Nav Error ????
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Nav Error ????
Question :
Flying back the other day from HKG to EWR via polar routing the aircraft (a B777-200) flew directly over the top of the Resolute Bay (CYRB) airport (a rare day when you can actually see it visually) yet the Nav Display depicted the airport (not the navaid) as being about 12 nm further in front of us - why would that be ?????
The ND indicated that GPS was updating position at that time and the nav data base indicated that it was current. This concerns me as what if God forbid we had needed to find the place for an emergency landing.
Flying back the other day from HKG to EWR via polar routing the aircraft (a B777-200) flew directly over the top of the Resolute Bay (CYRB) airport (a rare day when you can actually see it visually) yet the Nav Display depicted the airport (not the navaid) as being about 12 nm further in front of us - why would that be ?????
The ND indicated that GPS was updating position at that time and the nav data base indicated that it was current. This concerns me as what if God forbid we had needed to find the place for an emergency landing.
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poser indeed
6nm up and a ground distance of 12nm? That's a cone with a half-angle of 63 degrees at the apex. Or to put it another way 12nm out in 4800nm travelled is 0.25%.
I thought Resolute Bay was in Nunavut and only a dirt strip.
I seem to recall that Boeing positions change calculation mode at about 84deg North depending on model.
Continental do this run, that you?
I thought Resolute Bay was in Nunavut and only a dirt strip.
I seem to recall that Boeing positions change calculation mode at about 84deg North depending on model.
Continental do this run, that you?
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yeah
I thought so. But it may "qualify" to appear on the nav display if the sole restriction is length. I'd guess you'd have to be pretty Resolute to go there. Like a hill I climbed in Sitka called Bear Mountain. It had bears. Boeing did do a polar route article in their aero puff mag and CYRB was definitely not on the list of places for a divert! I think Shemya, Cold Bay and King Salmon were "approved" but I'd like to sit down and talk with the approver.
PS I wondered if there was a distant navaid. At N74 44.8 W094 59.7 there was an NDB, formerly a tall mast structure. At N74 43.6 W094 55.3 there is a VOR c/s YBR but neither of these is "distant" from the airstrip. Both show up on google maps with positional errors easily accounted for by the single decimal accuracy in minutes. The NDB may be there still but I have a notion it blew over or some such mishap.
PS I wondered if there was a distant navaid. At N74 44.8 W094 59.7 there was an NDB, formerly a tall mast structure. At N74 43.6 W094 55.3 there is a VOR c/s YBR but neither of these is "distant" from the airstrip. Both show up on google maps with positional errors easily accounted for by the single decimal accuracy in minutes. The NDB may be there still but I have a notion it blew over or some such mishap.
Last edited by mathy; 13th Aug 2010 at 14:06.
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Makes me wonder ?
Originally Posted by mathy
6nm up and a ground distance of 12nm? That's a cone with a half-angle of 63 degrees at the apex. Or to put it another way 12nm out in 4800nm travelled is 0.25%
As a "rule of thumb", as something disappears from view under the nose, it is one mile away for every thousand feet of altitude - so cruising at F380, something on the ground disappearing under the nose is 38 miles away. 12 miles is nothing at high altitudes.
Aviation Herald coverage here:
Crash: First Air B732 near Resolute Bay on Aug 20th 2011, impacted terrain
Airspeedintervention, your concern about having experienced a possible nav error and expressing that concern here then posting again having learned of the Resolute Bay tragedy, is commendable. The explanation given by other posters of the seeming discrepancy is probably correct, but so was your willingness to put your concern to your peers. One day that willingness could save lives, never lose it!
Crash: First Air B732 near Resolute Bay on Aug 20th 2011, impacted terrain
Airspeedintervention, your concern about having experienced a possible nav error and expressing that concern here then posting again having learned of the Resolute Bay tragedy, is commendable. The explanation given by other posters of the seeming discrepancy is probably correct, but so was your willingness to put your concern to your peers. One day that willingness could save lives, never lose it!
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Just a side note: When I'm driving 121mph and I'm exactly underneath a bridge, my GPS map still shows the bridge in front of me. There is a GPS graphics lag at high ground speeds. . . even on our glass cockpit screens! 
