Hygrometer

[Jan Olieslagers]

Why is an outside air thermometer common equipment, where I have never seen a hygrometer? Combining the information from both, a realistic warning for icing risk could be implemented; not hard to do with today's microcontrollers. Have I found a market niche?

[Danscowpie]

Have I found a market niche?


No.

That's what airfield have Stevensons Screens for - a well established procedure for doing the same thing at a much reduced cost.

[Floppy Link]

...not much use when it's stuck on the airfield and you're 56nm away at Fl065 in and out of the tops.

[BackPacker]

Are we referring to airframe ice or carb ice? I think Jan is referring to the latter.

Jan, there's an even better solution. Get a carb temperature gauge. That'll show you exactly when there's a risk of icing in the carbs. Or get rid of the carbs altogether - go injected.

[FlyingKiwi_73]

Hygrometers are notoriously slow to show change, also require regular calibration. they are fine for agriculture as the provide good trending etc.. but i'm not sure they would be acurate enough to detect imediate changes such as flight into icing conditions.

[Jan Olieslagers]

I was not thinking of any particular form of icing - I was wondering what can be the use of installing an OAT indication in the cockpit without an associated hygrometer. One can be perfectly safe with an OAT of -20°C, or in acute danger at +15°C. The calibration issue might be one explanation.

Nothing to do with conditions at aerodromes or other locations, of course.

[BackPacker]

or in acute danger at +15°C.

Airframe icing and carb ice are really two totally different beasts.

If you're in +15 OAT I would assume that you are reasonably safe from airframe ice, but indeed, carb ice is a real possibility.

[24Carrot]

If there is anything sophisticated available, I would expect to find it on a big Boeing or Airbus, as it must cost money to heat the leading edges for anti-ice purposes. Knowing the RH might allow less anti-icing.

Incidentally, even at +15C OAT you could still get airframe icing, eg if you had a sub-zero airframe following a descent, or if you were flying below clouds dropping super-cooled rain on you.

[BackPacker]

Well, weather balloons contain a very small package that continuously measures temperature, dew point, air pressure and maybe a few other environmental parameters, and transmits that to a ground station.

These packages are small, light and essentially expendable (because there's no guarantee that it will be found and sent back to the weather institute).

So the technology to measure temperature & dew point and thus relative humidity easily is available. Why it isn't used in aircraft... I have no idea.

[Ultranomad]

Hygrometers are slow and not so reliable. There are, however, various icing sensors, e.g. radioisotope-based ones, which contain a small dose of beta-active isotope and a particle counter. They are installed on Mi-8 helicopters and a few other Russian types. Even a very thin layer of ice on the sensing rod absorbs the radiation, reducing the count rate. Beta particles don't travel more than ~1 m in the air, so it's safe to the aircraft occupants, though on the ground you are supposed to put a lead sheath on it.

...Actually, on second thoughts, I think I know a way or two to design a responsive and reliable hygrometer, but it will certainly cost a lot more than the ones used in meteorology.

[FlyingKiwi_73]

WOW!:eek: now thats thinking. still would not like to be near it! lead undies for flying the russian choppers?

how fast does it decay? Hygrometers deffo not the way forward, you would want something that could do the calculation for you (knowing the OAT and Dew point) and do it thousands of times very quickly. is there not a glass panel EFIS that does this? i seem to remember readign about one somewhere?

[Ultranomad]

FlyingKiwi_73, RIO-3, the one on Mi-8, contains strontium-90, which has a half-life of 28 years. However, it's not the half-life that is primarily important but rather the quantity of the isotope.

There are also other physical properties of water that can be measured to assess air humidity, e.g. intensity of its infrared or NMR spectral lines, etc. Measuring them properly is a tricky business, but it's perfectly doable. It can even be done remotely - you have probably heard of cloud base measurements by a ground-based lidar.

[Jan Olieslagers]

Some interesting replies, thank you. I never imagined the radioactive bit, does sound slightly fearsome, that does. But allow me to reword the opener:

What good is an OAT gauge with no hygrometer to complement its data into useful information?

[FlyingKiwi_73]

Isn't OAT important for working out your TAS and density altitude?

[BackPacker]

Not to mention the chances of airframe icing...

As someone said earlier, yes, you can get airframe icing in a few well-understood conditions at +15C, but if the temperature drops below 0C, the chance all of a sudden become much greater.

It's also handy, if you keep an eye on the OAT during the ascent/descent, to find inversions and a few other weather fenomena that are highly dependent on temperature gradients.

And lastly, most aircraft performance tables, such as takeoff and landing distances, are temperature dependent.

So I wouldn't want to do away with the OAT just yet.

[24Carrot]

So I wouldn't want to do away with the OAT just yet.Agreed! If you want to miss mountains, Altimeter + OAT are best.:ok:

[Ultranomad]

After some reading on the subject, I realized I had been missing an obvious solution for ice detection currently in mass production. They stick out a thin piezoelectric probe, and once an ice begins to form on it, its vibration frequency changes due to a change in weight. Simple and beautiful.

[IO540]

Which icing are you chaps discussing?

Carb ice could be forecast by knowing the temp and DP, which is given in the ATIS (if there is ATIS) but you can also get electronic RH meters off the shelf; I have one for about £200 which I used to measure if keeping silica gel bags in the cockpit makes a decent difference (it does).

Carb ice can be avoided with a carb heater. But it is also aircraft type specific.

Structural ice comes from supercooled water droplets. These exist between 0C and about -15C (lower temp SLDs can be found in convective clouds), with the worst case around -5C. But you don't need a warning device. Just look out of the window onto the wing leading edge, and the elevator leading edge. At night you need a light (and deiced planes have such a light).