ATC "low altitude alert" on approach procedure?

Discussion in 'Cleared for the Approach' started by RussR, May 22, 2013.

  1. RussR

    RussR Cleared for Takeoff

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    Recently, I was flying the RNAV (GPS) RWY 7 into I19, Greene County, Ohio, in IMC (attached) - LNAV minimums.

    I was cleared direct to UYOKO for the straight-in, which is what I requested. I maintained 2700 until the FAF, WANKU, then began a descent to 1820 for the stepdown fix NINRE. Knowing that the cloud ceiling was at about 1900, I did descend a little faster than a truly stabilized approach would warrant, but nothing too amazing.

    On passing something like 2000, the controller came on the air, saying "Nxxx, low altitude alert, check altitude immediately, minimum altitude in your area is XXXX" (I forget, but it was something like 2000). I could hear the MSAW alert in the background as well. I responded something like "Roger" and simply continued the approach since I was breaking out and could see that I was at about the right altitude.

    I can't recall ever getting a low altitude alert from ATC while on an approach before - are those alerts usually disregarded or inhibited by ATC for aircraft on an approach (since obviously you have to go below the MVA at some point in your approach), or was something different this time? Or was this a controller error (she did seem to be in training)? Had the ceiling been lower, like 500 feet, it would have caused me some concern that my equipment was off, and I might no longer receive compliments on its accuracy.

    But she had properly confirmed my altitude/altimeter setting/Mode C readout on initial call, so it shouldn't have been an equipment problem on my side. And it read correctly on the ground after landing.

    Any wisdom on what happened here?
     

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  2. poadeleted20

    poadeleted20 Deleted

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    The MSAW system works on descent rate as well as altitude. On nonprecision approaches like that LNAV approach you were flying, it is not uncommon for a typical "drop and drive" descent rate to trigger that alert as you approach MDA. Double check everything anyway, but if you're on the profile, just press on.
     
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  3. Velocity173

    Velocity173 Final Approach

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    It's a judgement call and it depends on the controller. It's easier to just give the low altitude alert to cover their butt. It's not a situation where they're going to inhibit MSAW either. You're IFR doing an non-precision IAP, it's gonna go off if you dive at a rate that the computer is programmed to be excessive. Just acknowledge the controller and go on about your business.
     
  4. rbridges

    rbridges En-Route

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    didn't know that. I thought it would be based on minimum altitude for that section of the approach.
     
  5. bobmrg

    bobmrg En-Route

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    As I understand it, the MSAW program uses your present descent profile to project where you will be in 3 minutes (I think...maybe it's one minute). The machine doesn't know or care that you are going to level off, just that if you do not take some kind of action you are going to hit the terrain.

    Bob Gardner
     
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  6. roncachamp

    roncachamp Final Approach

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    MSAW alerts on dive and drive approaches are not unusual. The system is triggered by high descent rates; it doesn't know your intentions. When I receive the alarm I say nothing if the aircraft is above the MDA, I just monitor it a bit more closely. Non-pilot controllers tend to simply issue the alert.
     
  7. 35 AoA

    35 AoA Cleared for Takeoff

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    Sorry to resurrect dead thread, but I've run into this a few times recently, flying an LNAV only aircraft (for GPS approach purposes). Just so I am understanding this correctly, the only issue is the dive/drive descent rate, correct? In theory, if you were in a flying elevator, and you crossed the FAF and then dropped in a millisecond to MDA, you'd still have obstacle clearance, right? Or is there a maximum descent rate built into the assumptions of obs clrnc? I don't build the approaches, I only fly them, so apologies in advance if this is a dumb question.
     
  8. Velocity173

    Velocity173 Final Approach

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    The MSAW alerts aren't solely based on a minimum altitude. It uses an algorithm to compute a normal descent rate as well. You exceed the threshold of that rate on an IAP or VA, and she'll alarm. If you do an aggressive descent after the initial for the overhead, it'll alarm as well. Any tracked, assigned discrete code will set it off. Thankfully your 1200,4000 codes etc, won't. Same deal with conflict alerts. The computer computes an intercept ahead of time before radar separation is lost. Sometimes the radar room can be an annoying mess of bells and whistles going off.

    Like Steven said, some controllers, as long as you haven't busted MDA, aren't going to issue a safety alert.
     
    Last edited: Nov 28, 2017
  9. frfly172

    frfly172 Touchdown! Greaser!

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    Have recieved a low altitude alert,several times ,when decending at a rapid rate. Don’t bust MDA and there should be no problem. Remember the controllers have to cover their butts also.
     
  10. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser!

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  11. 35 AoA

    35 AoA Cleared for Takeoff

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    Ok cool, so in the most simple iteration of my question, my descent rate might be anything, but as long as I level off at MDA, I'm safe? And any "low alt alert" can just be disregarded? That's how I have run my show every time, and have responded (admittedly with a tinge of annoyance) with "I am descending to minimum descent altitude", at which point they just roger up. My assumption is that most controllers don't know we are flying LNAV, and much less realize that we cannot fly precision, and see a massive rate of descent at the FAF and just assume we are an airliner about to crash.
     
  12. RussR

    RussR Cleared for Takeoff

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    Not exactly, but in practice, yes. There is a concept in TERPS called the "7:1" area, where certain obstacles very near to the FAF or a final stepdown fix can be ignored for MDA consideration. On an RNAV (GPS) approach with LNAV minimums, this area starts 0.3 nm prior to the fix and extends for 1 nm towards the MAP. Vertically, it starts (usually) 250 feet below the FAF altitude and slopes downward at a 7:1 ratio for that 1 nm. If an obstacle is under that slope, it can be disregarded for MDA determination.

    Note that a 7:1 slope at 90 knots is about a 1300 fpm descent rate. So to have any kind of a problem you would need to start descending 0.3 nm prior to the FAF (this protects for early descents or equipment issues), and immediately establish that 1300 fpm descent. So, not likely in an actual airplane, short of a flying refrigerator or course. And even that would only be a problem if there actually happened to be obstacles pushing up against that limit.
     
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  13. Velocity173

    Velocity173 Final Approach

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    Well you want to at least acknowledge the safety alert. A "roger" will suffice. After the alert is issued, it's completely up to you on how to respond to it. You can keep descending at your current rate if you choose. There's a suggest climb / descent rate in the AIM but on final, ATC doesn't care.

    Just realize the warning is out of their control. It's calculated by the computer. Some interpret it as a mandatory warning that must be issued, while others will hold off on the warning unless you break the MVA/MDA. The controller's manual does refer to it as a judgment call but if they don't issue it and you crash, the investigation results won't be good for that controller.

    Personally if it was IMC and the aircraft is on final, I always issued it. Visual approaches and overheads, I generally didn't issue it. Really, if an aircraft has a true problem (spatial D / mechanical), a low altitude alert for a descent rate isn't going to do anything to resolve that problem. A low altitude alert for leaving an assigned MIA could've prevented this accident:

    http://www.super70s.com/super70s/tech/aviation/Disasters/72-12-29(Eastern).asp#Diagram
     
    Last edited: Nov 29, 2017
  14. aterpster

    aterpster Pattern Altitude

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    That is incorrect. An obstacle located within one mile past the FAF can be avoided in design with a 7:1 gradient. An obstacle accorded this treatment must be charted in the plan view. at 90 knots ground speed that would be approximately 1,300 fpm.
     
  15. luvflyin

    luvflyin Final Approach

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    I always wondered what the details were on 'assumed angle' of descent but never took the time to look it up. Thanks for the detail
     
  16. luvflyin

    luvflyin Final Approach

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    If Bitchin Betty went off I issued them to. I can't recall of anyone I ever worked with who didn't. Never did work a place though that had MSAW's going off 'in the pattern.' It wouldn't surprise me if changes in technology and procedure came out of that accident. Much of ATC procedure is written in blood, just like the FAR's are.
     
  17. Velocity173

    Velocity173 Final Approach

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    Oh it'll issue it in a holding pattern if the aircraft goes below the MVA. Tower stuff generally won't if it's a local VFR code. Some facilities like the one I worked, all the local codes were MSAW enabled.

    I was just using the Eastern L-1011 crash as to the importance of a safety alert. Don't even know if the facility back then had MSAW. Apparently they had to have two sweeps before issuing the alert. Never heard of the two sweep rule other than radar contact lost. If the controller would have been monitoring the flight, it would have been several sweeps anyway. It was almost a minute from when the aircraft started the descent to when the controller noticed it was 1,100 ft low. I find it hard to believe the NTSB didn't put any blame on the controller. No way that would get overlooked today.
     
    Last edited: Nov 29, 2017
  18. Palmpilot

    Palmpilot Touchdown! Greaser!

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    Good info to have. Does the same thing apply at stepdown fixes other than the FAF?
     
  19. aterpster

    aterpster Pattern Altitude

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    Yes. Could be a final segment step-down fix.
     
  20. 35 AoA

    35 AoA Cleared for Takeoff

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    Thanks for the info gents. Much appreciated!
     
  21. Velocity173

    Velocity173 Final Approach

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    Just stick to the PAR man and you won't have any problems. Stop using all this new gadgetry you've got your hands on now. Navy controllers need the work! :D
    #Navycontrollerlivesmatter
     
  22. luvflyin

    luvflyin Final Approach

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    Are those the same numbers that are used as an 'assumed angle' of descent for changes in MEA/MOCA on an Airway or in the initial/intermediate segements of an approach?
     
  23. aterpster

    aterpster Pattern Altitude

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    This 7:1 criterion is only for the NPA final approach segment. All other segments and airways must have full obstacle clearance for descent to a lower altitude.

    Just prior to the Secretary Brown crash at Dubrovnik the manager in charge of TERPs at the time had proposed to permit application of the 7:1 in other approach segments. He dropped the proposal after the Secretary Brown crash.
     
  24. pstan

    pstan Pre-takeoff checklist

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    Just out of curiosity, was the 7:1 slope ever an issue there?
     
  25. RussR

    RussR Cleared for Takeoff

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    No, but pilot actions and other TERPS criteria (ICAO, as implemented by Croatia) were factors. You can easily imagine how making any changes to obstacle clearance rules immediately following a high-visibility fatal accident where obstacle clearance was a factor, would not be a great idea. Even if the concepts and issues were not at all related.
     
  26. rocketflyer84

    rocketflyer84 Line Up and Wait

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    The altitude alert is quite common on stepdown approaches, especially if you do the “chop and drop” approach.

    Essentially the radar’s computer thinks you’re decending too fast (relative to a normal glideslope). It is startling and a bit distracting as you cross check everything but nothing to worry about too much if everything you were doing was legit. I think ATC is required to tell you if their screen flags you (even if they know what’s likely going on).
     
  27. Deke

    Deke Filing Flight Plan

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    So it's weird. I go into BOS often on Angel Flights and leisure. I get the low altitude alerts from tower going into BOS only on 22L. Haven't received it in other runways there. Most times I get I'm on a visual.....one time I flew the ILS was on GP and broke out at 1500 and stayed relatively on GP and received it. I was just watching another pilot on youtube on a visual and on a good GP and he received the same warning on the same runway. Many on this page have mentioned the settings used on their computer monitoring radar. That is correct....every time I've received this and heard others (airliners, etc) receive this it seems like a CYA thing for them. There is no action taken. I take videos often when going into Logan and the flight path is over some densely populated areas. My guess is that they get a lot of complaints.
     
  28. aterpster

    aterpster Pattern Altitude

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    I flew air carrier in there a whole lot. But, that was 1964-90. I recall, when the wx was good, and the winds would permit use of the 22s, we were lined up on a 20 mile final for 27 to avoid the noise.