Scudrunners

If you intend on flying VFR at night (or IFR, for that matter):

- Why is the dewpoint spread important?
- What causes the dewpoint spread to decrease?
- What's a visual telltale that you have dewpoint spread problems?
- What can happen when the dewpoint spread drops to zero?
- What is the minimum dewpoint spread for night VFR?  Why?

Hint: a melting blanket of snow can pump an
amazing amount of moisture into the air.

[quote author=Colonel Sanders link=topic=5330.msg13730#msg13730 date=1483976198]
If you intend on flying VFR at night (or IFR, for that matter):

- Why is the dewpoint spread important?
- What causes the dewpoint spread to decrease?
- What's a visual telltale that you have dewpoint spread problems?
- What can happen when the dewpoint spread drops to zero?
- What is the minimum dewpoint spread for night VFR?  Why?

Hint: a melting blanket of snow can pump an
amazing amount of moisture into the air.
[/quote]


Good and under discussed topic (as is all weather related).


Dewpoint changes can lead formation of fog and low ceilings.  Essential to monitor at night so not caught up above ground fog. Can form quickly. Track changes in spread before departure on a long CC. 


Could be wrong but I think one answer is an inversion certainly can cause trouble. Steep lapse rate usually means not an issue with ground fog unless it's a marine type layer blown onshore?  Could be errors in this comment as fog can form on a cold clear night too. Many types of fog.


Just off top of my head anyway.

Every pilot that gets a night rating ought to have
the previous covered in their training.  Amazingly
(to me, at least), they rarely do.

In addition to the above:

- What can go wrong during a night VFR takeoff?
What techniques and exercises can improve this?

- What can go wrong during a night VFR landing?
What techniques and exercises can improve this?

[img width=500 height=332][/img]

The above is the result of a puppy mill night VFR
landing attempt, of a fire-breathing 172, with all
aircraft and airport equipment functioning.

I got a quick lesson in dew point and diversions during my night rating cross country. We just made it to our second alternate as it closed in.

Fuel gauges were showing empty and the instructor nervously asked if I had dipped the tanks. I had, and we actually had lots of fuel, but lesson learned on dew
point.

And no, my instructor was not a 300 hour wonder who's balls just dropped. 

[quote author=Colonel Sanders link=topic=5330.msg13732#msg13732 date=1483978592]
Every pilot that gets a night rating ought to have
the previous covered in their training.  Amazingly
(to me, at least), they rarely do.

In addition to the above:

- What can go wrong during a night VFR takeoff?
What techniques and exercises can improve this?

- What can go wrong during a night VFR landing?
What techniques and exercises can improve this?

[img width=500 height=332][/img]

The above is the result of a puppy mill night VFR
landing attempt, of a fire-breathing 172, with all
aircraft and airport equipment functioning.
[/quote]


Takeoff:  (Well should be all takeoffs). Smoothly transition to a positive pitch attitude, confirmed with AI.  Must hold a consistent pitch atitude. Excursions could quickly lead to disorientation.  Immediate turns aren't a great idea until fully stable in the climb. Speed shouldn't change. Trim correctly.




Landing.  Visual illusions. Many comments possible but I don't like a long straight ins to a runway with black terrain preceding. Too hard to judge height. I find a proper base leg at one mile makes the approach easier. Constant angle on speed finals are a good idea. (Stable approach). Sudden changes aren't.

I'm going to guess first and see how bad my memories of this kind of thing are.

The further apart the dew point and temperature are the better. When the dew point is close to the temperature moisture will come out of the air and attach to things. (Can't remember the term for the percipitation molecules.)

If the moisture acumulates on a nice cold surface like the inside of a carburetor or on a cold wing this is bad. The sides of the carb are often cold enough to freeze water when it is a fairly nice day out. There is a nice chart showing ideal carb ice conditions. The best are like between 3C and 15C I think. Because if there is high humidity the dew point ends up close to the air temperature and here comes the moisture.

The dew point spread decreases when it is humid, some or lots of water in the air and the temperature drops. Warmer air holds more water. As the air cools off it gets closer to being unable to hold all the water.

Visual signs of dew point/temperature spread problems are fog, clouds and moisture on surfaces. Basically dew.

Once the dew point spread reaches zero it will likely result in percipitation.

I am not sure of the minimum dew point/temperature spread for night vfr. I know the reason for it would be the danger of icing becomes too high. I am going to gues around 4 or 5 degrees. Possibly a little more.

Ok, the first thing I was wrong about was how low the "severe icing danger at any power setting temperature." is. It goes below zero. Which I kind of knew but should have said -2C instead of +3c.

Here's "Take Five For Safety" including a carb icing chart.

http://www.tc.gc.ca/eng/civilaviation/p ... e-6016.htm

[quote author=David MacRay link=topic=5330.msg13736#msg13736 date=1483984998]
I'm going to guess first and see how bad my memories of this kind of thing are.

The further apart the dew point and temperature are the better. When the dew point is close to the temperature moisture will come out of the air and attach to things. (Can't remember the term for the percipitation molecules.)

If the moisture acumulates on a nice cold surface like the inside of a carburetor or on a cold wing this is bad. The sides of the carb are often cold enough to freeze water when it is a fairly nice day out. There is a nice chart showing ideal carb ice conditions. The best are like between 3C and 15C I think. Because if there is high humidity the dew point ends up close to the air temperature and here comes the moisture.

The dew point spread decreases when it is humid, some or lots of water in the air and the temperature drops. Warmer air holds more water. As the air cools off it gets closer to being unable to hold all the water.

Visual signs of dew point/temperature spread problems are fog, clouds and moisture on surfaces. Basically dew.

Once the dew point spread reaches zero it will likely result in percipitation.

I am not sure of the minimum dew point/temperature spread for night vfr. I know the reason for it would be the danger of icing becomes too high. I am going to gues around 4 or 5 degrees. Possibly a little more.
[/quote]


4-5 you're laughing. Monitor at 3, also rate of change, stable is ok, see 4-3-2 briskly better get on the ground.


I was caught once late in my IFR training, (with a 2000 hr instructor)  VFR rapidly became MVFR with maybe 500  foot ceilings and 1 mile, 10 minutes or so from airport. Could have done an approach but we knew what was where quite well,  so came in SVFR (first and only time ) I was really surprised how fast it developed.

Just getting into the night rating now so these are all good things for me to think on.

- Why is the dewpoint spread important?

It gives us a way to estimate the height at which clouds may form.

- What causes the dewpoint spread to decrease?

Either a increase in humidity or a drop in temperature.

- What's a visual telltale that you have dewpoint spread problems?

While I don't know a factual answer to this one my gut says that any signs of condensation are a warning sign. The plane is just one big condensation nucleus with variable pressures all over the place that can draw that moisture out into a liquid.

- What can happen when the dewpoint spread drops to zero?

Poofy white things appear. Ground may disappear.

- What is the minimum dewpoint spread for night VFR?  Why?

Depends how high you want to fly. At 1000' you need 500' vertical clearance so assuming a worst case 3°/1000' dry lapse rate you would need a 4.5° spread to expect clouds at 1500.

[quote author=Rookie Pilot link=topic=5330.msg13738#msg13738 date=1483985973]
..IFR training, (with a 2000 hr instructor)  VFR rapidly became MVFR with maybe 500  foot ceilings and 1 mile, 10 minutes or so from airport. Could have done an approach but we knew what was where quite well,  so came in SVFR (first and only time ) I was really surprised how fast it developed.
[/quote]It can be interesting around YYC and quite unpredictable west into the mountains and British Columbia. Moisture can move in pretty fast from far away. It's usually not a total surprise but it can be pretty shocking. Or at least impressive.

Especially on those days you are out side camping or something and they said it was going to be nice that weekend or even day but, I never checked the forecasts the day of.

The valleys west of here around Lake Louise can accumulate moisture and clouds in areas where the valley is quite wide, then narrows. It might stay for days, it might dissipate quickly and turn into a nice day. Sometimes it starts raining or snowing there. Depends on the winds, how much moisture and temperature changes.

I was coming home from Abbotsford once and it was nice leaving Golden, got cloudy just before the area around Sunshine Villiage ski resort. I could see the cloud from miles back collecting there, I had plenty of fuel so I decided to sight see a bit since the wind was blowing west and I was in clear blue skies I n the nice wide area where the number 1 highway intercects with the ice field valley. I was hoping the clouds would go away and it would clear up, after twenty minutes or so it did. So I was able to carry on home instead of returning to Golden.

One thing I delt with very poorly, was not trying to learn more from the flight service guys while we still had them. Instead, I enjoyed the luxury of relying on their mostly excellent skills to determine weather patterns and brief me. Once they closed the station at YBW I was left with what I still consider weak meteorology skills.

That bad part is I was probably not as far below average for someone with a few hundred hours as I should have been. Meteorology is pretty important for flight safety. Most of us have trouble with it, especially at first.