Mountain crossing question

[Htwjr]

I am planning my first mountain crossing trip from SC to KY.  The direct route crosses 6500' terrain with a substantial head wind.  How much clearance or what altitude would be required to avoid any problems with a mountain wave?  I realize that this is not even considered a mountain for you westerners but I wanted to have as smooth a flight as possible.  Thanks

[carusoam]

My favorite way to handle that is around 10,500' and 11,500’.

Mountain waves are dependent on windspeed.  

Using the above altitudes is more for a long glide time for figuring out what to do next.

Works for both the C and the O.

PP thoughts, not a CFI.

Best regards,

-a-

[Bob_Belville]

A lot depends upon the winds, a couple of thousand feet is usually adequate but be prepared to pull back and climb higher if your airspeed is decaying as you maintain altitude. I generally avoid Mt. Mitchell coming from the East. The mountain wave there is stronger than the ridges north of there which are almost as tall.

 

[Jeff_S]

Like Bob in NC, I fly over those mountains all the time out of Atlanta. 2000' above terrain is a good rule of thumb, but since they're not real tall to begin with you could go even higher depending on the weather. Mt. Mitchell is where you are most likely to see any major issues, but that probably isn't close to your route.

I've not personally experienced any true mountain wave action in the southern Appalachians. Not that it can't happen, but I think the general height and contours of the terrain make it less likely than out west. But when in doubt, approach any major ridge at a 45 degree angle and give yourself an escape in case you need it.

[carqwik]

Rule of thumb out west...2,000 feet for each 10 kts of wind at the top of the terrain.  So at 6,000 feet msl, if the wind is blowing at 30kts, then you need another 6,000 feet of altitude, 12,000 feet MSL.  Just a rule of thumb...but generally pretty good IMHO.  So if the wind is blowing at say 50 kts at 14k feet out West, it's not a good day to go across the mountains.

[Hank]

I cross southern WV / western NC at minimum of 7500 msl. Going near Mt. Mitchell on a windyndayncalls for higher. I found slow mountain wave going back from SnF at 11000, gentle 200-300' swings both ways. Trying to hold altitude can be difficult. The higher the winds, the higher you need to be. My C had precious little climb left above 10,000 but I managed to make 11.

Fly high!

[Browncbr1]

I usually cross through ashville airspace.  I crossed once when winds aloft at 3000 were 25kts so I went to 8500 and it wasn't too bad, but very rough below that.  I told myself, next time I would go higher or not go at all.  

[Mooneymite]

Aside from the turbulence factor, mountainous terrain is generally inhospitable should you lose your engine.  Altitude=options.

Edited February 1, 2017 by Mooneymite

[ArtVandelay]

Out west airlines get turbulence at 30000ft, I don't think you can fly over it, but I would be prepared to use Oxygen.

[Bob_Belville]

9 hours ago, Jeff_S said:

Like Bob in NC, I fly over those mountains all the time out of Atlanta. 2000' above terrain is a good rule of thumb, but since they're not real tall to begin with you could go even higher depending on the weather. Mt. Mitchell is where you are most likely to see any major issues, but that probably isn't close to your route.

I've not personally experienced any true mountain wave action in the southern Appalachians. Not that it can't happen, but I think the general height and contours of the terrain make it less likely than out west. But when in doubt, approach any major ridge at a 45 degree angle and give yourself an escape in case you need it.

Years ago, in my first M20E, flying from SE PA to Nashville TN in the winter, we looked at the Tri Cities Airport in the NE corner of TN for 50 minutes. We were at 10,000' but dumb and flying parallel to the ridge in the down wave making about 60 kts GS. Had I had enough sense to turn right for 20 miles I'd have gained about 100 kts.  

[BDPetersen]

Forecasting mountain waves involves several factors including surface winds, winds aloft, pressure gradient and topography. Route selection can make quite a dramatic difference, but figuring out that route is the challenge. My airline (which had red tails at the time) developed mountain wave deviation routes out west. A relative few miles change in course made for a much different ride. I wish I knew the specifics of route selection through the eastern mountains. It looks like airways that respect terrain may give a clue, but from Ohio to Florida I usually choose an end run south of BNA.

Waves tend to be problematic at two levels, 15,000 or below and, worse for jets, up at the tropopause which ranges from FL270 and up where the waves cause the trop to undulate and sometimes break as in "surfs up". Thus, I don't like mountain waves.

[peevee]

6 minutes ago, BDPetersen said:

Forecasting mountain waves involves several factors including surface winds, winds aloft, pressure gradient and topography. Route selection can make quite a dramatic difference, but figuring out that route is the challenge. My airline (which had red tails at the time) developed mountain wave deviation routes out west. A relative few miles change in course made for a much different ride. I wish I knew the specifics of route selection through the eastern mountains. It looks like airways that respect terrain may give a clue, but from Ohio to Florida I usually choose an end run south of BNA.

Waves tend to be problematic at two levels, 15,000 or below and, worse for jets, up at the tropopause which ranges from FL270 and up where the waves cause the trop to undulate and sometimes break as in "surfs up". Thus, I don't like mountain waves.

The red tail mtn wave routes and wave forecasts are still around, just with a different company stamped on them. They aren't very good. I see their forecast data every day.

[BDPetersen]

They worked well in my day. We seemed to be unscathed at the same time others were getting a roller coaster ride. Maybe they moved some mountains since then . . ,

[Bob - S50]

I think I once read a rule of thumb for mountain wave/turbulence.  1.5 times the tops.  So if the mountains are 6000', that would be 9000' or higher.  If they are 8000', it would be 12,000' or higher.  I've also heard it isn't usually a problems with wind less than 20 knots.  Take a look at windytv.com to see the winds.

However, as others have said, turbulence is not the only factor.  Having a place to land and time to find it if you lose your engine is another good reason for altitude.  I'm personally pretty comfortable crossing the Cascades near Seattle with only a couple thousand feet of clearance if I'm crossing near interstate 90.  Elsewhere, I usually want a bit more altitude unless I know there are several good valleys to land in.

Bob

(another former red tail)

[peevee]

1 hour ago, BDPetersen said:

They worked well in my day. We seemed to be unscathed at the same time others were getting a roller coaster ride. Maybe they moved some mountains since then . . ,

They love to route over some of the worst wave out there. One particular fix that should be avoided and they run right over it. 

 

Mountain wave is even more complex to forecast than you describe. It's an interesting phenomenon 

[BDPetersen]

Not worth using MS space for this. All I know is that when jet upset was a phenomena in the '60's and '70's, Dan Sowa and Capt Soderlind made statistically demonstrable, award winning improvements in the field that I thank for keeping my 37.2 year career right side up.

[larryb]

I cross the Sierra's around Truckee frequently. In this area, peaks are 9,000 and the pass where the highway is is at 7,200.  If winds are light I cross at 10,500 or 11,500. If winds are stronger I cross at 12,500 or 13,500. And if they are really strong, like > 40 kt, then I don't go.

One thing I have noticed in the crossings is that the mountain can "compress" the wind and increase it's speed. You may find your headwind is stronger 2,000 above the mountain than it is 4,000' above. 

Another thing to note, a 40kt wind doesn't necessarily mean a rough ride. I have had a smooth ride with winds like this, and only a couple bumps as I descend into the valley where the airport is, and where the winds are calm.

Right now, the mountain top winds are 60 gust 79 mph. I'm not going anywhere.

http://www.wrh.noaa.gov/rev/remotedata/newsqwsmt.php

Larry

[ArtVandelay]

I thought the general rule was if winds at peaks altitude is >20knots, it's a no go?

[peevee]

Just now, teejayevans said:

I thought the general rule was if winds at peaks altitude is >20knots, it's a no go?

I think most teach 30, at least for me 20-25 is serious reconsideration time, 30+ is a nogo

[N201MKTurbo]

A couple of things to consider.

Mountain waves are stronger if the barometric pressure is low.

The slope on both the windward and lee side makes a big difference. Steep sided ridges produce stronger waves. 

 

[Bob_Belville]

16 minutes ago, peevee said:

I think most teach 30, at least for me 20-25 is serious reconsideration time, 30+ is a nogo

This advice makes sense out west where NA aircraft are gasping for air above 10,000 and the slopes are steep. For our eastern mountains where Mt. Mitchell at ~ 6800 is the highest and for the most part the slopes are gentler, over 30 mph winds aloft might not be bad at all. Of course if I were in NH in the winter I would give little Mt. Washington (under 6300') a wide berth.

https://www.mountwashington.org/about-us/history/world-record-wind.aspx

[Htwjr]

Well we made it over and back across the mountains Thursday but it was outside my comfort zone in that I was dealing with three firsts all at once.  Fist time crossing the mountains, first time getting up close and personal with IMC and first time being concerned with icing.   Since the terrain on my direct route was 7000' I deviated south crossing 4000' terrain.  The headwinds were 25-30kts.  I crossed at 6500 but ran into some precip with the temps right around freezing.  Once across everything was fine.  On the way home I got above the clouds at 9500 and made good time 170kt + ground speed.  After we crossed the mountains we were between layers and ran into some light precip and started getting some ice on the windshield so I descended to warmer air diverting around the clouds and still made good time.  The ground speed on descent got up to 185kts.  I had flight following the whole way but in retrospect I should have filed IFR and considered getting a pop up clearance.  With my newly acquired instrument rating I felt fairly comfortable around the clouds.  The combination of mountains, clouds and ice not so much.  

[midlifeflyer]

On 2/1/2017 at 2:19 PM, peevee said:

I think most teach 30, at least for me 20-25 is serious reconsideration time, 30+ is a nogo

It depends a bit on where. In the Rockies we tended to use 25 at pass crossing altitudes because we needed to fly closer to terrain if normally aspirated and the craggy features would produce not only very strong turbulence but downdrafts exceeding piston climb capability.

it would be a bit better in the eastern  mountains, mostly due to the ability to climb higher to minimize the effects, but also due to the more rolling nature of the hills.

Of course, that's a pretty small difference, especially if you are planning to land at a mountain airport.

[Jeff_S]

On 2/5/2017 at 6:32 AM, Htwjr said:

Well we made it over and back across the mountains Thursday but it was outside my comfort zone in that I was dealing with three firsts all at once.  Fist time crossing the mountains, first time getting up close and personal with IMC and first time being concerned with icing.   Since the terrain on my direct route was 7000' I deviated south crossing 4000' terrain.  The headwinds were 25-30kts.  I crossed at 6500 but ran into some precip with the temps right around freezing.  Once across everything was fine.  On the way home I got above the clouds at 9500 and made good time 170kt + ground speed.  After we crossed the mountains we were between layers and ran into some light precip and started getting some ice on the windshield so I descended to warmer air diverting around the clouds and still made good time.  The ground speed on descent got up to 185kts.  I had flight following the whole way but in retrospect I should have filed IFR and considered getting a pop up clearance.  With my newly acquired instrument rating I felt fairly comfortable around the clouds.  The combination of mountains, clouds and ice not so much.  

Glad you had a successful trip and some good first experiences. Sounds like you handled the situation just fine, and that will give you confidence for the next time. Remember what they say: calm seas do not make experienced sailors!

As to IFR vs VFR, that's a personal choice. The challenge with IFR in the winter is that they can potentially put you into a cloud layer where you could pick up some ice. Of course, you can and should ask to get out of that immediately, but when flying VFR you can control that better. On the other hand, IFR gives you the comfort of knowing someone is helping you all along the route...in the mountains, there's always a chance that Flight Following will drop you if they can't get radar. It's really just a decision you have to make for each flight and for your personal comfort.

[astravierso]

Htwjr,

I have attached an older FAA document on Mountain Flying. AOPA also has an interactive presentation on the subject if you are a member.

Cheers,

Alan

tips_on_mountain_flying.pdf