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Boeing 727-51 DME

Old 13th Feb 2020, 16:11
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Boeing 727-51 DME

Does anyone know whether the DME on the Boeing 727-51 showed whole miles or tenths of a mile?
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Old 13th Feb 2020, 23:07
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The 100 series 727’s that I flew , I think, had the DME calibrated in miles and tenths. The -51 designation was the Boeing customer number and should tell you who the a/c was originally built for and help you to more specifically target your enquires. Good luck.

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Old 13th Feb 2020, 23:43
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Originally Posted by emeritus View Post
The -51 designation was the Boeing customer number and should tell you who the a/c was originally built for
Northwest.
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Old 14th Feb 2020, 10:10
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The 100 series 727’s that I flew , I think, had the DME calibrated in miles and tenths.
Many thanks. I am writing a book on Northwest Flight 305 of 11.24.1971. One of my research tasks was to determine the margin of uncertainty of the three DME reports (14m south of SEA, 19m south of SEA, and 23m south of PDX. I had been wondering why they reported whole miles and whether the instrument precluded a more precise reading.
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Old 14th Feb 2020, 17:08
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Originally Posted by DFS346 View Post
Many thanks. I am writing a book on Northwest Flight 305 of 11.24.1971. One of my research tasks was to determine the margin of uncertainty of the three DME reports (14m south of SEA, 19m south of SEA, and 23m south of PDX. I had been wondering why they reported whole miles and whether the instrument precluded a more precise reading.
Don't know of the event but are the reports you refer to recovered data from a flight data recorder or radio position reports from the crew?

If its recovered data then you would expect the accident report to give the data as accurately as it was recorded - i.e to tenths if the display showed that.

If it's a radio transcript then it is unlikely that pilots would report their DME range in units of less than a mile, even if that was displayed to them. We'd round up or down to the nearest whole unit (or perhaps a half if it was a very short range - eg on short finals where the half mile might be relevant).

OH
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Old 14th Feb 2020, 18:32
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Originally Posted by OvertHawk;
are the reports you refer to recovered data from a flight data recorder or radio position reports from the crew?
If it's a radio transcript then it is unlikely that pilots would report their DME range in units of less than a mile, OH
Many thanks OvertHawk. The event was a hijacking. The reports were from the crew, via ARINC. I infer that a report of e.g. 23 miles could be from a reading of between 22.5 and 23.5.
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Old 14th Feb 2020, 21:19
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D.B. Cooper.
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Old 15th Feb 2020, 10:35
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I was always told to give tower the “rounded down” whole number value. Apparently the logic is it gives the controller a bigger/safer margin to work with.
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Old 18th Feb 2020, 13:02
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Originally Posted by Duchess_Driver View Post
I was always told to give tower the “rounded down” whole number value. .
Many thanks Duchess Driver. Would that still apply if the flight was outbound?
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Old 18th Feb 2020, 23:02
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Again, usually while numbers only. The likelihood of being asked outbound is much less unless you’re operating procedurally in a non radar environment
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Old 19th Feb 2020, 11:00
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Thanks Emeritus, OvertHawk and Duchess_Driver.

The three DME reports that I'm studying were all outbound and were apparently transmitted at the initiative of the flight crew. All three reports are preserved in two documents:

(1) document 2013.5.23.1: “Digitized notes from Northwest Airlines executive George Harrison, who was present at Seattle-Tacoma International Airport during the hijacking of Northwest Airlines flight 305”,

(2) document 2013.5.23.4: “curator report for examination of Aeronautical Radio, Incorporated (ARINC) teletypewriter printout documenting communications between the flight crew of Northwest Airlines 305 and Seattle-Tacoma International Airport” [which apparently refers to Northwest flight operations at SeaTac, not ATC, since ATC transcripts have no record of these transmissions],

both documents held by the Washington State Historical Society.

The reports were as follows:

"0340 - 7000' 14DME V23" (handwritten note at 7:40 PM PST);
“305 OUT SEA 14 MILES ON V23 OUT SEA” (between time stamps for 7:34 and 7:42 PM PST)

"0344 - 7000' 19DME V23" (handwritten note at 7:44 PM PST);
“305 STILL AT 7THSD FT 19 DME S V23“ (between time stamps for 7:42 and 7:45 PM PST)

"0418 23 DME PDX 100/" (handwritten note at 8:18 PM PST);
“0422 23 DME S of PDX (DME 10,000) 165 IAS” (handwritten note at 8:22 pm PST);
no corresponding message on teletype tape.

Reconciling these numbers (if taken at face value) yields some implausible ground speeds (or implausible winds). For example position 1 to position 2 = 5 nm in 4 minutes = ground speed 75 knots. If 14 were 13.5 rounded up and 19 were 19.9 rounded down, the ground speed would be 96 knots, which is still hard to reconcile with a reported IAS of 160 knots. Hence my interest in establishing the uncertainties and/or rounding practice in the DME reports, and in identifying other sources of error.

I’d be grateful for any professional input as to how to reconcile these numbers.​​​​​​​
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Old 19th Feb 2020, 12:06
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Originally Posted by DFS346 View Post
For example position 1 to position 2 = 5 nm in 4 minutes = ground speed 75 knots​​​​​​​
How do you know that the distance from Position 1 to Position 2 is 5 nm ?

All I can see is that P1 is 5 miles closer than P2 to some unspecified DME.
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Old 19th Feb 2020, 14:02
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You're assuming that they were flying directly away from the beacon. If they were flying at an angle relative to the radial, your calculation is giving you the speed vector along the radial, while their actual speed may be anything from 75kt to Vne for that aircraft. Do you know if they were following a known route? V 23 may refer to one of the Victor airways in the US.
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Old 19th Feb 2020, 22:59
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Seattle Control cleared Flight 305 to proceed on the low level airway Victor 23, which is 8 nm wide between Seattle WA and Eugene OR. DME reports 1 and 2 are certainly with reference to the VORTAC at Seattle.
Where they were within the airway is a good question. According to Northwest Airlines and the FBI, USAF radar first picked up Flight 305 about 50 nm south of Seattle and about 1 nm west of the centerline of V 23; the radar plots show the airplane travelling in a straight line on a track of about 190.
I had assumed that, prior to the first radar plot, they had followed the centerline of V 23. That does not have to be the case. If for example at report 1 they were 4 nm east of the centerline, and at report 2, 4 nm west of the centerline, then the distance covered would have been 9.4 nm and the ground speed would have been 142 kt. That would be more compatible with the reported IAS of 160 kt. However I have not thought of a reason why they would track so far from the centerline.
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