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As a reminder, Turkish Airlines flight 1951 crashed in 2009 due to a faulty radio altimeter which set the autothrottle to idle before they touched down. This is

As a reminder, Turkish Airlines flight 1951 crashed in 2009 due to a faulty radio altimeter which set the autothrottle to idle before they touched down. This isn’t hot air or some sort of power struggle between FCC and FAA. Radio altimeter interference is a big deal.

mlindner 5y ago
We have GPS now combined with lots of other instruments. I'm not even sure radio altimeters are even used on many aircraft anymore.
- anamexis 5y ago
  GPS surely is not used for instrument landings, and especially not for altitude.
  - supernova87a 5y ago
    >> GPS surely is not used for instrument landings, and especially not for altitude. It is. And increasingly so in fact, as it allows more airports that don't have the resources to install and maintain ground-based equipment to have previously unavailable approaches. https://www.flyingmag.com/everything-you-need-to-know-about-rnav-gps-approaches/ https://www.flyingmag.com/everything-you-need-to-know-about-... However, it doesn't fall into exactly the same class as ILS approaches.
    - lisper 5y ago
      Instrument rated pilot here. A GPS LPV approach is essentially the same as an ILS [edit for clarification: a category 1 ILS]. It’s true that an LPV approach is classified as non precision but in practice an LPV approach is pretty much indistinguishable from a cat 1 ILS, which is classified as a precision approach.
      - ketzo 5y ago
        Hm, that's interesting to me. Is it just regulators (FAA I guess?) falling behind the times? Or is the GPS version really not adequate for some use case that is only satisfied by ILS (not sure what that is but I assume it's the "gold standard" landing approach?)
        c1yd3i 5y ago
        Airports served only by an RNAV (GPS) approach with LPV minimums have different alternate minimums (for planning purposes). That’s the only real distinction. LPV approaches are classified as non-precision due to an ICAO technicality over the lack of a ground transmitter at the airport. GLS approaches compensate for this.
        p_l 5y ago
        There are different categories of ILS approaches (one of them is actually impossible to implement at this time, as it requires tech that is at best in testing), and most importantly GPS is not considered "safe enough". Most use in heavy aviation is due to easier operation (especially with autopilot) but in better conditions only (i.e. at best equivalent of ILS Category 1)
        tjohns 5y ago
        GPS notoriously has poor altitude resolution. For this reason, GPS approaches are technically not considered "precision" approaches. WAAS (GPS plus a correction signal transmitted by separate geostationary satellites) improves the resolution to CAT I standards - an "LPV" approach. However, it's still not sufficient for CAT II/III approaches.
        jillesvangurp 5y ago
        You basically use the regular altimeter for altitude during such approaches. It's accurate enough to get to the decision height of a few hundred feet AGL after which you have to fly the rest of the approach manually anyway. That is true with CAT I ILS as well. A CAT III ILS approach requires very specialized equipment and training because you basically fly it all the way down to the runway with no requirement for visibility until moments before touchdown. However, GPS is a lot better when it is not being obstructed by buildings around you. Planes have the benefit of unobstructed paths to GPS satellites and would get a better quality and accuracy then you might get with a phone in a city. Also, using good antennas helps. So, the altitude accuracy would be fine mostly.
  - Aspos 5y ago
    It is not uncommon to get +/- 15m error on the vertical axis. There is a significant delay in the signal too, so can't be used for landing.
    - skimo8 5y ago
      This is why a recently calibrated altimeter is used for the altitude axis on GPS approaches. They also must have a constrained error margin to be certified, and it is factored into the buffered region around the approach.
    - creatornator 5y ago
      I think you're being downvoted because the imprecision you speak of is a result of: consumer GPS receivers having a limited polling rate, limited precision, and only using one GPS frequency. You can't extrapolate the limitations of GPS on say a phone, to limitations on a commercial plane. See the following from gps.gov: it can be accurate to within centimeters and with a high polling rate, but most consumer devices don't employ the necessary tricks. https://www.gps.gov/systems/gps/performance/accuracy/ https://www.gps.gov/systems/gps/performance/accuracy/
      - genewitch 5y ago
        Military gets more time resolution as well as no deliberate clock skew. Airliners won't have military GPS. Gps receivers for surveying require on the order of hours to resolve to "centimeters" using civilian GPS. The highest resolution GPS receiver I have still has +/-15ns jitter, which over the course of a day is good enough for maybe 12-20cm resolution, but the first hour the track is wild, 50-80 meters off in varying directions. My GPS is used as a time stamp source for lightning detection and location, so in aggregate with other detectors it can pinpoint a strike to within a meter or so anywhere in my hemisphere.
- itisit 5y ago
  > I'm not even sure radio altimeters are even used on many aircraft anymore. What misled you to think this? Radio altimeters are used throughout the aviation industry. Their ubiquity and criticality is the issue at hand. [0] [0] https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2021/SAFO21007.pdf https://www.faa.gov/other_visit/aviation_industry/airline_op...
- deleted 5y ago
  [deleted]
- op00to 5y ago
  Yes, I’m sure that few meters of vertical inaccuracy will not matter at all during landing.
- _moof 5y ago
  Radar altimeters are required for reduced minimums on ILS approaches.
magicalist 5y ago
> As a reminder, Turkish Airlines flight 1951 crashed in 2009 due to a faulty radio altimeter which set the autothrottle to idle before they touched down. Was it due to interference? Otherwise this seems specious.
- hn_throwaway_99 5y ago
  No, it wasn't, and I agree this is a specious example. The Wikipedia entry [1] has a good overview, as does this Medium post [2]. In the flight 1951 crash there were actually 2 altimeters for redundancy, but due to human error they were configured so that 1 faulty altimeter affected the autothrottle: > While on final approach for landing, the aircraft was about 2,000 ft (610 m) above ground, when the left-hand (captain's) radio altimeter suddenly changed from 1,950 feet (590 m) to read −8 feet (−2.4 m) height, although the right-hand (co-pilot's) radio altimeter functioned correctly. The voice recording showed that the crew was given an audible warning signal (landing gear warning horn) that indicated that the aircraft's landing gear should be down, as the aircraft was, according to the captain's radio altimeter, flying too low. This happened several times during the approach to Schiphol. The reason that the captain's radio altimeter was causing problems was the first officer making a mistake when arming the aircraft's autopilot system for a dual channel approach. > The Boeing 737NG type aircraft has two autopilot systems, which can work independently of each other (single channel) or together (dual channel). Turkish Airlines' standard operating procedure at the time stated that all approaches should be flown "dual channel" when available, but the inexperienced first officer forgot to arm approach mode in the aircraft's mode control panel (which controls the autopilot), meaning that the aircraft thought the pilots wanted to do a single-channel approach using control computer A (captain's autopilot), which had a radio altimeter failure. [1] https://en.wikipedia.org/wiki/Turkish_Airlines_Flight_1951 https://en.wikipedia.org/wiki/Turkish_Airlines_Flight_1951 [2] https://admiralcloudberg.medium.com/test-article-the-crash-of-turkish-airlines-flight-1951-440326b3bd9a https://admiralcloudberg.medium.com/test-article-the-crash-o...
  - nikcub 5y ago
    Want to plug your second source admiralcloudberg as a brilliant blog that is worth reading if you're into airline crashes and system failures. He's very in depth and detailed in his writing https://admiralcloudberg.medium.com/ https://admiralcloudberg.medium.com/
coderintherye 5y ago
From the report: https://catsr.vse.gmu.edu/SYST460/TA1951_AccidentReport.pdf https://catsr.vse.gmu.edu/SYST460/TA1951_AccidentReport.pdf "Interference as a singular cause appears to be extremely improbable." Interference is mentioned a few times in the report as likely not being the cause or at least sole cause. The only entity seeming to push interference as a cause was Boeing, which led to this remark: "We are not aware that interference has been conclusively identified as the explanation for the radio altimeter anomalies. In fact, some airlines have found that their incidence of radio altimeter anomalies decreased when they chose to install the new gaskets. If the conclusions here are intended to be DSB conclusions, rather than conclusions of Boeing and the airlines, the text should be revised to indicate they are DSB conclusions." in which they struck out the text "The negative values of the radio height were an indication of interference"

mlindner 5y ago

We have GPS now combined with lots of other instruments. I'm not even sure radio altimeters are even used on many aircraft anymore.

anamexis 5y ago
GPS surely is not used for instrument landings, and especially not for altitude.
- supernova87a 5y ago
  >> GPS surely is not used for instrument landings, and especially not for altitude. It is. And increasingly so in fact, as it allows more airports that don't have the resources to install and maintain ground-based equipment to have previously unavailable approaches. https://www.flyingmag.com/everything-you-need-to-know-about-rnav-gps-approaches/ https://www.flyingmag.com/everything-you-need-to-know-about-... However, it doesn't fall into exactly the same class as ILS approaches.
  - lisper 5y ago
    Instrument rated pilot here. A GPS LPV approach is essentially the same as an ILS [edit for clarification: a category 1 ILS]. It’s true that an LPV approach is classified as non precision but in practice an LPV approach is pretty much indistinguishable from a cat 1 ILS, which is classified as a precision approach.
    - ketzo 5y ago
      Hm, that's interesting to me. Is it just regulators (FAA I guess?) falling behind the times? Or is the GPS version really not adequate for some use case that is only satisfied by ILS (not sure what that is but I assume it's the "gold standard" landing approach?)
      - c1yd3i 5y ago
        Airports served only by an RNAV (GPS) approach with LPV minimums have different alternate minimums (for planning purposes). That’s the only real distinction. LPV approaches are classified as non-precision due to an ICAO technicality over the lack of a ground transmitter at the airport. GLS approaches compensate for this.
      - p_l 5y ago
        There are different categories of ILS approaches (one of them is actually impossible to implement at this time, as it requires tech that is at best in testing), and most importantly GPS is not considered "safe enough". Most use in heavy aviation is due to easier operation (especially with autopilot) but in better conditions only (i.e. at best equivalent of ILS Category 1)
      - tjohns 5y ago
        GPS notoriously has poor altitude resolution. For this reason, GPS approaches are technically not considered "precision" approaches. WAAS (GPS plus a correction signal transmitted by separate geostationary satellites) improves the resolution to CAT I standards - an "LPV" approach. However, it's still not sufficient for CAT II/III approaches.
        jillesvangurp 5y ago
        You basically use the regular altimeter for altitude during such approaches. It's accurate enough to get to the decision height of a few hundred feet AGL after which you have to fly the rest of the approach manually anyway. That is true with CAT I ILS as well. A CAT III ILS approach requires very specialized equipment and training because you basically fly it all the way down to the runway with no requirement for visibility until moments before touchdown. However, GPS is a lot better when it is not being obstructed by buildings around you. Planes have the benefit of unobstructed paths to GPS satellites and would get a better quality and accuracy then you might get with a phone in a city. Also, using good antennas helps. So, the altitude accuracy would be fine mostly.
- Aspos 5y ago
  It is not uncommon to get +/- 15m error on the vertical axis. There is a significant delay in the signal too, so can't be used for landing.
  - skimo8 5y ago
    This is why a recently calibrated altimeter is used for the altitude axis on GPS approaches. They also must have a constrained error margin to be certified, and it is factored into the buffered region around the approach.
  - creatornator 5y ago
    I think you're being downvoted because the imprecision you speak of is a result of: consumer GPS receivers having a limited polling rate, limited precision, and only using one GPS frequency. You can't extrapolate the limitations of GPS on say a phone, to limitations on a commercial plane. See the following from gps.gov: it can be accurate to within centimeters and with a high polling rate, but most consumer devices don't employ the necessary tricks. https://www.gps.gov/systems/gps/performance/accuracy/ https://www.gps.gov/systems/gps/performance/accuracy/
    - genewitch 5y ago
      Military gets more time resolution as well as no deliberate clock skew. Airliners won't have military GPS. Gps receivers for surveying require on the order of hours to resolve to "centimeters" using civilian GPS. The highest resolution GPS receiver I have still has +/-15ns jitter, which over the course of a day is good enough for maybe 12-20cm resolution, but the first hour the track is wild, 50-80 meters off in varying directions. My GPS is used as a time stamp source for lightning detection and location, so in aggregate with other detectors it can pinpoint a strike to within a meter or so anywhere in my hemisphere.
itisit 5y ago
> I'm not even sure radio altimeters are even used on many aircraft anymore. What misled you to think this? Radio altimeters are used throughout the aviation industry. Their ubiquity and criticality is the issue at hand. [0] [0] https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2021/SAFO21007.pdf https://www.faa.gov/other_visit/aviation_industry/airline_op...
deleted 5y ago
[deleted]
op00to 5y ago
Yes, I’m sure that few meters of vertical inaccuracy will not matter at all during landing.
_moof 5y ago
Radar altimeters are required for reduced minimums on ILS approaches.

magicalist 5y ago

> As a reminder, Turkish Airlines flight 1951 crashed in 2009 due to a faulty radio altimeter which set the autothrottle to idle before they touched down. Was it due to interference? Otherwise this seems specious.

hn_throwaway_99 5y ago
No, it wasn't, and I agree this is a specious example. The Wikipedia entry [1] has a good overview, as does this Medium post [2]. In the flight 1951 crash there were actually 2 altimeters for redundancy, but due to human error they were configured so that 1 faulty altimeter affected the autothrottle: > While on final approach for landing, the aircraft was about 2,000 ft (610 m) above ground, when the left-hand (captain's) radio altimeter suddenly changed from 1,950 feet (590 m) to read −8 feet (−2.4 m) height, although the right-hand (co-pilot's) radio altimeter functioned correctly. The voice recording showed that the crew was given an audible warning signal (landing gear warning horn) that indicated that the aircraft's landing gear should be down, as the aircraft was, according to the captain's radio altimeter, flying too low. This happened several times during the approach to Schiphol. The reason that the captain's radio altimeter was causing problems was the first officer making a mistake when arming the aircraft's autopilot system for a dual channel approach. > The Boeing 737NG type aircraft has two autopilot systems, which can work independently of each other (single channel) or together (dual channel). Turkish Airlines' standard operating procedure at the time stated that all approaches should be flown "dual channel" when available, but the inexperienced first officer forgot to arm approach mode in the aircraft's mode control panel (which controls the autopilot), meaning that the aircraft thought the pilots wanted to do a single-channel approach using control computer A (captain's autopilot), which had a radio altimeter failure. [1] https://en.wikipedia.org/wiki/Turkish_Airlines_Flight_1951 https://en.wikipedia.org/wiki/Turkish_Airlines_Flight_1951 [2] https://admiralcloudberg.medium.com/test-article-the-crash-of-turkish-airlines-flight-1951-440326b3bd9a https://admiralcloudberg.medium.com/test-article-the-crash-o...
- nikcub 5y ago
  Want to plug your second source admiralcloudberg as a brilliant blog that is worth reading if you're into airline crashes and system failures. He's very in depth and detailed in his writing https://admiralcloudberg.medium.com/ https://admiralcloudberg.medium.com/

coderintherye 5y ago

From the report: https://catsr.vse.gmu.edu/SYST460/TA1951_AccidentReport.pdf https://catsr.vse.gmu.edu/SYST460/TA1951_AccidentReport.pdf "Interference as a singular cause appears to be extremely improbable." Interference is mentioned a few times in the report as likely not being the cause or at least sole cause. The only entity seeming to push interference as a cause was Boeing, which led to this remark: "We are not aware that interference has been conclusively identified as the explanation for the radio altimeter anomalies. In fact, some airlines have found that their incidence of radio altimeter anomalies decreased when they chose to install the new gaskets. If the conclusions here are intended to be DSB conclusions, rather than conclusions of Boeing and the airlines, the text should be revised to indicate they are DSB conclusions." in which they struck out the text "The negative values of the radio height were an indication of interference"