QNH vs. QNE

[hawkeyeted]

"I was flying into Almaty and noticed on the IAP that there were two altimeter figures; one for QNH, the other QNE. I understand QNH to be the MSL altitude having compensated for atmospheric pressure changes. I also understand that QNE is the altitude in "standard" pressure (1013/29.92). (All altimeter settings are QNE once above transition altitude/FL.) I'm a little perplexed why list QNE as an available altimeter setting in the approach. I'm also not sure why the significant difference in MDA based on the altimeter setting. (I think the the QNH MDA was ~5000 ft, but the QNE MDA was ~3000 ft). I ALWAYS land using QNH, but can someone much smarter than me tell me what the heck is going on?"

[mvm]

Hey Ted all my charts for Almaty indicate QFE not QNE...i would think flying around that close to the ground QNE may get you into big trouble...(CFIT)...as far as the differences Ted QFE will cause your altimeter to show airfield pressure hence your altimeter will indicate 0 on touch down...so when the chart shows a QFE in meters(800 on the chart) take that number multiply by 3.28 for feet...in this case 2624ft then add the field elevation to that..in Almaty's case 2234ft which gives us a total of 4848ft which is pretty close to the QNH in feet of 4900 as the MDA listed for the ils entry for rwy 05 someone can correct me if i'm wrong here but i know many Russian airfields require QFE settings on approach i think they believe it gives them at least an approximation of a radio altimeter sometimes with tragic results in the past especially when the PIC forgets to reset to QNH on a go around!!! ..i hope i explained myself sometimes i have trouble explaining things...thank god i'm not a teacher

[hawkeyeted]

"Mike, I went back to my charts and checked. You are correct, they are QFE not QNE. That said, does that mean a QFE approach is similar in fasion to the radio altimeter, meaning that although you are using the barometeric altimeter, there altimeter is referencing AGL vice MSL? That would seem to be the case since your altimeter would read "0" vice airport elevation? (Do I have that right?)"

[dave colavecchio]

"You are correct that QFE will have your altimeter read 0 upon landing. Some airlines still use this method. The conversion can be tricky if you make an error. I think it's more of a military thing. American Airlines used it (might still) until one night while landing an MD-80 at Bradley International Airport where I work. http://www.ntsb.gov/ntsb/brief.asp?ev_i ... 4839&key=1 What the report doesn't tell is that the winds were 160@45 and the pressure was dropping like a stone.(The tower controllers abandoned the cab because they were scared by the wind strength) Their is no ILS to 15 at Bradley. They have ILS's to 33, 6, and 24. They made the conversion 20 minutes out and by the time they reached the airport it was 76 feet off. The hit trees on top of a ridge about 2 miles from the threshold.The starboard engine turned into a wood chipper and failed. The gear was damaged. The aircraft landed (crashed) about 1/4 mile from the threshold and came to rest on the runway after crashing through the 33 ils antenna. A couple of shear moments of terror for the crew and passengers. The gear, leading edge, engines were all replaced and the aircraft was returned to service."

[mvm]

"That is quite correct Ted...QFE references AGL for the airfield that the altimeter setting was issued for..and QNH is referenced/corrected to sea level and i know from past experience that many european countries will still issue QFE settings when you approach the transition level and as Dave has mentioned and i also hinted at.. many airliners have experienced this type of mishap with QFE settings....as a Russian pilot i knew once stated and i'm quoting here "Assuming an error of "QNH set at TL" in a QFE operation during emergency divert scenario, the result shall be "staying high equal to field elevation""