QNH vs Standard
 

Climbing o 6,oooft 1023 QNH. Another Aircratf descends to FL70. Clearance between reduced by 280ft. Or vice versa. Would this scenerior be possible. Any ATCOs care to comment?
Thanks

BYOD - the situation you describe would be safe because FL70 would equate to 7280ft thus giving 1280ft of vertical separation above 6000ft.

If the QNH is less than 1013 separation between 6000ft and FL70 would be less than 1000ft. In the London TMA many of the SIDs terminate at 6000ft therefore the lowest Flight Level we use is the first whole one which guarantees at least 1000ft separation above 6000ft.

eg QNH 1010mb lowest FL we use above 6000ft is FL80.

I apologize for being pickey,

but

10 mb difference (1023mb vs 1013mb) equates to 300ft;

therefore ac at FL70 is actually 1300 ft above you at 6000ft (1023mb)

6000ft on 1013 is not separated from fl70, ask srg. It has to be 1012 for 1000ft separation

rej

1 mb = about 28ft, but is taught rounded up to 30ft, easier to do!


:)

10 DME ARC

Cheers

17 years in the occupation and never too old to learn.

I would hate to think how many controllers have had tickets suspended etc for not having enough separation due to rounding up to 30 mb.

ILS 119.5

RTFQ

It was 1013 vs 1023
@ 28 ft = 280 ft even at 28ft per milibar, 6000ft on 1023mb equates to 5720ft on 1013mb

:confused:

I stand to be corrected, but isn't the level change of 1 hpa inversely proportional to gravity and air density (which is in turn inversely proportional to absolute temperature). I don't know what range of likely level changes can be expected of 1 hpa difference, but I would expect that cold air near the ground would equate to fewer feet than warm air up high?

Any REAL boffins out there?

I doubt if many ATCOs remember the maths, nor do they need to. At every ATC unit I've served in there's been a dirty great card with the levels available against QNH values. When certain QNHs are reported everyone hollers "we've lost a level...", or something similar.

Right as has already been explained,

Draw your line, Sea level 1023 up to another line 6000ft = transition Alt.

Next line for 1013mb 300ft above Sea level, pressure decreases with height, then up another 6000ft = FL60, transition Level, another 1000ft above would be FL70.

Hence FL70 is 1300ish ft above Transition Altitude 6000ft, or as said before 1280ft.

Sorry to get basic but it always worked back in the college!


:)

Heathrow Director

We in the mil side of terminal ops use this type of calculation many times daily as we operate in Class G (& a few in Class D) on a variety of altimeter settings including QFE, aerodrome QNH, RPS and SAS.

If our controllers can't do this simple maths then they might as well abandon the occupation.

Yes... and a simple thing to remember, which I was taught 40 years ago... When you wind the subscale UP the altitude indicated increases. That's the bottom line and it solves every altimeter problem.

ILS 119.5 you said

"6000ft on 1013 is not separated from fl70, ask srg. It has to be 1012 for 1000ft separation"

Think you find it has to be 1014! on 1012 6000ft isnt separated from FL70 either 1013 and below isnt. all for that .2 something of a millibar! LOL

When above trans. alt. do you not HAVE to use Flight Levels?

Regards,

JC

JeroenC:

If I understand your question correctly, you are asking for confirmation that you always have to use FL above the Trans Alt.

This is not quite correct, as it is approved practice (different SOPs notwithstanding) to set QNH and fly by reference to that when cleared to an altitude below the Transition Altitude even if you are physically still above it (i.e. in FL area). You must be not expecting level reports to be requested until past the T. Alt, however.

There has been some discussion on other threads about the advisability of setting QNH very early in the descent (or 1013 in the climb) because of the risk of getting an intermediate level off or level check request and the danger of levelling at the wrong level or reporting the wrong one. Some airline SOPs have other criteria for when to change for this reason.

Class G airspace, below transition and VFR, unless directed to do otherwise, pilots often set whatever pressure is most relevant to their task, look out the window and avoid other ac.

Above Transition, pilots flying VFR are "encouraged" to fly quadrantals (1013mb). Ask a dozen pilots what they do and why and I think you might get a dozen different answers.

rej's answer is correct for the UK but rules are different in other States.

Speaking from the controller's perspective, a controller can assign a level based on any datum he or she chooses as long as the prescribed separation is achieved. In the airspace around an airport it is quite conceivable that an altitude might be assigned above TA.

Good practice says stick to the standards operating practices where you can but if needs must ......