An air carrier flight crew suspects that the driftdown calculations for the route to bjx may be faul...

Attributes
ACN	342887
Time
Date	199608
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : nld
State Reference	FO
Altitude	msl bound lower : 31000 msl bound upper : 33000
Aircraft 1
Controlling Facilities	artcc : mmex
Operator	common carrier : air carrier
Make Model Name	Fokker 100
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	342887
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Chart Or Publication
Air Traffic Incident	other

Narrative:

Location: over nld, mexico. Flts to bjx from dfw have an en route driftdown decision point that is just 8 mi south of nld. This seems pretty useless to me as the terrain that would be a factor in the event of an engine failure starts at mty and would continue to be a factor until near slp. If terrain is not a factor, then why have the decision point? If it is a factor, the decision point should be between mty and slp where it would be of use. Callback conversation with reporter revealed the following information: the reporter said that his company had recently been using a computer generated driftdown calculation that is occasionally erroneous. In this case, the decision line is drawn before the aircraft would get over any high terrain. The captain inquired of his company if this meant that the high terrain south of the present go/no-go line was not a problem for the F100 aircraft at normal cruise altitudes of FL310 and FL330. He has not received an answer yet. The minimum off-route altitude along this portion of the route is as high as 14200 ft near slp and 12800 ft at his destination, bjx. The reporter said that his normal single engine cruise altitude on a standard day is around 21000 ft.

Original NASA ASRS Text

Title: AN ACR FLC SUSPECTS THAT THE DRIFTDOWN CALCULATIONS FOR THE RTE TO BJX MAY BE FAULTY BECAUSE THE DECISION LINE IS DRAWN BEFORE HIGH TERRAIN IS ENCOUNTERED.

Narrative: LOCATION: OVER NLD, MEXICO. FLTS TO BJX FROM DFW HAVE AN ENRTE DRIFTDOWN DECISION POINT THAT IS JUST 8 MI S OF NLD. THIS SEEMS PRETTY USELESS TO ME AS THE TERRAIN THAT WOULD BE A FACTOR IN THE EVENT OF AN ENG FAILURE STARTS AT MTY AND WOULD CONTINUE TO BE A FACTOR UNTIL NEAR SLP. IF TERRAIN IS NOT A FACTOR, THEN WHY HAVE THE DECISION POINT? IF IT IS A FACTOR, THE DECISION POINT SHOULD BE BTWN MTY AND SLP WHERE IT WOULD BE OF USE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT HIS COMPANY HAD RECENTLY BEEN USING A COMPUTER GENERATED DRIFTDOWN CALCULATION THAT IS OCCASIONALLY ERRONEOUS. IN THIS CASE, THE DECISION LINE IS DRAWN BEFORE THE ACFT WOULD GET OVER ANY HIGH TERRAIN. THE CAPT INQUIRED OF HIS COMPANY IF THIS MEANT THAT THE HIGH TERRAIN S OF THE PRESENT GO/NO-GO LINE WAS NOT A PROB FOR THE F100 ACFT AT NORMAL CRUISE ALTS OF FL310 AND FL330. HE HAS NOT RECEIVED AN ANSWER YET. THE MINIMUM OFF-RTE ALT ALONG THIS PORTION OF THE RTE IS AS HIGH AS 14200 FT NEAR SLP AND 12800 FT AT HIS DEST, BJX. THE RPTR SAID THAT HIS NORMAL SINGLE ENG CRUISE ALT ON A STANDARD DAY IS AROUND 21000 FT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

An air carrier flight crew suspects that the driftdown calculations for the route to bjx may be faulty because the decision line is drawn before high terrain is encountered.