Topic Author
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Is This Serious? Is It Possible?

Wed Apr 04, 2001 11:39 pm

Look at what I read on a newspaper yesterday:

It was an article having to do with the longest commercial route, JFK-HKG. Since it is a polar route, the journalist wrote that:
1.- During the polar flight, since it was above the pole, the autopilot is turned off because the computers on board would not know where to fly to. This sounds logical, but, is it imperative to fly exactly above the magnetic pole? Or could it be done some dozens of miles away? Would then the computers be affected by the unability to decide?

2.- Due to extreme cold while over the pole, the pilots must have a continuous control of the temperature in order to prevent fuel from freezing. If the temperature descend a bit too much, they increase the speed... in order to increase air drag... so the wing temperature raises!!!!
Altough I could be making a mistake, I'm sorry but this sounds to me like ignorant-journalist-pretending-to-be-specialist's b*llsh*t. You know what I mean: he/she has read about drag, temperatures resulting from it, maybe something about the temperature in a supersonic airplane's nose, made a cocktail with polar ground winter temperatures and liquids density in the cold, and take his/her own (logical?) conclusions...
Actually I have never flown over the pole, and cannot imagine how cold is the air there at FL390. But I've found myself flying over the Capricorn Tropic during southern summer, FL410, with outside temperature —53ºC, and in whatever season, FL320 to FL370, usual temperatures being —37ºC to —45ºC.

Professional opinions?

Thanks a lot in advance.

Best turbulences
Posts: 372
Joined: Wed Mar 21, 2001 12:53 am

RE: Is This Serious? Is It Possible?

Thu Apr 05, 2001 12:46 am

Hi Turbulence,
I agree with you to some extent. I have made several polar flights as a Flight Engineer, and the autopilot issue seems to have been greatly "dumbed down" by the reporter. The acft I fly switches to a "Grid Mode" when operating above or below 70' N/S lat to accommodate longitude convergence above that level. The autopilot off may be a precaution for an Inertial Navigation System (INS); they tend to get a little confused when you cross geographic hemispheres, and poles (i.e. true north). Magnetic reference systems get confused around the poles. We used INS and monitored with GPS, and autopilot worked just fine.
As far as the fuel issue is concerned, the fuel grade determines its freezing point, but it never really freezes, it just turns to”slush" or "jelly", making it difficult to pump. The procedure we use, regardless of location, is if the fuel temp remains below it's freezing temp for more than 2 hours, we operate all pumps, and intentionally transfer between tanks to maintain a desirable viscosity. If it is cold enough to cause a problem, you will no long before engine flameout by monitoring fuel pressure and filter differential pressure. The statement about descending to increase fuel temp is partially true. According to my performance books, you would have much better luck descending than speeding up, simply increasing speed will increase fuel consumption more than descending to a warmer but less efficient altitude.
The coldest I have ever seen was at 390 around Thule, Greenland- around -72c.

"Never trust a clean Crew Chief"
Posts: 201
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RE: Is This Serious? Is It Possible?

Thu Apr 05, 2001 2:44 am

I could be wrong but here it goes.

1. Large planes generally navigate using GPS,inertial guidance systems, gyros etc. not a magnetic compass so I don't see what effect the (Magnetic) North pole would have on these systems. Frankly I don't think any.

2. The outside air temp in the Flight Levels probably isn't any different whether you are at the North (or South) Pole or the Equator, which is pretty darn cold as you mentioned.

RE: Lower Alt. Over N. Pole

Thu Apr 05, 2001 8:49 am

I do know that pilots will usually fly @ lower flight levels (FL310, FL330) while over the North Atlantic and while flying over the Polar Routes to prevent ice freezing, I guess???

RE: Is This Serious? Is It Possible?

Thu Apr 05, 2001 7:37 pm


"The outside air temp in the Flight Levels probably isn't any different whether you are at the North (or South) Pole or the Equator, which is pretty darn cold as you mentioned. "

I think this is not true. Temperatures at higher altitudes relate to the ground temperature underneath them and decrease at a constant rate going higher, relative to ground temp. That is why it would make sense, like Jet Joc said, for aircraft to fly lower above the Poles...


RE: Is This Serious? Is It Possible?

Thu Apr 05, 2001 8:39 pm

Hi all,Regarding the fuel temp subject :
-1/although the outside temperature at fl 390 is fairly predictable around -50 c,you could still find bubbles of very cold air,especially on extreme latitudes and sometimes at quite lower ones ;I remember a -80 c a few years ago over Turkey.
-2/the problem is with long range airliners that have been cold soaked for hours at altitude with their fuel temp steadily going down to the level of the OAT+Ram rise pertaining to their cruising Mach number.To illustrate the point,let's take an airplane flying M.85 in a -60 c atmosphere.The temp rise equals 31 deg,so the fuel temp will be,after a time,-29 c.In order to reach the fuel freezing point @ -46 c,the OAT should be -74 c,which is not uncommon over Siberia ;Of course,the crew would have long done something before reaching that fuel temp extreme.Going down to a warmer altitude is probably the safest solution as the benefit of increasing the Mach to .87 would only improve the temp rise by one or two degrees.
Still using my old trusted CR-3 computer,an airplane cruising at M.80 would have the same problem happening at OAT= -71 c.

As for the Nav problem,the point is when you fly right over the pole,you have a discontinuity in your nav data.But more important,as your individual units are probably not exactly coherent,you'd find yourself with a big conflict on your hands Embarrassmentne IRS going south another still going north with Irs longitudes that could be 180 degrees apart.That's why one does'nt fly right over the pole but some 10~15 degrees off it.

Very interesting thread.
Cheers all
CX Flyboy
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RE: Is This Serious? Is It Possible?

Fri Apr 06, 2001 2:53 am

We have had a Operations Manual Bulletin issued by Boeing to us about polar operations, basically, they state:

Number CAT-50(R6)
Date January 1, 1999

Background Info. Part A (B-HOO through B-HOU)

With LNAV and an autopilot engaged, airplanes in service have experienced a roll of up to 30 degrees bank angle to the left and right as the North Pole is passed. Investigation has shown this is caused by a momentary error in the FMC computed desired course as the polar waypoint is passed. Flight testing indicates no undesired steering commands occur if the airplane is operated between 89 degrees North and 89 degrees South latitude.

In addition, erroneous wind indications have been observed on the ND and the EICAS advisory NO LAND 3 and NO AUTOLAND have been displayed on different occasions in the region of the North Pole. This may occur when the IRUs go through the polar heading reversal at different times, causing the AFDS to report a heading fault.

These messages may also be displayed when the airplane position is north of 73 degrees North latitude or south of 60 degrees south latitude and the Heading Reference Switch is moved from True to Norm.

It is possible these messages may disappear from EICAS, or never appear on EICAS, with the fault still latched in AFDS. In this case, LAND 3, LAND 2, or NO AUTOLAND may be annunciated on final approach and subsequently the autopilot may disconnect. The latched fault can be completely cleared by simultaneously cycling the sutopilot and both flight directors off then on after all 3 IRU positions are out of the polar regions.

(Most of the next part is the same for B-HOV and on, and B-HUA and on, except for this paragraph.....)

The Honeywell IRU Navigation Performance Upgrade Program (NavPUP), in conjunction with upgraded flight control computers, corrects these anomalies and allows unrestricted polar navigation. Existing fleet airplanes can be modified by Boeing Service Bulletin 747-34-2443.

Operating Instructions, Part A.

Do not operate north of 89 degrees north latitude and south of 89 degrees south latitude.

Anytime the airplane is operated north of 73 degrees north and south of 60 degrees south, disconnect the autopilot and simultaneously turn both flight directors off momentarily to clear a possible fault after all 3 IRU positions are outside the polar region ( across 73 north or 60 south).

Operating instructions Part B. (Newer aircraft)

Do not operate north of 89 degrees north latitude and south of 89 degrees south latitude.

Disregard display of FMC computed winds when near a pole.
Wow, typing that tired me out!
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RE: Is This Serious? Is It Possible?

Sat Apr 07, 2001 12:09 am

The issue of the fuel is a concern primarily from flights from the US over the pole. There are primarily two types of jet fuel used in commercial service: Jet A and Jet A1. Jet A is the primary fuel that is distributed in North America. Jet A1 is distributed throughout the rest of the world. Jet A has a higher freeze point than Jet A1. The freeze point on jet fuel is not when it turns to a solid block of kerosene, but the temperature at which the waxy paraffins go back into solution. This effects those airlines that are operating between the US to HKG over the pole. The concern is that the boost pumps or fuel controls could become clogged and cause a loss of power. The operators have set up equipment that can measure the freeze point of the actual fuel that is loaded into the airplane. With the dispatch paperwork, the crew is given a slip that has the fuel's freeze point. The crew must monitor the fuel temperature during the flight and if it gets within 4 deg C of the freeze point, they must take some action. The action taken may be to speed up, descend, or even alter course.

RE: Is This Serious? Is It Possible?

Sat Apr 07, 2001 1:29 am

Pardon my ignorance on this, but a simple question -

What about a Fuel Heater?

If the primary issue of cold weather fuel use is paraffin solids clogging the fuel lines or filter, why not affix a small electrical resistance heater to the outside of the fuel line near the initial tank exit point? This could heat the fuel up by 10 or 20 degrees, enough to lower any worrisome issues over fuel solidification.

Also, What about fuel additives?

I used to own a diesel car (1982 Olds 98), and had problems with cold-weather fuel solidification (It stalled out at -15 degrees f). The truck stop had an additive that would lower the operating temperature range, which I was told was just pure Kerosene. Would this not solve the issue as well?

-- Kevin

Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

RE: Is This Serious? Is It Possible?

Sat Apr 07, 2001 4:15 pm

Yep, there is a very real concern for fuel "freezing" on polar routes. I've had the opportunity to see the results of test flights conducted by 777 operators for these routes.

If the fuel gets too cold, the solution is to lose some altitude. If the fuel temperature is suspect, the crew is to assume the total air temperature is the fuel temp.

As for the heater, the 777 has a fuel/oil heat exchanger.

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