AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Groundschool Question...          AirTalk.org Forum Index -> Aviation (Students) View previous topic :: View next topic   Author Message news.btinternet.com Guest Posted: Sat May 08, 2004 2:58 pm    Post subject: Groundschool Question... Hi, I have a question which I'm having trouble answering, anyone care to help? An aircraft flies from aerodrome "A", where QNH is given as 1020mb, to aerodrome "B", where QNH is given as 999mb. Aerodrome "A" is 800ft above MSL and aerodrome "B" is 500ft above MSL. If the altimeter sub scale is not changed from 1020, what is the altimeter indication on landing? (1mb = 30ft) Well, my stab at it is this: On the ground at Aerodrome 'A', the altimeter would read the elevation of the airfield - 800ft as the altimeter is set to aerodrome A's QNH value. The change in pressure between aerodrome 'A' and aerodrome 'B' is -21mb and where 1mb = 30ft that would be 630ft change. Because it's going to a lower pressure, that would be +630ft. So the altimeter would register the change of pressure and read 800ft + 630ft = 1430ft. The only thing is the stated answer to the question is 1130ft so I've got something wrong here - probably discounting the height of aerodrome B above MSL. Any ideas? Many thanks, Paul, 2hrs into PPL London, UK Back to top Jürgen Exner Guest Posted: Sat May 08, 2004 3:14 pm    Post subject: Re: Groundschool Question... news.btinternet.com wrote: Quote: Hi, I have a question which I'm having trouble answering, anyone care to help? An aircraft flies from aerodrome "A", where QNH is given as 1020mb, to aerodrome "B", where QNH is given as 999mb. Aerodrome "A" is 800ft above MSL and aerodrome "B" is 500ft above MSL. If the altimeter sub scale is not changed from 1020, what is the altimeter indication on landing? (1mb = 30ft) Well, my stab at it is this: On the ground at Aerodrome 'A', the altimeter would read the elevation of the airfield - 800ft as the altimeter is set to aerodrome A's QNH value. The change in pressure between aerodrome 'A' and aerodrome 'B' is -21mb and where 1mb = 30ft that would be 630ft change. Because it's going to a lower pressure, that would be +630ft. So the altimeter would register the change of pressure and read 800ft + 630ft = 1430ft. The only thing is the stated answer to the question is 1130ft so I've got something wrong here - probably discounting the height of aerodrome B above MSL. Any ideas? You missed that B is 300 ft lower than A. So if there were no change in pressure then at landing your altimeter would read 500ft MSL But because of the lower pressure it is 500ft + 630ft = 1130ft. jue Back to top Paul Banks Guest Posted: Sat May 08, 2004 4:53 pm    Post subject: Re: Groundschool Question... Jürgen, Thanks for this. I understand this much better now. Paul PPL Student 2hrs, England, UK From: "Jürgen Exner" <jurgenex (AT) hotmail (DOT) com> Subject: Re: Groundschool Question... Date: 08 May 2004 16:14 news.btinternet.com wrote: Quote: Hi, I have a question which I'm having trouble answering, anyone care to help? An aircraft flies from aerodrome "A", where QNH is given as 1020mb, to aerodrome "B", where QNH is given as 999mb. Aerodrome "A" is 800ft above MSL and aerodrome "B" is 500ft above MSL. If the altimeter sub scale is not changed from 1020, what is the altimeter indication on landing? (1mb = 30ft) Well, my stab at it is this: On the ground at Aerodrome 'A', the altimeter would read the elevation of the airfield - 800ft as the altimeter is set to aerodrome A's QNH value. The change in pressure between aerodrome 'A' and aerodrome 'B' is -21mb and where 1mb = 30ft that would be 630ft change. Because it's going to a lower pressure, that would be +630ft. So the altimeter would register the change of pressure and read 800ft + 630ft = 1430ft. The only thing is the stated answer to the question is 1130ft so I've got something wrong here - probably discounting the height of aerodrome B above MSL. Any ideas? You missed that B is 300 ft lower than A. So if there were no change in pressure then at landing your altimeter would read 500ft MSL But because of the lower pressure it is 500ft + 630ft = 1130ft. jue Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AirTalk.org Forum Index -> Aviation (Students) All times are GMT Page 1 of 1   Jump to: Select a forum Aviation----------------alt.aviationAviation safety EnhancementPlanes, gliders and flying in AustraliaAviation et pilotage (Français)rec.aviationAviation (Aerobatics)Hot air, gas balloons and airshipsHang-gliding, paragliding, foot-launched flightAircraft Design & BuildingInstrument Flight RulesAviation (Classifieds)Military AircraftNaval AviationAviation (Miscellaneous Topics)Owning AirplanesAviators (General Discussions)Aviation related productsAircraft RestorationHelicopters and rotary wing aircraftsFlight SimulationSailplanesAviation (Students)Light aircraftFlying related activities in the UK Airports----------------alt.airportsEdinburgh Airport Glasgow Airport Humberside Airport Luton Airport Aberdeen Airport Glasgow Prestwick Airport  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.