Question regarding frost and frost accumulation
 

So I'm hoping to pick a few brains, get some idea of other people's experiences and understanding of frost accumulation on a helicopter. There are a lot of educational sources regarding icing, but not a lot regarding frost and the effect on helicopters as opposed to fixed wing aircraft.


Currently in my area, we're sitting at 32F outside with a dew point of 31F making it about 98% saturated. I've read (fixed wing pilots say) that frost can't form on moving objects, and it's only a danger if the aircraft has frost already accumulated on the surface of an aircraft and only forms on stationary items.
Personally I have a hard time believing that with the wind chill and flying at an altitude where the aircraft is passing through air below freezing and at the same altitude where the temp and dewpoint are matched, that frost wouldn't accumulate. However there's no Icing AIRMET predicting icing conditions, and the skies are clear.


Am I being overly paranoid? If the helicopter is in a heated hangar, would you push it out and fly in these conditions where the temp and dewpoint are matched at just below freezing temperatures?


Any insight would be greatly appreciated.
Thanks

My opinion...no visible moisture, I will fly in it. Visible moisture...I'm not going if the OAT is below (or forecast below) +3C.

Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed rather than dew.

The dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, forming more liquid water. The condensed water is called dew when it forms on a solid surface, or frost if it freezes. The condensed water is called either fog or a cloud, depending on its altitude, when it forms in the air.

If the temp and dewpoint are the same it means the air has reached 100% saturation, the air may be clear, but caution is required because you may suddenly find yourself coping with anything from mist to thick, thick fog should the temp drop ever so slightly. Hence the rule mentioned by LRP re visible moisture and temp, the particular temp will be spelled out in your aircrafts manual.

One day, many years ago, i take off with an Alouette II early in the morning with a blue sky... After take off, i got just the climbing speed 45 kts and the bubble go totally withe in a few seconds. i have just the time to turn my head looking by the windows, open the vent in front and doors vent and the white disappear...Not frost , but satured air hotter inside than outside....

To follow on from Megan's points - if the dew point and temperature of the air are the same you would normally expect cloud/fog to form but that will only happen if there are hydroscopic nuclei available (dust, salt, etc) for the water to condense on to.

From Bobakat's story, it is quite common to end up with the windscreen freezing over if you descend from clear, cold air (cold airframe) into warmer, moist air.

Most airliners are not allowed to get airborne with visible frost on the wings (some exceptions on some aircraft if above the wing fuel tanks).

If you don't have visible frost on the blades then you will be OK to fly (remaining VMC) but avoid cloud/fog or visible moisture below 5 degrees C.

Kinetic heating keeps the tips warm enough.

Just an aside on wind-chill as mentioned in the OP - 3wind chill is only relevant to an object which is trying to maintain its temperature ABOVE the ambient air temperature. Wind chill is a measure of the extra energy required to sustain a given temperature when there is a cooler wind.

If you have an object which is trying to maintain its temperature at 10degC in an air temperature of 10degC the wind will make no difference to the required energy.

If you have an object which is trying to maintain a temperature of 20degC in an ambient air temperature of 10degC then the amount of energy required will vary with the wind speed.

If you have an object which is NOT trying to maintain its temperature, but is in an ambient air tempeerature of 10degC, then no amount of wind will cause it to get colder than 10degC.

Humans feel wind-chill because our bodies try to sustain a constant temperature, but airframes don't...

PDR

I know I shall get slated for this, but I never bothered about hoar frost on the blades, and can honestly say that I never noticed any effect.

Never did either, usually it is gone after the exhaust heat is recirculated on run-up. But then there are those pesky regulations:

§135.227 Icing conditions: Operating limitations.

(a) No pilot may take off an aircraft that has frost, ice, or snow adhering to any rotor blade, propeller, windshield, stabilizing or control surface; to a powerplant installation; or to an airspeed, altimeter, rate of climb, flight attitude instrument system, or wing, except that takeoffs may be made with frost under the wing in the area of the fuel tanks if authorized by the FAA.

LRP - agreed, letting the blades 'thaw' in the exhaust gas is just as effective as airliners having de-icing before take off.

In days of old -

Fog - freezing to static items - hoar frost or rime ice.

Aircraft in the hangar.

Drive down the road - large private property - to verify where the fog ends in 8/8 blue.

About 1km.

Push helicopter (H500) from hangar - light engine - depart.

Fly legally at speed to avoid obstacles if required - Helicopter VFR ~ 40 knots.

Avoid flying higher than you would care to fall as auto is probably an issue.

Watch torque requirement rising and hoping it does not become an issue before exit from fog. Did happen on occasion.

Exit to 8/8 blue area and endure asymmetric ice shedding.

Hope the big bits don't hit anything important (i.e.T/R)

T/R was never an issue as heated by exhaust - not applicable to all types of course! If yours gets covered in black stuff you should be OK.

2p.

Ensuring that the cockpit transparencies are clear before takeoff can be more difficult to achieve (and more dangerous if they are not) than a bit of hoar frost on the MRBs.

I can think of at least two helicopter serious accidents where misted canopies were the likely main cause.

We used to keep a fan heater in the cockpit for that.

And in really cold weather there almost always is an inversion.
Taking of and climbing i to warmer air will have the same effect on the windscreen.

Addendum - we never operated with front doors on!!

Yes, so did we when parked outside a nice hangar. But not possible when sitting in a field in the early hours of a very cold night, waiting for your clients to leave the party....

But the things you might bump into are near the ground.... ;)