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Comment by sk5t | Hacker News Reader

> 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.

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).

> There's some altitude where 358kt indicated is Mach 0.9.

Not really: http://www.aerospaceweb.org/question/atmosphere/q0112.shtml

Those tables make no reference to indicated airspeed.

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.

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.

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.

I'm not familiar with that one. Does that mean to take 300mg?

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.

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.

kg/N varies, but by less than your normal margin of error. That's vastly different from deciding to use newtons to weigh "kilograms indicated" on your trip to mars when they give a vastly different number.

My point was that we don't weigh ourself in 'newtons' despite that being what we are actually measuring. We make some reference conversion to a much more intuitive unit, kilograms. We rely on the fact that it's a pretty constant conversion between the two.

Is this really much different than that? I don't think so. The aerodynamics of the plane (stresses on the structure, performance of the aircraft w.r.t. stall, etc.) all remain ~constant for a given IAS which is why it's used. We're just making a dynamic pressure measurement and converting it to a speed in such a way that it factors in all the environmental conditions that influence what we actually care about - how the plane responds in the air.

The N/Kg conversion might be off by a factor of what, 1% in extreme cases (Denver)? Whereas IAS can easily be a factor of two or more different from the plain English sense of "airspeed".

My point is that airspeed is a vector measured relative to something. In this case, it's set to a specific situation (25C/101.3kPa/0% humidity) and then measured relative to the stagnation pressure at that point because then no matter those variables, the plane reacts more or less the same way for the pilot, who is the one who needs to understand what's happening. Your 'plain English' sense of 'airspeed' is pretty irrelevant to the most critical parts of flight - takeoff, landing and not exceeding the plane's design limits.

Moreover, it also means that if you're flying a very basic plane, you can use external stimuli to understand what the other variables are doing and account for them (also noting that IAS converges to the other 'air speeds', within reason, as you approach the ground). You have markers on runways, wind socks, etc. to all get a feel for what your IAS translates to on the ground and thus how much speed you're going to have when you hit the runway. That seems far more useful to me than having some kPa display on the dash.

> Your 'plain English' sense of 'airspeed' is pretty irrelevant to the most critical parts of flight - takeoff, landing and not exceeding the plane's design limits.

I know the pilot needs a measure that behaves like IAS. I just think it's misleading to call the thing we measure "airspeed" when it can be so radically different from, well, airspeed.

> You have markers on runways, wind socks, etc. to all get a feel for what your IAS translates to on the ground and thus how much speed you're going to have when you hit the runway. That seems far more useful to me than having some kPa display on the dash.

I don't understand this argument. Surely if you were flying in a plane that displayed kPa those same markers etc. would give you exactly the same sense for how a given kPa reading translates into speed on the ground. All that would change would be that the conversion factor would be slightly different. Is "110 knots" any more intuitive (to a non-pilot) as a measure of everyday speed on the ground than "20 pascals"?

It depends on the scales one uses. You're right for most bathroom scales, but not for balance-beam scales as one tends to find in physicians' offices; those measure mass and are accurate in a wide variety of accelerated reference frames (and in particular are insensitive to anisotropies in the Earth's gravitational field).

Accurate weight-measuring scales rely on m = F_w/g, where m is mass, F_w is the normal force delivered by the scale's measuring surface to the measured object, and g is almost always g_n := 9.806 m/s^2 per ISO 80000-3:2006 (item 3-9.2) or similar standards. (Tbh the ISO document does not do much more than offer up a defined value.) This relation only holds (and only approximately) when the scale is used as intended by its designer, and that condition picks out a set of preferred (and accelerated) reference frames. A "contact camp" supporter (like Baez[1]) would say that the measurement of weight is accurate in a much wider variety of frames, crucially including in local free fall.

Modern weight-measuring scales will almost always use SI units internally, but most can supply a variety of output units (the one in my bathroom can show kg, stone, lb, Newtons, and the value read from the piezo element package. It sadly has no internal bubble level or equivalent, and no accelerometer, and its software doesn't make use of data from devices within the mobile device it runs on (which could be placed on the scale between my feet). Finally, it was expensive enough to do any or all of the above, so I won't recommend it).

More colloquially:

http://www.livestrong.com/article/343183-the-differences-bet...

[1] http://math.ucr.edu/home/baez/physics/General/Weight/whatIsW... -- and revisiting this wonderful page is the real reason for this reply. :-)

The aircraft has a maximum speed rating, beyond which the structure may fail. That speed rating is based on the aerodynamic forces on the aircraft, which are proportional to the indicated airspeed, not the true airspeed. That's why the instrument panel almost universally shows IAS. That way the pilot doesn't need to perform an altitude-based nonlinear conversion in his head during an emergency to see that he's about to kill everyone on board.

Just so you know the forces are actually proportional to the square of equivalent airspeed which is indicated airspeed adjusted for the compressibility of the air at the sensor. Indicated airspeed is still a better number than true airspeed for approximating the aero forces

So long as we are being pedantic: The sensor just reads dynamic pressure. To get IAS from the pressure signal it is sufficient to solve the equation dynamic_pressure = density * indicated_airspeed^2, where density is taken to be air at a reference temperature at sea level (mumble mumble except for transonic and supersonic conditions mumble). In order to compute true airspeed, you need to measure the absolute pressure and temperature of the air you're traveling through in order to calculate the density.

The aerodynamic forces are proportional to the dynamic pressure, which is the native reading on the sensor. But when you relate dynamic pressure to indicated airspeed, the do-not-exceed airspeed doesn't change, so you can stick a single red line on the IAS gauge that's always valid.

Uh, I think they have a red line on the gauge, anyway. I just saw an unmanned prototype break up in flight partially because of the lack of said red line, so I'm kinda just assuming that passenger aircraft have one.

When it comes to redline, there are some factors which relate to true airspeed, not indicated. Smaller aircraft (maybe larger too, I'm just limiting it to "smaller" due to my own ignorance) at higher altitude tend to be limited by flutter (a situation where positive feedback from movements of the wings or other surfaces shakes the plane apart) which mostly depends on true airspeed. In my plane, for example, never-exceed speed (Vne) is 143kts indicated at sea level, 121kts indicated at 10,000ft, and a mere 73kts indicated at 36,000ft.

The 143kts figure is marked on my airspeed indicator, but above 10,000ft I have to remember to refer to it elsewhere if I want to go fast.

FYI

  dynamic_pressure = 0.5 * ref_density * equivalent_airspeed^2

Its not a function of indicated airspeed. Indicated airspeed needs to be corrected for the compressibility of air to obtain the equivalent airspeed. At low airspeeds/altitudes, the indicated airspeed and equivalent airspeed are very close, so your equation would hold up just fine.

As a result at higher altitudes and faster speeds, the red line on an indicated airspeed gauge would be variable to account for the effects of compressibility.

When coining new terms at my day job (software engineer) I try to avoid terms like "indicated air speed" for the reason that part of the term – "indicated" in this case – is easily ignored by those less familiar with the terminology, because it seems superfluous. (In this case, "indicated" to a layperson simply means "it was shown on a dial". In that interpretation, it carries little information and hence gets ignored.)

We had a similar problem at my day job (a storage company) – there were two kinds of entities, one called a "volume" and one called a "storage volume". The were related (hence the similar names) but not the same thing. Of course, those less familiar with the difference kept dropping "storage" from "storage volume" to cause all sorts of confusion.

My solution? Eradicate "storage volume" from our vocabulary and replace it with the pseudo-acronym "SV", which stood for nothing. Confusion ended.

Indicated airspeed is not a term for laypeople. Its meaning and significance is well understood by every pilot and aero engineer.

Those less familiar with the terminology generally don't fly aeroplanes. Indicated air speed is different from true air speed and ground speed, but this is essential complexity - pilots need to know it.

Indicated air speed is determined by measuring pressure in the pitot tube (https://en.wikipedia.org/wiki/Pitot_tube) and is shown on the air speed indicator, which on an A330 is on the Primary Flight Display (https://en.wikipedia.org/wiki/Primary_flight_display). Important speeds such as stall speed (https://en.wikipedia.org/wiki/Stall_(fluid_mechanics)#Speed) and never exceed speed have constant values of indicated air speed.

True air speed can be calculated if you know the air pressure, which depends on the altitude and outside air temperature. Altimeters work by measuring air pressure.

It's a good idea when reporting something to take into account the readership, and to a lay audience explain things which are counter-intuitive.

That seems sensible, but it's way too late to make the change in aviation. You've got a century or so of tradition behind this terminology. The good news is that if you're in a position to make any use of an "indicated air speed" number, then you'll have been taught what it means early on. It's not obvious for laymen, but it's not intended for laymen either.

As a pilot, you cannot escape knowing this terminology. Knowing how your instruments do their measurements ("indicated vs true air speed", among others) is absolutely integral to the job, if you don't want to fly into a formation of "cumulo-granite" somewhere along the line.

One is what your pitot tube is telling you - which is a combination of altitude, weather conditions, wind, and airspeed.

The other is how much air is flowing over your wings - which is what actually keeps you aloft.

To quote the ace fighter pilot Yogi Berra - "In theory, theory and practice are the same. In practice, they are not".

You cannot escape pressure-altitude. It has way more implications than this. Starting with your ability to get off the ground.

IIRC, true airspeed is not especially useful. Ground speed is important for navigation (although hard to get accurately without GPS) and IAS tells you the forces acting on the aircraft.

"KIAS" is literally "knots indicated air speed"

"KTAS" would be "knots true air speed"

It's no more a clue than "mph" would be a clue for "miles per hour", IMO

Yes, my suggestion is that KIAS is an okay UOM, and knots is inappropriate/confusing.

However, unlike the xkcd strip about HO model trains, I'm not ready to mount a ferocious defense with regard to this thing I just heard about.

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...

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

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.)

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

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-indic...

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?

"Up to" ;)

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).

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.

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.

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.

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/a...

Indicated airspeed, as sibling comments point out, is not true airspeed. The air is moving at 500 but the stagnation pressure is the same as if it were moving only 300 at sea level so the indicated speed will be 300 +- calibration errors.

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