Narrative:

I am flying pairing xx. It's a 14 day trip with 10 flying legs; 1 commercial leg and 1 DH leg. That's 12 legs in 14 days! Total flight time for the trip is 66.11 hours; tafb is 317.21. It's day 9 of the trip. During the past 9 days I have flown 7 legs; one commercial flight and had layovers consisting of no more than 24 hours and as little as 12 hours. It's a busy trip to say the least; and the rest of the trip is the same. I have 5 days left until it's over; and still another 4 legs to go including the DH leg. What I'm writing about is my leg today from ZZZZ-vabb. It started out with a 24 hour layover; waking up @ xa:45 am for a xb:45 am show. The first leg was ZZZ1-ZZZ; then sitting in ZZZ for 3 hours and then flying ZZZ-vabb. Duty day was approximately 12 hours and flight time was approximately 7.5 hours. Weather was clear; wind 070/08; temp 28. We had briefed the emrak 1A for runway 27. Considering the tailwind for runway 27 so we briefed the usage of autobrakes 4 and to be aware. However; at approximately 100 miles from vabb the airport switched runways from 27 to runway 9; possibly because of the tailwind. Mumbai control was holding us high; 32;000 feet and then 25;000 feet where we were then told to 'orbit' one time to the left; and finally requesting us to expedite descent to 15;000 feet as traffic finally passed us. We quickly reprogrammed the FMC for the change of arrivals and runway. (Emrak 1B/ rwy09) mumbai approach had us on a heading of 270 downwind and passing the airport off my left side @ 9;000 feet. They told us to expedite descent to 2;600 feet and slow to 180 knots. (Kinda hard to do; descend & slow; but we complied). ATC then instructed us to turn left 150 and intercept runway 09 localizer. We captured the localizer at approximately 15 DME and 4;000 feet and proceeded inbound. We then were cleared for the ILS 09 approach. Glide slope intercept altitude is at 2;600 feet. At approximately 12 DME and established inbound and following the localizer/glide slope down; we received a lot of movement in the localizer and a degraded G/south indication. We continued; both monitoring the approach. (We did not receive an EICAS message or egws at any time during the flight; only a 'line' running horizontal through the G/south FMA.) I then took a look at my jepp ILS 09 approach plate to see if there was a G/south inop procedure or altitude crossings. Considering we were VFR and could see the water below us as well as the shore line ahead both of us felt good about continuing. When I looked up; it was then I realized we had flown below the G/south intercept altitude of 2;600 feet and were now @ 1;400 feet. The autopilot had lost the ability to maintain the G/south. I then quickly selected flch and the plane ascended back to G/south intercept altitude of 2;600 feet. At which time the first officer questioned mumbai tower if the ILS was working. 'Affirmative' was their reply back. We recaptured the G/south and continued to a full stop on runway 09. Mumbai tower never once questioned our altitude or reason for asking if the ILS was working. This was the first experience in my 27 years of commercial flying that I have ever had an altitude or ILS G/south deviation. It was a good learning experience for both myself & the first officer; as we discussed this for a lengthy time once parked at the gate and later at the hotel. We explored different explanations for this and found in the [manual] as a good reference. I attribute the constant 24 hour layovers; flying 7 legs along with one commercial flight in the past 9 days; and my momentarily looking down at my jepp plate at just the wrong time; all contributing factors for this deviation. One lesson I did learn was to always have one pilot fly the plane while the other pilot solve the problem. Two heads down are not a good thing!!

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Original NASA ASRS Text

Title: While on the VABB ILS 09; the crew received a lot of movement in the localizer and a degraded GS indication which resulted in a descent to 1;400 feet; well below the 2;600 feet glideslope intercept altitude.

Narrative: I am flying pairing XX. It's a 14 day trip with 10 flying legs; 1 commercial leg and 1 DH leg. That's 12 legs in 14 days! Total flight time for the trip is 66.11 hours; TAFB is 317.21. It's day 9 of the trip. During the past 9 days I have flown 7 legs; one commercial flight and had layovers consisting of no more than 24 hours and as little as 12 hours. It's a busy trip to say the least; and the rest of the trip is the same. I have 5 days left until it's over; and still another 4 legs to go including the DH leg. What I'm writing about is my leg today from ZZZZ-VABB. It started out with a 24 hour layover; waking up @ XA:45 am for a XB:45 am show. The first leg was ZZZ1-ZZZ; then sitting in ZZZ for 3 hours and then flying ZZZ-VABB. Duty day was approximately 12 hours and flight time was approximately 7.5 hours. Weather was clear; wind 070/08; temp 28. We had briefed the EMRAK 1A for runway 27. Considering the tailwind for runway 27 so we briefed the usage of autobrakes 4 and to be aware. However; at approximately 100 miles from VABB the airport switched runways from 27 to runway 9; possibly because of the tailwind. Mumbai control was holding us high; 32;000 feet and then 25;000 feet where we were then told to 'orbit' one time to the left; and finally requesting us to expedite descent to 15;000 feet as traffic finally passed us. We quickly reprogrammed the FMC for the change of arrivals and runway. (EMRAK 1B/ rwy09) Mumbai approach had us on a heading of 270 downwind and passing the airport off my left side @ 9;000 feet. They told us to expedite descent to 2;600 feet and slow to 180 knots. (kinda hard to do; descend & slow; but we complied). ATC then instructed us to turn left 150 and intercept runway 09 localizer. We captured the localizer at approximately 15 DME and 4;000 feet and proceeded inbound. We then were cleared for the ILS 09 approach. Glide slope intercept altitude is at 2;600 feet. At approximately 12 DME and established inbound and following the localizer/glide slope down; we received a lot of movement in the localizer and a degraded G/S indication. We continued; both monitoring the approach. (We did not receive an EICAS message or EGWS at any time during the flight; only a 'line' running horizontal through the G/S FMA.) I then took a look at my Jepp ILS 09 approach plate to see if there was a G/S inop procedure or altitude crossings. Considering we were VFR and could see the water below us as well as the shore line ahead both of us felt good about continuing. When I looked up; it was then I realized we had flown below the G/S intercept altitude of 2;600 feet and were now @ 1;400 feet. The autopilot had lost the ability to maintain the G/S. I then quickly selected FLCH and the plane ascended back to G/S intercept altitude of 2;600 feet. At which time the First Officer questioned Mumbai tower if the ILS was working. 'Affirmative' was their reply back. We recaptured the G/S and continued to a full stop on runway 09. Mumbai tower never once questioned our altitude or reason for asking if the ILS was working. This was the first experience in my 27 years of commercial flying that I have ever had an altitude or ILS G/S deviation. It was a good learning experience for both myself & the First Officer; as we discussed this for a lengthy time once parked at the gate and later at the hotel. We explored different explanations for this and found in the [manual] as a good reference. I attribute the constant 24 hour layovers; flying 7 legs along with one commercial flight in the past 9 days; and my momentarily looking down at my Jepp plate at just the wrong time; all contributing factors for this deviation. One lesson I did learn was to always have one pilot fly the plane while the other pilot solve the problem. Two heads down are not a good thing!!

Data retrieved from NASA's ASRS site 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.