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A330 flight control laws saved Voyager, inquiry finds

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Re: A330 flight control laws saved Voyager, inquiry finds

#161

Post by raattgift »

NamTaf wrote: post #95
Dylan16807 wrote: post #58

Earlier quoted context omitted.

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.…

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. :-)

Re: A330 flight control laws saved Voyager, inquiry finds

#162

Post by mcguire »

knute wrote: post #140
illys wrote: post #135

Earlier quoted context omitted.

Other figures are also surprising: "a maximum indicated air speed of 358kt (662km/h), or Mach 0.9", while at 30000ft, Mach 0.9 is typically 983.7 km/h(source among others: https://www.globalaircraft.org/converter.htm ) Maybe "indicated" makes a difference, but so wide?

The outside of the airplane has a forward-pointing tube called a pitot tube. As the airplane travels forward, air is compressed into this tube. By measuring the pressure, you can derive the airspeed. However the higher you go, the lower the ambient air pressure is and so the "indicated airspeed" is not at all accurate. However indicated airspeed is still very useful as it measures the air resistance the aircraft is e…

Indicated airspeed is not related to ground speed, but indicated airspeed is the important number, as far as keeping the aircraft in the air is concerned.

Re: A330 flight control laws saved Voyager, inquiry finds

#163

Post by Pxtl »

knute wrote: post #140
illys wrote: post #135

Earlier quoted context omitted.

Other figures are also surprising: "a maximum indicated air speed of 358kt (662km/h), or Mach 0.9", while at 30000ft, Mach 0.9 is typically 983.7 km/h(source among others: https://www.globalaircraft.org/converter.htm ) Maybe "indicated" makes a difference, but so wide?

The outside of the airplane has a forward-pointing tube called a pitot tube. As the airplane travels forward, air is compressed into this tube. By measuring the pressure, you can derive the airspeed. However the higher you go, the lower the ambient air pressure is and so the "indicated airspeed" is not at all accurate. However indicated airspeed is still very useful as it measures the air resistance the aircraft is e…

On the subject of indicated airspeed, does the mach number have the same meaning? That is, if the aircraft had reached an indicated airspeed of Mach 1, would they break the sound barrier and experience a sonic boom?

Re: A330 flight control laws saved Voyager, inquiry finds

#164

Post by mcguire »

xenophonf wrote: post #156

I was curious, so I found a picture of an A330-200 cockpit here: http://www.airplane-pictures.net/photo/251517/cs-tom-tap-por... I'd like to know what kind of camera is going to fall against the pilot's stick and jam it forward!

Judging by the perspective, the seats are all the way back. When occupied, the end of the armrest should be somewhere close to the sidestick.

Re: A330 flight control laws saved Voyager, inquiry finds

#165

Post by brandmeyer »

drewkett wrote: post #81
brandmeyer wrote: post #71

Earlier quoted context omitted.

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…

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.

Re: A330 flight control laws saved Voyager, inquiry finds

#166

Post by rtkwe »

rconti wrote: post #99
rtkwe wrote: post #50

Earlier quoted context omitted.

> “With his feet on the flight deck roof, the co-pilot reached down and attempted to disengage the autopilot by pulling back on his sidestick,” In this case I don't think they really knew that the issue was the jammed stick before the co-pilot pulling back on his stick gave the flight computer priority. It's hard to really say from the information given exactly when they discovered the camera was jammed and removed i…

They also said the captain was pulling back on the stick as well.. or at least ATTEMPTING to. Maybe they're indicating that the stuck camera (which seems to have come unstuck before recovery was complete?) prevented him from doing so. It would be really weird for both pilots pulling back on the stick to result in a dual input error situation.

Not that weird I think the net force on the Captain's stick was probably still forward with the camera jammed there with him trying to pull back.

Re: A330 flight control laws saved Voyager, inquiry finds

#167

Post by [deleted] »

tropo wrote: post #43
jsmthrowaway wrote: post #35

Earlier quoted context omitted.

The computer worked to prevent the aircraft from entering an overspeed condition by pitching up and attempting to recover from the dive even before that dual stick input was an issue. It was intervening 13 seconds into the incident, as the aircraft threatened to exceed safety tolerances. The dual stick input created another issue after that intervention began. The automation very possibly saved lives here. The oversp…

Nope, it wouldn't happen in a Boeing aircraft. This is a well-known Airbus defect that was a contributing factor to the crash of Air France Flight 447. Boeing uses traditional controls. It's like a race car steering wheel on a big movable stick. It is really easy to see how the control is positioned. Airbus uses a small joystick off to the side. It is force-based like a Thinkpad pointing stick, so it barely moves whe…

[deleted]

Re: A330 flight control laws saved Voyager, inquiry finds

#168

Post by mikeash »

brandmeyer wrote: post #165
drewkett wrote: post #81

Earlier quoted context omitted.

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…

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.

Re: A330 flight control laws saved Voyager, inquiry finds

#169

Post by digler999 »

hh2222 wrote: post #2

That will likely cut down on flight deck videos on youtube.

Thats what came to my mind too. Which is sad. I definitely dont want passengers exposed to more risk, but I'm also against "its policy" kind of rules that squelch people from having fun and exposing more people to aviation.

Re: A330 flight control laws saved Voyager, inquiry finds

#170

Post by digler999 »

huh333i wrote: post #54
duskwuff wrote: post #39

Earlier quoted context omitted.

> If you get too careless it will even run the engines up to full takeoff thrust and microblog it to you. What does "microblog" mean in this context? I assume it doesn't mean that the A330 isn't posting about its engine thrust on Twitter. (Yet.)

The thrust FMA (flight mode annunciator) will show "A. FLOOR" alpha floor or "TOGA LK" (takeoff/go-around thrust lock). The former means Fifi just took over and is applying full thrust for an immediate high AoA situation. The latter means the emergency is over but Fifi is so white knuckled and panting that she's ignoring lever position and keeping full thrust. I say microblog because you have to make a conscious effo…

When thrust is applied by the computer, do the throttle levers move on their own or stay in their original positions ?
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