QNH Confusion

[Student Pilot]

So the term Global positioning system doesn't cover GNSS? Typical CASA why use one word when 4,398,298 will say the same thing.

 

 

[Mazda]

It's not just CASA. Kawasaki trade marked "Jet Ski" so anything other brand must be called a "Personal Water Craft", not a "Jet Ski"!

 

 

[Guest brentc]

In practice, I continously set the altitude to correspond with the GPS altitude , and that is so accurate that (when everything is working) you couldn't do better.

With all due respect Nev, not a good idea! Following this principle you are placing yourself and others at risk who are flying off the area QNH as you have mentioned. You have the possibility to be several hundred feet off the mark!

 

Also regarding your comment:

 

'Just a little "watch it" here,also. Give the lower level of the CTA steps a 200' avoidance, as the IFR types can fly right on that height. '

 

This is not entirely correct as airspace is often + or - 500 ft from the even thousand, therefore by default you will generally be 500ft away from the IFR traffic, plus if you fly your 200ft below the CTA and you are using your GPS altitude (which may be inaccurate) which may put you at an incorrect pressure height you may end up flying on an IFR cruising level! Remember too that not all IFR traffic flies in CTA, so you can just as easily come across something fast and IFR at 4,000 ft!

 

 

[djpacro]

Thanks for the reminder, brentc; somewhere in my memory cells is some information on the difference between pressure altitude and geometric height - temperature variation from ISA was a significant factor as I recall.

 

 

[facthunter]

.reality.

 

brentc, There is no tolererance whatever on DME steps with CTA boundaries. The IFR aircraft can fly at the lower limit. It is suggested that other aircraft should separate themselves by 200 feet to provide separation. This was from a CASA approved training manual (from memory).

 

I would like to retract some of my previous post, and apologise to brentc.

 

Taking the point raised by two contributors, The quoted phrase cannot stand alone, and doesn't read right. The GPS derived height can only be a second reference which could alert you to a discrepancy (ie incorrect setting) and possibly an out of date QNH. The altimeter presumes a standard atmosphere which could be expected to produce significant variations, on occasions, the higher you fly, depending on the difference between the actual atmosphere and the ISA atmosphere. Nev..