The relevance is that you may get an unexpectedly high rate of descent after passing the bend.
I think we are splitting hairs. Look at some numbers. The 11mb diff in QNH, taking the change in the slope from 6000ft to 3000ft, is 10% of the altitude delta, which taking the small angle approximation (or calculus of small changes) is a 10% change in the descent angle, and most will struggle to notice that. My KFC225, if you set say – 500fpm, might do 500, or 400, or 600. That is +/- 20%. Unless you do a descent in PIT mode (which uses the KI256 pitch feedback, AFAIK – I don’t think there is an “angle sensor” in the KC225), it will be done barometrically and will never be that accurate. Flying is just not that precise… The things an autopilot can do precisely is e.g. altitude hold but that’s because it has two loops: the internal baro, and the KEA130A encoding altimeter gray code input for the long term definitive feedback. For VS you don’t have a precise reference; you would need precision VS feedback. That will never be right either, due to temp and density changes…
A descent will never be a straight line in the vertical plane, with any GA avionics. You could do it with GPS altitude.
Anyway, no matter how you shake this, setting the final QNH at the start of the descent, avoids all this.
Just got this by email:
Peter wrote:
I still don’t understand the relevance of setting 1024 or 1013, with a descent from FL080 to 3000ft, and a TA of say 6000ft. It means that it won’t be a straight line, but the “bend” at 6000ft will never be visible because the delta is 11mb which is worth about 300ft.
The relevance is that you may get an unexpectedly high rate of descent after passing the bend. When you choose your desired descent rate in the GPS box, it will compute a Flight Path Angle (FPA) based on a constant QNH. If the altimeter setting is increased during the descent, the aircraft will suddenly be above the profile and the AP will increase the descent rate to rejoin the descent profile.
If you have chosen a 500 fpm descent (say), the target altitude is 5000 ft and you reset the altimeter from 1013 to 1024 when passing 6000 ft, the GPS suddenly finds that it is 300 ft above the profile and has to increase the descent rate to 650 fpm. (Possibly more depending on how fast it wants to rejoin the profile.) If you don’t react by pulling back on power, the airspeed will increase which may be a bad thing.
Peter wrote:
That’s what most people do, and it is safest, but it may not be the “right way”.
It depends on your SOP. Both ways are permitted.
On many a/c it is the law of 3.
1/ When told to descend from a flight level and given a QNH you set the barometric setting on the A/P (eg KAP 140) set the rate of descent then the altitude you wish to descend to
2/ Change the eg G1000 from barometric to QNH setting + target altitude.
3/ The stand by altimeter will probably have been left on the QNH perhaps from depart. Change this to new QNH
Check all is as expected. On an ILS it might also be worth calculating the difference eg 1013 to 1024 = approx 330ft so you are likely to pick up the GS about a nm earlier than on a standard day.
On climb and when switching to 1013 you might want to check that the altitude on your standby altimeter is coherent with your other 2 settings.
ie in the case above about 330ft lower than you would be in altitude.
But as already written. Change setting when you are given the QNH on the way down and the FL on the way up.
I still don’t understand the relevance of setting 1024 or 1013, with a descent from FL080 to 3000ft, and a TA of say 6000ft. It means that it won’t be a straight line, but the “bend” at 6000ft will never be visible because the delta is 11mb which is worth about 300ft.
An AP descent is in VS, PIT or IAS mode and none of those will know about the altimeter subscale.
This is AP specific, some GFC have extra modes, the VNV function on these fancy autopilots will have magenta glidepath (calculated from altimeter altitude & target altitude) then you descent on VPATH mode (not VS, IAS, PIT), if you don’t have barometric input to navigator, it will use GPS altitude…
PS: this is for cruise vertical navigation, there is also Baro-L/VNAV in approach which goes up & down as you move altimeter setting
You can move altimeter at FL80 to 1024 on receiving descent clerance to 3000ft QNH
That’s what most people do, and it is safest, but it may not be the “right way”. The obvious issue is if you get a descent from say FL80 to 3000ft, the TA is 6k, and you get a cancellation on the 3000ft and a request to level off at FL70. Then you need to reset the subscale from 1024 to 1013. But this is rare.
Just esthetics, it does not make much difference (maybe some difference if you are flying VNAV to millimeters on autopilot)
I don’t understand. An AP descent is in VS, PIT or IAS mode and none of those will know about the altimeter subscale.
In theory, the altimeter should go from 1013 to 1024 as you descend through the TA,
You can move altimeter at FL80 to 1024 on receiving descent clerance to 3000ft QNH if level cruise is not anticipated, no need to swap it at the transition altitude
Just esthetics, it does not make much difference (maybe some difference if you are flying VNAV to millimeters on autopilot)
In much of the UK, one cannot comply with it due to CAS and MSA. In the south east especially, the flying altitude is driven entirely by CAS bases and MSA, and this is the case whether you are VMC or IMC. The CAA (enforcement department) is telling people to allow 200ft below CAS base which right away implies not flying at any semicircular level. And if you are flying somewhere where you could obey these “rules” then pretty soon you have to descend due to a piece of Class A in the way.
Then throw in UK PPL training which gets most people to fly at 2000ft, and you really do not want to be flying on any round number.
Re transition altitude, which is a totally different topic, this is ignored because if you are under ATC control and flying high enough for the TA to be relevant, you are probably on an IFR flight plan, probably with London Control, and on descent they give you instructions: “descent FL080”, or “descent 3000ft QNH 1024” so again there is no need to know the TA, and the altimeter gets moved from 1013 to 1024 when they give you the latter instruction. In theory, the altimeter should go from 1013 to 1024 as you descend through the TA, but nobody I know bothers because it is easy to forget, and doing a bust is infinitely more serious.
Peter wrote:
First time I heard a lot of the stuff above, like 3700ft being non ICAO and thus prohibited.
Depends on the height. SERA.5005(g) says that uncontrolled VFR above 3000 ft AGL shall – unless otherwise specified by the competent authority – fly at semicircular levels, both on QNH and on standard setting.
For uncontrolled IFR, SERA.5025(a) applies and it is even stricter – uncontrolled IFR shall always use semicircular levels unless the competent authority has specified otherwise for flight at or below 3000 ft AGL only!
3700 ft is not a semicircular level so it is not permitted for VFR OCAS if the ground is at less than 700 ft (unless otherwise specified by the competent authority) and never for IFR. I’m pretty sure ICAO Annex 2 says the same thing, although I haven’t checked.
Doesn’t matter…
I agree that this regulation is frequently inappropriate.. And of course the airspace structure may occasionally make it “difficult” to comply with.
