Xav8tor
Wingback Tin Kicker
- Joined
- Jan 15, 2005
- Posts
- 52
Do you go 100 Above TDZE Prior to MAP on LOC/DME?
I read a copy of the factual reports (Operations, FDR, CVR, etc.) on the J32 accident last year in IRK. According to investigation interview statements made by pilots, managers and an FAA inspector, a lot of pilots are under the impression that, upon seeing the approach lights during a non-precision approach, it is permissible to continue descent from MDA to 100 feet above the touchdown zone – REGARDLESS of the aircraft’s distance to the threshold. In other words, after crossing the FAF and doing the old chop and drop to MDA, for example, at two miles DME from the runway/MAP you see the ALS, (but not the VASI or runway lights), they thought it was permissible (and safe) to go ahead and keep on going down to 100 feet while still that far out.
I know how I have always understood, applied, and taught that reg and related procedures, but I am curious to know what you guys (and girls) think and how you interpret and apply it, assuming you use standard 121 (i.e., airline "style") profiles and procedures. I re-read the TERPS and know that what a lot of people assume about obstacle clearance and tolerances is more than a little optimistic. Take a 75 foot altimeter error, toss in another 20 feet for a reading a little off due to rapid changes in baro pressure or measurement error, add in another 20 or 30 feet of altitude variation due to wind/rough air, etc., and the permissible obstruction clearance guarantee even as close in as the MM is at best a few feet.
But in this example, we are talking 1 to 1.5 nm from the threshold and 100 above TDZE in slightly rolling terrain. What do you think about when planning and briefing an approach such as this? Also, the IRK LOC/DME 36 NOS plate shows an approx. 100 foot obstacle almost on, just barely right, of the LOC/extended centerline about 3/4 to a mile out. Jepps don’t chart final controlling obstacles, or any approach obstructions, lower than 400 AGL, so the Jepp for that approach looks like a clear shot. The reason I noted that is I am thinking, even if I was taught and thought that going to 100 above TDZE 1.5 out upon seeing the approach lights (but not a VASI or the runway) was kosher, I might have second thoughts if the plate I had in front of me showed a 95 foot tall clump of trees between me and the runway.
The final, and perhaps most, interesting thing, one that really threw me for a loop reading these factual reports, is that the NTSB seems to have found that the crew of the same flight under similar weather conditions the night before the accident almost did the exact same thing and had to do an abrupt pull up of sorts to avoid hitting the trees at about the same place.
Comments and clarification anyone? I’ve done a good bit of accident analysis and flew 32’s on similar approaches hundreds of times, and did probably close to 1000 hours of II instruction. With all of that, I am still scratching my head on this trying to figure out what everyone was thinking, and I am mainly talking about those on up the chain here. It’s not like the crew weren’t paying attention and just flew it in. From what I am reading, they thought the continued descent was the right thing to do and were trained that way, maybe going all the way back to their instrument rating and in flight school (at an aviation college). One guy (I think it was someone from a FSDO) said dropping to 100 AGL two miles out was perfectly safe and legal and obstacle clearance was guaranteed by the TERPS, which is of course, not quite correct.
Who has a clue in this scenario? What would you do (have done)? Not Monday morning quarterbacking or second guessing a deceased crews’ decision, but trying to understand how it could happen and more importantly, WHY? Are most pilots operating on those assumptions, interpretations of the regs, and NP approach procedures described above?
Thanks.
I read a copy of the factual reports (Operations, FDR, CVR, etc.) on the J32 accident last year in IRK. According to investigation interview statements made by pilots, managers and an FAA inspector, a lot of pilots are under the impression that, upon seeing the approach lights during a non-precision approach, it is permissible to continue descent from MDA to 100 feet above the touchdown zone – REGARDLESS of the aircraft’s distance to the threshold. In other words, after crossing the FAF and doing the old chop and drop to MDA, for example, at two miles DME from the runway/MAP you see the ALS, (but not the VASI or runway lights), they thought it was permissible (and safe) to go ahead and keep on going down to 100 feet while still that far out.I know how I have always understood, applied, and taught that reg and related procedures, but I am curious to know what you guys (and girls) think and how you interpret and apply it, assuming you use standard 121 (i.e., airline "style") profiles and procedures. I re-read the TERPS and know that what a lot of people assume about obstacle clearance and tolerances is more than a little optimistic. Take a 75 foot altimeter error, toss in another 20 feet for a reading a little off due to rapid changes in baro pressure or measurement error, add in another 20 or 30 feet of altitude variation due to wind/rough air, etc., and the permissible obstruction clearance guarantee even as close in as the MM is at best a few feet.
But in this example, we are talking 1 to 1.5 nm from the threshold and 100 above TDZE in slightly rolling terrain. What do you think about when planning and briefing an approach such as this? Also, the IRK LOC/DME 36 NOS plate shows an approx. 100 foot obstacle almost on, just barely right, of the LOC/extended centerline about 3/4 to a mile out. Jepps don’t chart final controlling obstacles, or any approach obstructions, lower than 400 AGL, so the Jepp for that approach looks like a clear shot. The reason I noted that is I am thinking, even if I was taught and thought that going to 100 above TDZE 1.5 out upon seeing the approach lights (but not a VASI or the runway) was kosher, I might have second thoughts if the plate I had in front of me showed a 95 foot tall clump of trees between me and the runway.
The final, and perhaps most, interesting thing, one that really threw me for a loop reading these factual reports, is that the NTSB seems to have found that the crew of the same flight under similar weather conditions the night before the accident almost did the exact same thing and had to do an abrupt pull up of sorts to avoid hitting the trees at about the same place.
Comments and clarification anyone? I’ve done a good bit of accident analysis and flew 32’s on similar approaches hundreds of times, and did probably close to 1000 hours of II instruction. With all of that, I am still scratching my head on this trying to figure out what everyone was thinking, and I am mainly talking about those on up the chain here. It’s not like the crew weren’t paying attention and just flew it in. From what I am reading, they thought the continued descent was the right thing to do and were trained that way, maybe going all the way back to their instrument rating and in flight school (at an aviation college). One guy (I think it was someone from a FSDO) said dropping to 100 AGL two miles out was perfectly safe and legal and obstacle clearance was guaranteed by the TERPS, which is of course, not quite correct.
Who has a clue in this scenario? What would you do (have done)? Not Monday morning quarterbacking or second guessing a deceased crews’ decision, but trying to understand how it could happen and more importantly, WHY? Are most pilots operating on those assumptions, interpretations of the regs, and NP approach procedures described above?
Thanks.
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