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Altimeter at high altitude?

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Hello all,I am probably missing something can't figure out what. Here it goes:After passing 18000 ft, I press on STD and the alt gets reset to 29.92 as it should. I manually reset the standby one to same. When flying on VATSIM I get reports of being off by 2-400 ft from declared cruising altitude.If I press "B" to reset it via FSX the barometer becomes 30-something and after descending a few hundred feet I get to the "real" flight level.I would rather use the NGX controlls and not see 30.25 at FL360... What am I missing?Thanks

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Are you using Radio or Barometer when your at cruise? Also there is an option in the NGX FMC where you can Sync the Captain, First Officer and Standby Barometers. (If you change one they all change) Since you said you had to change the Standby one that makes me think you do not have the sync option enabled. If don't then you need to change all 3 manually.Captains SideFirst Officers SideStandy

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Thanks Paul for taking the time to chat me on VATSIM and assist. I got the right procedure to follow, please follow up with any additional info when available.Best regards,

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Hi guys we finally figured out 3 things. First his Barometers were not synched.Second he was wanting to know why when his aircraft is set at FL370 for example why on Vatsim it shows you at say 37,373 and not exactly 37,000Also he was not aware that you should have the Radio/Baro Knob in the Radio position on approach.As far as I know you want the Radio when on approach so you know the exact height your above the airport. But at what altitude do you switch from Baro to Radio and vice versa? Im guessing that you would do it at the transition altitude of 18,000 in the US and 6000 in Europe?Can somebody explain this in detail for us? I know how it works I just do not know the exact procedures.Thanks

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I always keep mine on baro and only change to radio if doing a cat 2 or 3 approach. I don't think radio or baro has any effect when at cruise altitude. Its only for setting mda on approach.


Sean Green

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Hi guys we finally figured out 3 things. First his Barometers were not synched.Second he was wanting to know why when his aircraft is set at FL370 for example why on Vatsim it shows you at say 37,373 and not exactly 37,000Also he was not aware that you should have the Radio/Baro Knob in the Radio position on approach.As far as I know you want the Radio when on approach so you know the exact height your above the airport. But at what altitude do you switch from Baro to Radio and vice versa? Im guessing that you would do it at the transition altitude of 18,000 in the US and 6000 in Europe?Can somebody explain this in detail for us? I know how it works I just do not know the exact procedures.Thanks
In answer to the first question - that is perfectly normal. Altimeters are internally calibrated such that their reading of height above sea level is only exactly accurate when the atmospheric pressure is 29.92 inches of mercury. (or 1013 millibars, also known as "hectopascals".) Since the actual atmoshperic pressure is rarely exactly 29.92 in/hg on any given day or hour, the altimeter has the ability to be externally corrected by setting the ACTUAL pressure at the moment into the unit - the "altimeter setting" always given to landing or departing aircraft by the tower - also known as QNH.By setting the actual current pressure, the altimeter's reading is compensated to read the the true MSL altitude, at the aircraft's current location, at whatever current atmospheric pressure exists at that time.Imagine that the aircraft is parked on the ramp at an airport which is exactly 1000 feet MSL. Imagine also, that the current pressure really is exactly 29.92, and that the barometric setting in the altimeter has been set to 29.92. In this scenario, the altimeter will read... yes, 1000 feet exacly.Now, without changing anything in the aircraft, picture the situation if the atmospheric pressure goes up - say from 29.92 to 30.30 in/hg. This could easily happen if a cold front has passed, and colder, denser air is moving into the area. What do you suppose the altimeter will do? (Remember, it is still set to 29.92).The altimeter reading will go down by several hundred feet. The higher-pressure air makes the altimeter "think" that it has descended, since in reality, an altimeter is just a sensitive air pressure gauge. Now, if the pilot resets the barometric setting on the altimeter to match the actual pressure outside the aircraft (30.30 in/hg) the altimeter wil once again read the correct altitude of 1000 feet.Just the opposite will happen if the actual pressure decreases below 29.92... say down to 29.40 in/hg. In this instance, the altimeter will read higher than 1000 feet, since the falling pressure is interpreted by the altimeter as an ascent. Once again, when the barometric knob is set to the match the actual atmosheric pressure, the altimeter will once again read correctly.This is why it is extremely important to always make sure that the altimeter is set to the current barometric pressure of the nearest airport when preparing to land or takeoff. Obviously, an error of several hundred feet could be deadly when operating IFR with no view of outside terrain.Once up in cruise at higher altitudes, however, knowing ones exact height above the ground is less important than knowing that all aircraft operating in the same airspace are all using the SAME barometric setting on their individual altimeters. As long as they all set their barometric setting to 29.92 in/hg, then even though individual altimeters might be several hundred feet "off" in their reading of true altitude above sea level - they will all be "off" by the same amount - so that two aircraft passing in opposite directions with a thousand foot separation truly will be 1000 feet apart.By the way, the altimeter reading when the barometric knob is set to 29.92 is called "pressure altitude".When you see someone on Vatsim who is flightplanned for FL 330, but showing an altitude of 33,350 you can be sure of three things. (1.) The pilot (correctly) has his altimeter set to the STD setting of 29.92 (2.) The pilot is using ASE, REX or some other program that injects real weather into the sim. (3.) The actual pressure outside the aircaft being injected by the weather program is something other than 29.92 in/hgThe reason that the error shows up in the transponder mode C reading on Vatsim is due to a the fact that both FS9 and FSX transmit the aircraft's true MSL altitude in transponder mode C. In a real aircraft, the mode C altitude encoder is "fixed" at 29.92 at all times, so when operating above the transition altitude (where all altimeters are set to the STD 29.92 in/hg) the mode C should be the same as what what the pilot sees on his altimeter in the cockpit.Now as for the second question - the radio/baro knob has nothing to do with the aircraft's reported altitude on ATC radar, or what the pilot sees on the altimeter in the cockpit - it simply selects whether the "minimums" callout on an instrument approach will be based on MSL altitude from the altimeter (in the case of a non-precision approach, or a CAT I ILS), or will be based upon actual height above ground, as measured by the radar altimeter (only used during a CAT II or CAT III ILS).

Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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That's why I never hear minimums call out when landing in fog at Zurich the last few nights, should be fixed now :-)


-Iain Watson-

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Thanks Jim for the amazing answer,Now this all makes much more sense. I did not include REX in my logic.... Much appreciated!Florin

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Just for info, radio altitude become "alive" at 2500 fts (radio altimiter works continously, the data is shown only at 2500 ft of radio altitude), so, if you want to use the radio minimums and you are checking at wich altitude you want to do, you can use this altitude or below. But, as it does not affect any information pre, during or post flight, you can set it to baro or radio whenever you want. Normally I think it depends on the approach you do, as you are not sure of what you will do on final, when you set the minimums, set also if you want a baro or radio one.


Regards

Andrea Daviero

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Went looking for a more info on this and came across the article below which gets even more technical. Yeah its for the Airbus but it applys to all aircraft and is a good read. http://www.airbus.co..._TECH-SEQ01.pdfI still can't believe how far off on all this I was for a simple little thing such as an altimeter lol. Yeah I know how to set it but it gets a little technical and complex when you got Boeing, Mc Donnell Douglas and Airbus. Somewhere somebody told me long ago to use QFE when landing or doing Circuits and QNH rest of the time. So In the NGX I just assumed Radio as QFE and Baro as QNH?To make this a little simpler unless the chart specifies otherwise your are better off leaving the NGX at BARO and the MD-11 at QNH?

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Regarding the Baro vs Radio, here is the deal:When performing ILS approaches, the charts will give you a DH in feet above threshold elevation. To set up for this, you switch to radio and set it to the documented decision height (usually ~200 feet).When performing VOR or LDA approaches, you get an MDA (Minimum Descent Altitude) instead of a DH, which is a BARO altitude (MSL) you should not descend below unless you have a visual on the runway. So when flying a VOR approach (for example), you set the knob to BARO and punch in the required MDA. This will give you the "Minimums" callout at MDA, and you can then land or go around as required.Hope that clears this up._O.K._

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So then I was correct and you should use Radio and QFE for ILS approaches?
Not exactly.ILS Cat I uses baro (since you reach minimums relatively far out from the runway, and a cat I ILS does not guarantee that the terrain in front of the runway will be "flat" enough to make a radio dh meaningful). ILS Cat II and III use radio.As for QFE, this is never used in IFR operations, at least not in the US.---Martin Boehme

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And to make you even more puzzled, you would normally only use Radio Mins on ILS with CAT II or CAT III (low visibility procedures), as with CAT I minimum is 200 feet, and that being a bit off the runway, terrain can be somewhat off.But this really has nothing to do with setting the "altimeter" right, other than baro minimums will get their numbers from it.Now back to QNH QNE and QFE.QFE is the altimeter setting that will give you "0" when on the runway. Useful for pattern work maybe, but not much else (unless you are in russia, where it is used in place of QNH).QNH is what is used in most of the world in altitudes closer to ground and would show you your altitude above mean sea level (in standard atmosphere), that is, it is corrected for weather influence - pressure systems - what your weatherman talks about.QNE, or Standard, also the STD button in NG, is 29,92 InHg, or 762mmHg, or 1013(,25) hPa or mBar, is what is used above Transition Altitude, so that all aircraft use the same setting worldwide. Theoretically you could use QNH in place of it, but that would mean you would have to change the altimeter OFTEN, like every couple minutes, since you travel fast on cruise level. Not an issue on approach, sincea) you travel slowerB) you generally only get down in close vicinity of the airportc) you benefit from terrain clearance, that is not an issue in cruise.Now there are some more pressure settings, corrected for temperature and such, but to piltos, those are not important to know.

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