6 ms·

> with a maximum indicated air speed of 358kt (662km/h), or Mach 0.9, recorded What? 358 knots is much closer to mach 0.5 in air. 358 meters/second is close.

by sk5t 10y ago

> with a maximum indicated air speed of 358kt (662km/h), or Mach 0.9, recorded What? 358 knots is much closer to mach 0.5 in air. 358 meters/second is close... 662 mph is close.

sk5t 10y ago
Further confusing numbers: > “Up to 24% of the aircraft occupants were rendered temporarily unfit for duties following the incident,” the report adds, referring to 25 passengers and seven crew members. > “Without the excellent technology of the Airbus A330 flight control laws, the outcome could have been very different, with the realistic potential for the loss of the aircraft and 198 of our people,” MAA director general AM Richard Garwood says in his summary of the incident. 24% of 198 is not 25+7 but 47-ish. What am I missing here?
- deleted 10y ago
  [deleted]
- colanderman 10y ago
  "Up to" ;)
mikeash 10y ago
Indicated airspeed is based on the force of the oncoming air, and decreases as air gets less dense. There's some altitude where 358kt indicated is Mach 0.9. Edit: before I get more replies attempting to lecture me on how wrong I am, please look up the difference between true airspeed (how fast the air is moving past the airplane) and indicated airspeed (a measurement derived from the difference between static and dynamic pressure on the aircraft that only matches how fast the air is moving when you're at sea level).
- lutusp 10y ago
  > There's some altitude where 358kt indicated is Mach 0.9. Not really: http://www.aerospaceweb.org/question/atmosphere/q0112.shtml http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
  - mikeash 10y ago
    Those tables make no reference to indicated airspeed.
- sk5t 10y ago
  Thanks for the clarification. Is it normal to refer to kts with the assumption this is related to pressure on the airframe and not actual airspeed? The reference to the 365KIAS rating later in the article provides only a clue IMO.
  - mikeash 10y ago
    It literally says "indicated air speed." There's no need for interpretation or assumption. If I saw a raw airspeed without and qualifier, I'd probably assume it meant indicated speed. But I fly small planes where that's the only airspeed available. If you want true airspeed, you have to take what's indicated and convert manually.
    - sk5t 10y ago
      So it does. I still find this disunion of UOM disagreeable, in the way that "in thousandths, take 300 grams of acetaminophen twice daily" is troubling.
      - mikeash 10y ago
        I'm not familiar with that one. Does that mean to take 300mg?
        Filligree 10y ago
        I hope so. It sure as hell can't mean 300g, and anything other than mg would be crazy, but I'm not feeling very confident here.
      - NamTaf 10y ago
        Why? Do you take issue to the fact that scales show you a measurement in kg rather than N despite it varying as you move around the world (and potentially with changes in temperature, depending on the type of scales)? Velocity/mass is far more intuitive than pressure/force even if they don't apply exactly the same in all cases. It's about what works easiest for the users of the system.
- djrogers 10y ago
  That's not correct. At 33,00 ft, Mach 0.9 is about 600Kts, the altitude charts don't go high enough for Mach 0.9 to be 358Kts - by that point yo haven't enough air to transmit sound. And if you're implying that 358kts indicated could be 600kts actual with the right tailwind - that's true, but it still wouldn't be Mach 0.9, as the speed of sound applies only to airflow over the airplane. No matter the tailwind, the airflow will remain subsonic. If this weren't the case, passenger liners could go supersonic with a 200kt tailwind all day long...
  - mikeash 10y ago
    It has nothing to do with wind. 358kts indicated at 30,000ft translates to about 670kts true airspeed, for example. It's purely a result of how airplanes measure speed. https://en.m.wikipedia.org/wiki/Indicated_airspeed https://en.m.wikipedia.org/wiki/Indicated_airspeed
  - jsmthrowaway 10y ago
    Neither is this. An airplane measures indicated airspeed by sampling dynamic pressure. This happens to be useful because it has a direct correlation to various important V-speeds, such as the speed where a wing in a certain configuration generates enough lift for forward flight and so on. This is why an aircraft's V-speeds are given in KIAS. (Put another way, you get the density altitude calculation for free by relying on IAS, as the airplane gets more "inaccurate" with altitude but in actuality "accurate" from the perspective of fluids traveling over surfaces.) As an airplane gains altitude, IAS dramatically falls versus true airspeed due to this measurement artifact. Complicating this calculation for someone approaching it physically, as you have: by regulation all aircraft calibrate to the same altimeter pressure (29.92 inHg) beyond 18,000 feet. If you think about this physically, that means a whole bunch of airplanes indicate 35,000 feet but in actuality linger around 34,000 or 36,000 or something depending on the actual atmospheric pressure. That also affects how airspeed is calculated, meaning you can't say "well, at 33,000 feet, Mach 0.9 is blah." That computation is a little more nuanced based on the way aircraft sensors are set up and, until computers, required a whiz wheel. 'mikeash is correct, here. You're correct for TAS. IAS is a weird aerospace-specific thing where it's actually helpful to be wrong. I know. To the point of the article, too, the computer worked to avoid exceeding Vne and probably started intervening after passing Vno, which are both figures given in KIAS because both rely on how thick the air is. (Everything in this comment applies to subsonic flight, and the entire calculus changes when you're >=Mach 1.)
- DonaldFisk 10y ago
  Thin and very cold air. The altitude was 33000 feet (FL330). Assuming a ground temperature of 15 celsius and a drop in temperature of 2 degrees per 1000 feet gives an OAT of -51 celsius. My flight computer converts 358KIAS @ 33000feet @ -51celsius to 620KTAS, which is Mach 0.94. FL = flight level (hundreds of feet) KIAS = knots indicated air speed KTAS = knots true air speed OAT = outside air temperature
lutusp 10y ago
The speed of sound varies with altitude. Nevertheless, at 30,000 feet, Mach 1 is 1,091 km/h, so the original statement is not correct (mach 0.9 at 30,000 feet would be about 982 km/h).
- sk5t 10y ago
  My takeaway/learning is that "knots indicated airspeed" is a measure of pressure due to the aircraft mushing its way through the atmosphere, and is vaguely in proportion to actual_airspeed * altitude, and from my non-pilot's perspective ought to be expressed in bars or torr or something else meant for pressure and not velocity or distance.
  - jsmthrowaway 10y ago
    The airplane flies based on how much of that mush is doing work on various parts of it, which means "I stay in the air" is a complex calculation of speed and pressure. Factoring in the slowly-decreasing density of that mush to those very important speeds saves calculation time, which is why pilots stick with indicated as the baseline. It's a known-wrong measurement with some valuable uses, and true airspeed and ground speed are usually available in the cockpit in real time. They're just not flown by.
  - mannykannot 10y ago
    You are not going to like 'density altitude' then, and the terms 'light year' 'parsec' and 'electron volt' will blow your mind. Despite the efforts of standards bodies to regularize everything, people continue to use measures that match their purpose.
  - mcguire 10y ago
    IAS is calculated from the difference between the outside air pressure as measured by a "static" port on the side of the aircraft and the pressure measured by the pitot tube pointed in (hopefully) the direction of travel. It could be measured as pressure, but it is proportional to true airspeed under standard temperatures and pressures. The short answer is that bus left about a hundred years ago. See about page 10-24: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch10.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a...
noxryan 10y ago
358KIAS is in fact (or is close) to the Vne (never exceed) speed of an A330. This is an indicated airspeed limit while mach numbers are based off TAS (True Airspeed). Here's a link to help explain the differences between IAS/CAS and TAS: https://www.decodedscience.org/airspeed-of-an-aircraft-indicated-airspeed-ias-and-true-airspeed-tas/5035 https://www.decodedscience.org/airspeed-of-an-aircraft-indic...

by sk5t 10y ago

> with a maximum indicated air speed of 358kt (662km/h), or Mach 0.9, recorded What? 358 knots is much closer to mach 0.5 in air. 358 meters/second is close... 662 mph is close.

sk5t 10y ago

Further confusing numbers: > “Up to 24% of the aircraft occupants were rendered temporarily unfit for duties following the incident,” the report adds, referring to 25 passengers and seven crew members. > “Without the excellent technology of the Airbus A330 flight control laws, the outcome could have been very different, with the realistic potential for the loss of the aircraft and 198 of our people,” MAA director general AM Richard Garwood says in his summary of the incident. 24% of 198 is not 25+7 but 47-ish. What am I missing here?

deleted 10y ago
[deleted]
colanderman 10y ago
"Up to" ;)

mikeash 10y ago

Indicated airspeed is based on the force of the oncoming air, and decreases as air gets less dense. There's some altitude where 358kt indicated is Mach 0.9. Edit: before I get more replies attempting to lecture me on how wrong I am, please look up the difference between true airspeed (how fast the air is moving past the airplane) and indicated airspeed (a measurement derived from the difference between static and dynamic pressure on the aircraft that only matches how fast the air is moving when you're at sea level).

lutusp 10y ago
> There's some altitude where 358kt indicated is Mach 0.9. Not really: http://www.aerospaceweb.org/question/atmosphere/q0112.shtml http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
- mikeash 10y ago
  Those tables make no reference to indicated airspeed.
sk5t 10y ago
Thanks for the clarification. Is it normal to refer to kts with the assumption this is related to pressure on the airframe and not actual airspeed? The reference to the 365KIAS rating later in the article provides only a clue IMO.
- mikeash 10y ago
  It literally says "indicated air speed." There's no need for interpretation or assumption. If I saw a raw airspeed without and qualifier, I'd probably assume it meant indicated speed. But I fly small planes where that's the only airspeed available. If you want true airspeed, you have to take what's indicated and convert manually.
  - sk5t 10y ago
    So it does. I still find this disunion of UOM disagreeable, in the way that "in thousandths, take 300 grams of acetaminophen twice daily" is troubling.
    - mikeash 10y ago
      I'm not familiar with that one. Does that mean to take 300mg?
      - Filligree 10y ago
        I hope so. It sure as hell can't mean 300g, and anything other than mg would be crazy, but I'm not feeling very confident here.
    - NamTaf 10y ago
      Why? Do you take issue to the fact that scales show you a measurement in kg rather than N despite it varying as you move around the world (and potentially with changes in temperature, depending on the type of scales)? Velocity/mass is far more intuitive than pressure/force even if they don't apply exactly the same in all cases. It's about what works easiest for the users of the system.
djrogers 10y ago
That's not correct. At 33,00 ft, Mach 0.9 is about 600Kts, the altitude charts don't go high enough for Mach 0.9 to be 358Kts - by that point yo haven't enough air to transmit sound. And if you're implying that 358kts indicated could be 600kts actual with the right tailwind - that's true, but it still wouldn't be Mach 0.9, as the speed of sound applies only to airflow over the airplane. No matter the tailwind, the airflow will remain subsonic. If this weren't the case, passenger liners could go supersonic with a 200kt tailwind all day long...
- mikeash 10y ago
  It has nothing to do with wind. 358kts indicated at 30,000ft translates to about 670kts true airspeed, for example. It's purely a result of how airplanes measure speed. https://en.m.wikipedia.org/wiki/Indicated_airspeed https://en.m.wikipedia.org/wiki/Indicated_airspeed
- jsmthrowaway 10y ago
  Neither is this. An airplane measures indicated airspeed by sampling dynamic pressure. This happens to be useful because it has a direct correlation to various important V-speeds, such as the speed where a wing in a certain configuration generates enough lift for forward flight and so on. This is why an aircraft's V-speeds are given in KIAS. (Put another way, you get the density altitude calculation for free by relying on IAS, as the airplane gets more "inaccurate" with altitude but in actuality "accurate" from the perspective of fluids traveling over surfaces.) As an airplane gains altitude, IAS dramatically falls versus true airspeed due to this measurement artifact. Complicating this calculation for someone approaching it physically, as you have: by regulation all aircraft calibrate to the same altimeter pressure (29.92 inHg) beyond 18,000 feet. If you think about this physically, that means a whole bunch of airplanes indicate 35,000 feet but in actuality linger around 34,000 or 36,000 or something depending on the actual atmospheric pressure. That also affects how airspeed is calculated, meaning you can't say "well, at 33,000 feet, Mach 0.9 is blah." That computation is a little more nuanced based on the way aircraft sensors are set up and, until computers, required a whiz wheel. 'mikeash is correct, here. You're correct for TAS. IAS is a weird aerospace-specific thing where it's actually helpful to be wrong. I know. To the point of the article, too, the computer worked to avoid exceeding Vne and probably started intervening after passing Vno, which are both figures given in KIAS because both rely on how thick the air is. (Everything in this comment applies to subsonic flight, and the entire calculus changes when you're >=Mach 1.)
DonaldFisk 10y ago
Thin and very cold air. The altitude was 33000 feet (FL330). Assuming a ground temperature of 15 celsius and a drop in temperature of 2 degrees per 1000 feet gives an OAT of -51 celsius. My flight computer converts 358KIAS @ 33000feet @ -51celsius to 620KTAS, which is Mach 0.94. FL = flight level (hundreds of feet) KIAS = knots indicated air speed KTAS = knots true air speed OAT = outside air temperature

lutusp 10y ago

The speed of sound varies with altitude. Nevertheless, at 30,000 feet, Mach 1 is 1,091 km/h, so the original statement is not correct (mach 0.9 at 30,000 feet would be about 982 km/h).

sk5t 10y ago
My takeaway/learning is that "knots indicated airspeed" is a measure of pressure due to the aircraft mushing its way through the atmosphere, and is vaguely in proportion to actual_airspeed * altitude, and from my non-pilot's perspective ought to be expressed in bars or torr or something else meant for pressure and not velocity or distance.
- jsmthrowaway 10y ago
  The airplane flies based on how much of that mush is doing work on various parts of it, which means "I stay in the air" is a complex calculation of speed and pressure. Factoring in the slowly-decreasing density of that mush to those very important speeds saves calculation time, which is why pilots stick with indicated as the baseline. It's a known-wrong measurement with some valuable uses, and true airspeed and ground speed are usually available in the cockpit in real time. They're just not flown by.
- mannykannot 10y ago
  You are not going to like 'density altitude' then, and the terms 'light year' 'parsec' and 'electron volt' will blow your mind. Despite the efforts of standards bodies to regularize everything, people continue to use measures that match their purpose.
- mcguire 10y ago
  IAS is calculated from the difference between the outside air pressure as measured by a "static" port on the side of the aircraft and the pressure measured by the pitot tube pointed in (hopefully) the direction of travel. It could be measured as pressure, but it is proportional to true airspeed under standard temperatures and pressures. The short answer is that bus left about a hundred years ago. See about page 10-24: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch10.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a...

noxryan 10y ago

358KIAS is in fact (or is close) to the Vne (never exceed) speed of an A330. This is an indicated airspeed limit while mach numbers are based off TAS (True Airspeed). Here's a link to help explain the differences between IAS/CAS and TAS: https://www.decodedscience.org/airspeed-of-an-aircraft-indicated-airspeed-ias-and-true-airspeed-tas/5035 https://www.decodedscience.org/airspeed-of-an-aircraft-indic...