GRID MORA
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GRID MORA
Quick question for you airline types.
RAPID D over mountainous terrain. Do you have to cold weather correct GRID MORA. simple YES or NO. But, you must show where it is written and the method for making such a correction if your answer is yes. Use this route as the basis for your answer.
London - Vancouver. RAPID D occurs over Greenland not under radar control. Unable to reach Iceland on HF.
AND.........GO.
RAPID D over mountainous terrain. Do you have to cold weather correct GRID MORA. simple YES or NO. But, you must show where it is written and the method for making such a correction if your answer is yes. Use this route as the basis for your answer.
London - Vancouver. RAPID D occurs over Greenland not under radar control. Unable to reach Iceland on HF.
AND.........GO.
Does your Jepp chart change the Grid MORA change when it's cold? No? Then you should 'amend' it.
Any cold error correction is going to be a rough guess anyway as you don't know (normally) the ground elevation which can be around 10k over the Greenland Plateau or sea level with 10k peaks along the coast and the exact correction is dependant on the column of air height, and the temp at the ground station (which you don't have).
As a quick and dirty rule of thumb when heading down, i'd look at my ISA D, times it by the published MORA (in thousands) then times by 4 and add that to the published MORA.
ie. ISA-20, published MORA 12k. 20*4 = 80. 12*8=96. Add the zero=960. I'd level out at 13k and once the ECAMs sorted and follow up plan of attack, then refine further by taking the temp lapse rate for Greenland, and multiplying it by the air pressure of the aircraft. Then add 1 to this result and divide by the sum of the lapse rate times the pressure at ground level times 15. Take the log of the result, Multiply by ISAD and then divide by the lapse rate. Add this to the MORA.
PS. I've assumed Jepp's Grid Mora, giving 2000' clearance in Greenland. And if you want references, check the bit in the Jepps where is says to correct altitudes in cold weather, referencing the (obsolete?) ICAO 8186 in all circumstances except radar control... Airbus have similar opinions, giving low altitude calculations and a high(er) altitude temp correction graph, more fine tuned but something you can't commit to memory.
Any cold error correction is going to be a rough guess anyway as you don't know (normally) the ground elevation which can be around 10k over the Greenland Plateau or sea level with 10k peaks along the coast and the exact correction is dependant on the column of air height, and the temp at the ground station (which you don't have).
As a quick and dirty rule of thumb when heading down, i'd look at my ISA D, times it by the published MORA (in thousands) then times by 4 and add that to the published MORA.
ie. ISA-20, published MORA 12k. 20*4 = 80. 12*8=96. Add the zero=960. I'd level out at 13k and once the ECAMs sorted and follow up plan of attack, then refine further by taking the temp lapse rate for Greenland, and multiplying it by the air pressure of the aircraft. Then add 1 to this result and divide by the sum of the lapse rate times the pressure at ground level times 15. Take the log of the result, Multiply by ISAD and then divide by the lapse rate. Add this to the MORA.
PS. I've assumed Jepp's Grid Mora, giving 2000' clearance in Greenland. And if you want references, check the bit in the Jepps where is says to correct altitudes in cold weather, referencing the (obsolete?) ICAO 8186 in all circumstances except radar control... Airbus have similar opinions, giving low altitude calculations and a high(er) altitude temp correction graph, more fine tuned but something you can't commit to memory.
Last edited by compressor stall; 26th Sep 2014 at 14:10.
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Try "Air Traffic Control">DOC8168>Altimiter setting procedures
Don't have the paragraph in front of me, but it says that unless you are under radar vectors, you should correct all MOC altitudes.... Or something along those lines
Don't have the paragraph in front of me, but it says that unless you are under radar vectors, you should correct all MOC altitudes.... Or something along those lines
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To all thank you so far. I do realise the requirement for cold temp correction. I guess what I'm getting at is, everybody kind of has their own thing. You know, 4% here this chart here etc. But no where is it definitively written to follow any certain procedures. If it is written that what I'm looking for. Come on guys im a pilot, if it ain't written in black and white, it doesn't exist.
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If you are a pilot, then apply common sense. Simple question - Is a chart corrected for below ISA temperatures and below standard pressures, or does is state elevation as being above MSL? If you believe the former then do nothing. If not, make some corrections. We use 40 feet per degree below ISA plus 30 feet per HPa below standard. We typically over-correct because we correct for the entire column of air between or cruising level and MSL and not just the layer between us and the ground.
CS - You missed the bit where do you take away the square root of the number you first thought of.
CS - You missed the bit where do you take away the square root of the number you first thought of.
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You shouldn't have to see it written down to decide whether or not to make a temperature correction. It gets down to -30 in Alaska and there are several 20,000 foot(or nearly so) mountains within an hour of Anchorage. If you don't make a correction, you may have a serious problem because those mountains do exist(written in black and white). Fortunately in this case, significantly lower terrain is nearby. Might as well have TERRAIN selected on one of the ND's as there are not many thunderstorms in that region.
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Originally Posted by richardhead
Come on guys im a pilot, if it ain't written in black and white, it doesn't exist.
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Ok. Yes of course there are many little tricks, "rules of thumbs" etc etc....thank you. What boggles my mind is. For something as important as obstacle clearance in the event of a Rapid decompression there is nothing written defining a procedure to be used. Now before you go off again, I know what I would do I know what half of Pprune would do. What I am looking for is a procedure in writing. 
everything is defined in this business, so trust me when the lawyers come looking for the Captain and the answer to their question is, " well we just use a rule of thumb, or we basically guess." You better have plenty of time and money on your hands. I just can't believe there is nothing written even on a Jeppesen chart or manual.....
Thanks all for your input, honestly.
everything is defined in this business, so trust me when the lawyers come looking for the Captain and the answer to their question is, " well we just use a rule of thumb, or we basically guess." You better have plenty of time and money on your hands. I just can't believe there is nothing written even on a Jeppesen chart or manual.....Thanks all for your input, honestly.
Richardhead
But there is! 
It then lists the values. It's all in the aforementioned reference.
Pittdown man
That tongue in cheek process really is actually the most accurate formula for working out cold temperature correction. It even takes into account the variation in the value of 'g' with altitude. ICAO doc 8186 refers.
I just can't believe there is nothing written even on a Jeppesen chart or manual.....
Requirement for Temperature Correction
The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10° C below standard tempera-ture as measured at the altimeter setting source. This is safe for all altimeter setting source alti-tudes for temperatures above -15°
For colder temperatures, a more accurate correction should be obtained from Tables 1 and 2 below. These tables are calculated for a sea level aerodrome. They are therefore conservative when applied at higher aerodromes.
NOTE 1: The corrections have been rounded up to the next 5m or 10 ft increment.
NOTE 2: Temperature values from the reporting station (normally the aerodrome) nearest to the position of the aircraft should be used.
The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10° C below standard tempera-ture as measured at the altimeter setting source. This is safe for all altimeter setting source alti-tudes for temperatures above -15°
For colder temperatures, a more accurate correction should be obtained from Tables 1 and 2 below. These tables are calculated for a sea level aerodrome. They are therefore conservative when applied at higher aerodromes.
NOTE 1: The corrections have been rounded up to the next 5m or 10 ft increment.
NOTE 2: Temperature values from the reporting station (normally the aerodrome) nearest to the position of the aircraft should be used.
Pittdown man
That tongue in cheek process really is actually the most accurate formula for working out cold temperature correction. It even takes into account the variation in the value of 'g' with altitude. ICAO doc 8186 refers.
