5 ms·

Does anyone else wonder why they need to actually fly the devices instead of just extrapolating from the electromagnetic interference test results required by t

by sunir 15y ago

Does anyone else wonder why they need to actually fly the devices instead of just extrapolating from the electromagnetic interference test results required by the FCC? It feels as if they believe in voodoo rather than physics. Is there a reason they require device specific tests?

sage_joch 15y ago
That's like asking why integration tests are needed when individual modules have already been unit tested.
roadnottaken 15y ago
Exactly. I thought that was precisely why the FCC regulated the type of signals that consumer devices emit.
raganwald 15y ago
Simulations only go so far. What if there was some sort of weird interaction between an iPad and the magnetic field of the earth at altitude, or cosmic rays, or something—I obviously don’t know what. But whatever it is, you don’t want to do all your tests in a “simulation,” then have some scientists say “Oops, our bad, we forgot to simulate lower pressure or increased gamma rays or what-have-you,” when we can simply fly the device and all of our testing equipment. Now excuse me, I have to return to preparing for the Invasion of Moghron from the Frozen Seas of Neptune
harshreality 15y ago
Perhaps they aren't willing to assume that the electronics in planes are as shielded as they're supposed to be. What confuses me is why they don't ensure that planes' electronics are properly shielded rather than testing a bunch of electronics that emit very weak to weak EM. If a cell phone could create problems for a flight during take-off, what about a lunatic pointing a HERF weapon at the plane?
- HeyLaughingBoy 15y ago
  Shielding is only part of the battle; the other part is what they're doing here: controlling incoming emissions. Nothing can be perfectly shielded in the real world and instruments or controllers may need very long cables to connect to their respective sensors and actuators making shielding difficult. Now add in that many of these instruments actually need to process an RF signal and so won't work if shielded, and it becomes a difficult problem to solve.
deleted 15y ago
[deleted]
Anechoic 15y ago
Is there a reason they require device specific tests? It's the "unknowns" not accounted for in the models that are the (potential) issue. If you have time read through this Ars Technica thread where a bunch of these questions are hashed out: http://arstechnica.com/civis/viewtopic.php?f=23&t=1163338 http://arstechnica.com/civis/viewtopic.php?f=23&t=116333... For those without the time, I'll highlight one post from page 5: "Unintentional radiators. The reason they ask you to power down ALL electronics, and not just turn off radios, is that the oscillators (the clock that runs all digital devices) on the circuit board can act as miniature radios, in and of themselves, via clock signals on circuit traces. They emit at the clock frequency, and in some cases many higher level harmonics of those frequencies. As I've stated several times, it's actually pretty easy to mitigate against known frequencies and signalling techniques...it's the ones you don't know about that are the problem. EMI can be downright spooky. As an example: I was once testing a medical device (for the Medical Device Directive) that was required to failsafe, since it would literally be touching a patient (it was a combination pulseoximeter and a few other things all rolled into one. There were numerous tests for both emissions and immunity, and things were going along ducky until we noticed two separate failure modes that weren't considered failsafe. At the time, we were testing ESD immunity up to 20kV. It would pass one time, and not another. We thought maybe we had a bad unit, so we got a few more from the manufacturer. That entire week we kept trying to figure out how to make the failure repeatable, without luck. My coworker and I went in over the weekend, and could simply not make any of the units fail at all, with the exact same test. That's what triggered our thought process...what else could be causing the issue during the work week but not on a weekend? Other immunity tests! Turns out, a dozen or so meters away, a different device was undergoing a different test...that wasn't required for the MDD. It was a conducted immunity test (it may have been Electric Fast Transients, can't recall) but the actual test signal was leaking out of that lab, and into our lab, via AC lines in the building. Our chief engineer submitted a proposal to add whatever test it was to the MDD, but I don't know whatever became of that. I've seen simple clocks inside electronic gear cause CPUs to go haywire...in effect a single system interfering with itself. I've also seen extremely low power, yet very high frequency harmonics invade and corrupt function of another device, several meters away."
- SlowButEffectiv 15y ago
projektx 15y ago
Because its for airplanes, up in the air, with hundreds of people on them, sitting on thousands of gallons of jet fuel hurtling through the atmosphere over a densely populated area near you.
baddox 15y ago
I'm pretty sure there have been enough people that ignore the rules and leave their devices on for us to be reasonably sure there's no substantial risk.
- tzs 15y ago
  Isn't it possible that 300 passengers using devices at the same time could pose substantially more risk than just a handful of people using devices?
  - baddox 15y ago
    It's possible, but it's also possible that each passenger has ten iPads in their carry-on bag. Will they perform a test flight with 3,000 iPads? Not to mention how many more iPads could be stored in checked luggage.
    - mkramlich 15y ago
      3000 iPads * $800 per iPad = $240k cost of plane itself + cost of all the human lives on it = many millions, or "priceless", right? seems cheap in comparison. plus they don't need to perform this test on every plane before every single flight. only once or a few times, per model permutation. If you spread the testing cost across all the operational instances where they'd be de-risked, I bet it would be pennies per flight, at most.
Estragon 15y ago
It's admirable and entirely understandable that they should want to move to a policy grounded in direct empirical evaluations. Beats the hell out of the current situation, and I bet if it was your responsibility you would do the same. Some risks (hundreds of lives, hundreds of millions of dollars) are bound to trigger excessive caution.
mkramlich 15y ago
"In theory, there is no difference between theory and reality. In reality, there almost always is."

by sunir 15y ago

sage_joch 15y ago

That's like asking why integration tests are needed when individual modules have already been unit tested.

roadnottaken 15y ago

Exactly. I thought that was precisely why the FCC regulated the type of signals that consumer devices emit.

raganwald 15y ago

Simulations only go so far. What if there was some sort of weird interaction between an iPad and the magnetic field of the earth at altitude, or cosmic rays, or something—I obviously don’t know what. But whatever it is, you don’t want to do all your tests in a “simulation,” then have some scientists say “Oops, our bad, we forgot to simulate lower pressure or increased gamma rays or what-have-you,” when we can simply fly the device and all of our testing equipment. Now excuse me, I have to return to preparing for the Invasion of Moghron from the Frozen Seas of Neptune

harshreality 15y ago

Perhaps they aren't willing to assume that the electronics in planes are as shielded as they're supposed to be. What confuses me is why they don't ensure that planes' electronics are properly shielded rather than testing a bunch of electronics that emit very weak to weak EM. If a cell phone could create problems for a flight during take-off, what about a lunatic pointing a HERF weapon at the plane?

HeyLaughingBoy 15y ago
Shielding is only part of the battle; the other part is what they're doing here: controlling incoming emissions. Nothing can be perfectly shielded in the real world and instruments or controllers may need very long cables to connect to their respective sensors and actuators making shielding difficult. Now add in that many of these instruments actually need to process an RF signal and so won't work if shielded, and it becomes a difficult problem to solve.

deleted 15y ago

[deleted]

Anechoic 15y ago

Is there a reason they require device specific tests? It's the "unknowns" not accounted for in the models that are the (potential) issue. If you have time read through this Ars Technica thread where a bunch of these questions are hashed out: http://arstechnica.com/civis/viewtopic.php?f=23&t=1163338 http://arstechnica.com/civis/viewtopic.php?f=23&t=116333... For those without the time, I'll highlight one post from page 5: "Unintentional radiators. The reason they ask you to power down ALL electronics, and not just turn off radios, is that the oscillators (the clock that runs all digital devices) on the circuit board can act as miniature radios, in and of themselves, via clock signals on circuit traces. They emit at the clock frequency, and in some cases many higher level harmonics of those frequencies. As I've stated several times, it's actually pretty easy to mitigate against known frequencies and signalling techniques...it's the ones you don't know about that are the problem. EMI can be downright spooky. As an example: I was once testing a medical device (for the Medical Device Directive) that was required to failsafe, since it would literally be touching a patient (it was a combination pulseoximeter and a few other things all rolled into one. There were numerous tests for both emissions and immunity, and things were going along ducky until we noticed two separate failure modes that weren't considered failsafe. At the time, we were testing ESD immunity up to 20kV. It would pass one time, and not another. We thought maybe we had a bad unit, so we got a few more from the manufacturer. That entire week we kept trying to figure out how to make the failure repeatable, without luck. My coworker and I went in over the weekend, and could simply not make any of the units fail at all, with the exact same test. That's what triggered our thought process...what else could be causing the issue during the work week but not on a weekend? Other immunity tests! Turns out, a dozen or so meters away, a different device was undergoing a different test...that wasn't required for the MDD. It was a conducted immunity test (it may have been Electric Fast Transients, can't recall) but the actual test signal was leaking out of that lab, and into our lab, via AC lines in the building. Our chief engineer submitted a proposal to add whatever test it was to the MDD, but I don't know whatever became of that. I've seen simple clocks inside electronic gear cause CPUs to go haywire...in effect a single system interfering with itself. I've also seen extremely low power, yet very high frequency harmonics invade and corrupt function of another device, several meters away."

SlowButEffectiv 15y ago

projektx 15y ago

Because its for airplanes, up in the air, with hundreds of people on them, sitting on thousands of gallons of jet fuel hurtling through the atmosphere over a densely populated area near you.

baddox 15y ago

I'm pretty sure there have been enough people that ignore the rules and leave their devices on for us to be reasonably sure there's no substantial risk.

tzs 15y ago
Isn't it possible that 300 passengers using devices at the same time could pose substantially more risk than just a handful of people using devices?
- baddox 15y ago
  It's possible, but it's also possible that each passenger has ten iPads in their carry-on bag. Will they perform a test flight with 3,000 iPads? Not to mention how many more iPads could be stored in checked luggage.
  - mkramlich 15y ago
    3000 iPads * $800 per iPad = $240k cost of plane itself + cost of all the human lives on it = many millions, or "priceless", right? seems cheap in comparison. plus they don't need to perform this test on every plane before every single flight. only once or a few times, per model permutation. If you spread the testing cost across all the operational instances where they'd be de-risked, I bet it would be pennies per flight, at most.

Estragon 15y ago

It's admirable and entirely understandable that they should want to move to a policy grounded in direct empirical evaluations. Beats the hell out of the current situation, and I bet if it was your responsibility you would do the same. Some risks (hundreds of lives, hundreds of millions of dollars) are bound to trigger excessive caution.

mkramlich 15y ago

"In theory, there is no difference between theory and reality. In reality, there almost always is."