SWA Low Altitude Alert-TPA
07-26-2024, 10:14 AM
#21
Gets Weekends Off
Joined APC: Sep 2014
Posts: 726
So now I'll ask you: Why is the autopilot still engaged at 500' on a visual approach with 0 in the altitude selector?
No wonder this is becoming a weekly news event.
07-26-2024, 11:44 AM
#22
Gets Weekends Off
Joined APC: Jul 2005
Posts: 261
The following is from a Boeing perspective.
Setting zero in the MCP provides no intrinsic value. However, setting the TDZE elevation in the MCP is a technique to ensure the airplane is on the proper glide path to the runway by verifying that the green arc is at the runway threshold. Since no published minimums exist for pure visual approaches such as an RNAV Visual approach (OpSpec C081), the MCP must be set to some altitude below the airplane altitude until the FMC is on approach logic. Modern Boeing FMCs sequence to "on approach" after the first waypoint of the approach or approach transition, 2000' AGL, or for the 747-400(NGFMC)/-8/777/787, when flaps out of UP. At this point, when the airplane is more than 300' below the Missed Approach Altitude (MAA), the MCP altitude is set to the MAA. No matter what was initially set in the MCP altitude (0, TDZE, 500' HAA, etc.), the MCP is set to the MAA well before crossing the final fix, rendering the initial MCP setting a moot point (moo for Joey fans). If using IAN, then the MAA is set at glide path capture.
The real question for the OKC and TPA incidents is what was selected in the FMC. Modern FMC database memory is large enough to contain selectable VFR APPR guidance for all runways. If an instrument approach isn't available, the VFR APPR may be selected and flown using LNAV/VNAV. Again, the MCP altitude is immaterial since it is set to an arbitrary MAA when flaps are out of UP, etc., as described above. This isn't throwing stones at an airline or second-guessing, but someone interested in how this happened in a modern 737 Max airplane capable of GLS, RNP, RNP AR, LPV, IAN, and ILS approaches, and VFR APPR availability.
Setting zero in the MCP provides no intrinsic value. However, setting the TDZE elevation in the MCP is a technique to ensure the airplane is on the proper glide path to the runway by verifying that the green arc is at the runway threshold. Since no published minimums exist for pure visual approaches such as an RNAV Visual approach (OpSpec C081), the MCP must be set to some altitude below the airplane altitude until the FMC is on approach logic. Modern Boeing FMCs sequence to "on approach" after the first waypoint of the approach or approach transition, 2000' AGL, or for the 747-400(NGFMC)/-8/777/787, when flaps out of UP. At this point, when the airplane is more than 300' below the Missed Approach Altitude (MAA), the MCP altitude is set to the MAA. No matter what was initially set in the MCP altitude (0, TDZE, 500' HAA, etc.), the MCP is set to the MAA well before crossing the final fix, rendering the initial MCP setting a moot point (moo for Joey fans). If using IAN, then the MAA is set at glide path capture.
The real question for the OKC and TPA incidents is what was selected in the FMC. Modern FMC database memory is large enough to contain selectable VFR APPR guidance for all runways. If an instrument approach isn't available, the VFR APPR may be selected and flown using LNAV/VNAV. Again, the MCP altitude is immaterial since it is set to an arbitrary MAA when flaps are out of UP, etc., as described above. This isn't throwing stones at an airline or second-guessing, but someone interested in how this happened in a modern 737 Max airplane capable of GLS, RNP, RNP AR, LPV, IAN, and ILS approaches, and VFR APPR availability.
07-26-2024, 04:53 PM
#23
Gets Weekends Off
Joined APC: Sep 2014
Posts: 726
The following is from a Boeing perspective.
Setting zero in the MCP provides no intrinsic value. However, setting the TDZE elevation in the MCP is a technique to ensure the airplane is on the proper glide path to the runway by verifying that the green arc is at the runway threshold. Since no published minimums exist for pure visual approaches such as an RNAV Visual approach (OpSpec C081), the MCP must be set to some altitude below the airplane altitude until the FMC is on approach logic. Modern Boeing FMCs sequence to "on approach" after the first waypoint of the approach or approach transition, 2000' AGL, or for the 747-400(NGFMC)/-8/777/787, when flaps out of UP. At this point, when the airplane is more than 300' below the Missed Approach Altitude (MAA), the MCP altitude is set to the MAA. No matter what was initially set in the MCP altitude (0, TDZE, 500' HAA, etc.), the MCP is set to the MAA well before crossing the final fix, rendering the initial MCP setting a moot point (moo for Joey fans). If using IAN, then the MAA is set at glide path capture.
The real question for the OKC and TPA incidents is what was selected in the FMC. Modern FMC database memory is large enough to contain selectable VFR APPR guidance for all runways. If an instrument approach isn't available, the VFR APPR may be selected and flown using LNAV/VNAV. Again, the MCP altitude is immaterial since it is set to an arbitrary MAA when flaps are out of UP, etc., as described above. This isn't throwing stones at an airline or second-guessing, but someone interested in how this happened in a modern 737 Max airplane capable of GLS, RNP, RNP AR, LPV, IAN, and ILS approaches, and VFR APPR availability.
Setting zero in the MCP provides no intrinsic value. However, setting the TDZE elevation in the MCP is a technique to ensure the airplane is on the proper glide path to the runway by verifying that the green arc is at the runway threshold. Since no published minimums exist for pure visual approaches such as an RNAV Visual approach (OpSpec C081), the MCP must be set to some altitude below the airplane altitude until the FMC is on approach logic. Modern Boeing FMCs sequence to "on approach" after the first waypoint of the approach or approach transition, 2000' AGL, or for the 747-400(NGFMC)/-8/777/787, when flaps out of UP. At this point, when the airplane is more than 300' below the Missed Approach Altitude (MAA), the MCP altitude is set to the MAA. No matter what was initially set in the MCP altitude (0, TDZE, 500' HAA, etc.), the MCP is set to the MAA well before crossing the final fix, rendering the initial MCP setting a moot point (moo for Joey fans). If using IAN, then the MAA is set at glide path capture.
The real question for the OKC and TPA incidents is what was selected in the FMC. Modern FMC database memory is large enough to contain selectable VFR APPR guidance for all runways. If an instrument approach isn't available, the VFR APPR may be selected and flown using LNAV/VNAV. Again, the MCP altitude is immaterial since it is set to an arbitrary MAA when flaps are out of UP, etc., as described above. This isn't throwing stones at an airline or second-guessing, but someone interested in how this happened in a modern 737 Max airplane capable of GLS, RNP, RNP AR, LPV, IAN, and ILS approaches, and VFR APPR availability.
Setting TDZE (even though I personally don't like it) is justifiable with the logic as explained above.
But not even I, zerozero, would dial ZERO.
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