accuracy and precision, not pilot capability. [0] The pressure difference between 5K MSL and 10K MSL at standard conditions is 14.6 kPa. The pressure difference between 30K MSL and 35K MSL at ISA is 6.3 kPa. For a given amount of aircraft-to-aircraft variability in their precision altitude sensing equipment, the resulting difference in actual altitude is more than double in RVSM airspace than in the lower altitude range above. That's the reason for RVSM: there is less change in pressure with change in altitude, coupled with a very busy altitude range (such that controllers would have an operational need to pass traffic overhead with only vertical separation rather than being able to use vectoring to achieve lateral separation between aircraft). It's not a linear relationship, but if I take an airplane with a 0.75 kPa absolute error in one direction and pass traffic with a 0.75 kPa absolute error in the other direction 1000' indicated above them, at low altitude, that 1.5 kPa total error is a little over 500 feet while IFR-IFR separation is 1000 feet minimum outside of RVSM. (These aircraft would likely be right on the border of passing a non-RVSM static system check.) If I take those same two aircraft into the mid flight levels and pass one over the other at 30K and 31K feet, the total error is around 1200 feet, which is why non-RVSM aircraft cannot be separated by 1000 feet in RVSM airspace, because you don't know that they'll miss each other. Improve the accuracy and precision of the static system and improve the examination criteria, making the airplane RVSM-capable, and now you can pass that traffic over each other at 1000' of indicated separation and be sure they'll miss. [0] - There is a pilot training requirement, which is focused on knowing the rules for RVSM and does not involve a checkride.
- ceejayoz 3y ago
  Still missing the point. You’re talking about getting different aircraft to agree between each other. The post upthread expressed surprise at an aircraft maintaining a steady altitude to within tens of feet. That’s been a thing for many decades.
  - sokoloff 3y ago
    I think you're at least partially missing the point. For autopilots servo'd to pressure altitude, holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa (which is roughly the private pilot checkride standard as-tested). Modern autopilots are actually better at holding altitude to a very tight tolerance than ancient, analog autopilots. Both can hold standards well within the PPL ACS.
    - Dylan16807 3y ago
      > holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa "more" difficult is obviously true, but the difficulty of holding an altitude is only a small part of the overall difficulty of RSVM. In other words, RSVM is much more about accuracy than precision, and the claim was that planes were "probably fairly precise already". The reason they needed upgrades was to improve the accuracy, not so much to improve the precision.