The 'Nevada Triangle'

[poteroo]

Watched 'Aircrash Confidential' on 9GEM on Wednesday night.

 

Whilst it used the Steve Fossett 'mystery' disappearance, then location of the wreckage a year later, as it's lead in story - the real story was about mountain wave effect on aircraft in the Sierra Nevada mountains between Nevada and California.

 

The story made the amazing statement that some '2000 aircraft were believed to have been lost' due to failing to maintain terrain clearance when crossing this region. Did I hear right?

 

It also stated that it wasn't until after the Steve Fossett loss that meteorologists proved that a massive leeside downdraft was responsible. This surprised me, because the effect of mountain wave winds on aircraft trying to climb over terrain (upwind) has long been known, and pilots trained to avoid it. We certainly have these effects in Australia during winter frontal weather over the Stirling Range in WA, and over the Great Divide in the Eastern states. Of course, it is a more serious threat to aircraft when the terrain is high - say, over 7000 ft, when many aircraft are at full throttle height and cannot increase their rate-of-climb.

 

And, look up 'the Nevada triangle' on Dr Google if you want to read more.

 

 

[old man emu]

It was a mountain wave that got Shirley Strachan

 

Media releases: 20 December 2001 - Shirley Strachan accident highlights mountain wave dangers

 

 

[poteroo]

I think the most newsworthy Australian accident was the Southern Cloud' (?) in the late 30's in the Southern Highlands of NSW. Couldn't outclimb the ranges and that's understandable when you look at the diagrams.

 

Modern BoM forecasts are detailed about lower level turbulence and windshear due to strong winds over mountainous areas. The pre-requisites for serious effects are winds of >25kts and blowing onto the range at less than 30 degrees off perpendicular. Prudent pilots just don't fly in winds > 30 kts in the lower levels, and if they are obliged to, they avoid the higher mountainous area via diversion. Even with lower speed winds, its unwise to fly straight toward the range or ridgeline, and to try to cross with less than 2000ft under you. The diagrams illustrate why.

 

 

 

 

 

[facthunter]

Ignore at your peril. Learn all this stuff and give yourself a chance of flying into old age.. Nev

 

 

[turboplanner]

Ignore at your peril. Learn all this stuff and give yourself a chance of flying into old age.. Nev

Was MET taught in RA in your time as an instructor FH?

 

 

[facthunter]

Covered to a minimum extent (Not impressed) in the recommended (at the time) training books. I recall marking a batch of met exam papers. The exam was produced by the RAA.. Some of the answers were quite remarkable, but they could vary widely as there were no formal ground school lessons . was up to the individual to peruse the recommended material. Not unusual and we are talking about mature people who are not forced to fly planes and should be quite motivated.

 

Getting good reference material for the MET was never easy if you wanted to get well into it. I used a lot of stuff written for glider pilots There's probably more about now. BOM etc? At ATPL level you once you [passed, were an "accredited MET observer". That could actually come in handy. I've always been a bit keen on met but there's met and there's met for carrying fuel . Nev

 

 

[DenisPC9]

I The diagrams illustrate why.

Thanks, that explains several flights I was on coming back into Kundiawa in the late 70s.

 

I cant remember the name of the Outstation I was visiting but on the return leg, it was as if the C172 was a surfboard on a cresting wave. The bloody thing just took off and "slid" down the face of whatever carried it over what appeared to be an invisible hump. The pilot appeared to have no control over what was happening, until a way over and down the slide. He then gently pulled back on the stick and we resumed normal flight.

 

We had a visitor from down South, and we tried to arrange for them to go out on at least 1 charter. She nearly crapped herself when the plane "did its thing".

 

When we landed I asked the pilot what was that? From memory he said something about a "standing wave" and he regularly did that on that particular run. Then I thought he took it in fairly high when we headed out (to the Outstation).

 

 

[Methusala]

In the early days a hang glider pilot called Dennis Pagen wrote a number of well illustrated booklets on the subject. "Micro-meteorology for Hang Glider pilots" covered localised effects of wind at lower levels.

 

 

[poteroo]

When we landed I asked the pilot what was that? From memory he said something about a "standing wave"

It likely was a mountain wave. The major PNG wave was over the Owen Stanleys (10-13500 ft) when the NW monsoon was blowing against them - (Dec-Feb). From the N side, (Kokoda/Poppondetta). you needed over 15,000 to get over the Gap area, so we chickens usually went around to the E/SE where the range was lower, (nr Safia). Tried it in the turbo Aztec one day, (empty), and succeeded in making -1500 fpm on climb. Time to quit!!

 

 

[poteroo]

Was MET taught in RA in your time as an instructor FH?

Not that I remember either. I learned 'on-the-job' from check pilots. It was much likelier to stick with you when you actually experienced the loss of altitude despite being at climb power.

 

 

[Old Koreelah]

...The pre-requisites for serious effects are winds of >25kts and blowing onto the range at less than 30 degrees off perpendicular...

Great diagrams, Poteroo.

The Liverpool Range is good place to see those effects. The first diagram shows why, when crossing the range with a steady airflow, you hit a band of strong turbulence miles past the divide. The third diagram explains why, when I was flying home from the recent Scone Warbirds show, the headwind was much stronger down low, in the lee of the range. As I climbed higher, out of the accelerating air pouring over the divide, it reduced.

 

 

[turboplanner]

Not that I remember either. I learned 'on-the-job' from check pilots. It was much likelier to stick with you when you actually experienced the loss of altitude despite being at climb power.

If an organisation doesn't teach Meteorology, just refers student pilots to a book, I don't think those students have a long term prayer if they get as far as regular cross country flying in all weathers, and over distances that include different topography, and two or three climate conditions in the one day.

At the same time, without sitting down and researching it, my gut feel is that fatalities from failing to manage the weather conditions encountered is roughly the same percentage as GA pilots who have been trained and tested. This could be due to the fact that most RA pilots are still flying close enough to base, that they are familiar with local weather patterns and low level bolt holes. It could also be that when they do the occasional long trips, they are doing them on their holidays where a day on the ground doesn't matter.

 

I did Met in night training with the other students at about my stage, and we had a great instructor who reinforced the names of all the cloud types and what was to be expected from each, and about the wind flow over mountain ranges, and where it was safe to fly, about fog and how it could cut you off from your training field, about frost and how it could disrupt the laminar flow over the top of the wing and affect your take off distance on that dawn departure, and about ice. At the end of it, when we had passed, he told us we were Licenced Meteorologists, which impressed us no end (but I can't recall ever seeing a piece of paper.

 

That training has saved me getting in too deep on quite a number of occasions.

 

 

[facthunter]

Postponing the trip when weather is suss is the best option for sport flyers. When you do a regular op( RPTschedule ) or charter you are expected to go regardless, and make it if you are legally able to do it. The biggest breakthrough in safety was the installation of weather radar so you can assess the Build ups and navigate around them. I did a lot of night flying before the radar became universal so I've seen both situations personally. Hail for instance will completely wreck most planes. A high vertical development Cb will break one up. On a moonless night you can't see this stuff till the lightning flash illuminates it all. when you are right in it. In the Timor sea at the right time of the year,the tops are around 60,000 feet and aircraft sometimes just disappeared. The weather can be very extreme. near the thermal equator.. Nev

 

 

[skippydiesel]

Going back to the program - I know its for general consumption, to this end certain points are dramatised (to the hilt & beyond) but when the commentate is untenable to distinguish between air speed & ground speed, one wonders WTF!!

 

 

[Nico13]

Last wednesday I was scheduled to do a ferry flight from Lethbridge to Stawell down here in Vic but the wind on the day was trying to take the roof off the hangar. I had looked at all the weather on AV Plan and noted the comment, "severe MTW", in the long list of conditions.

 

Well I wasn't going anywhere before that but that was a new one for the list down here. Mountain Wave over the Grampians would have been something else.

 

I think I might have wound up off the east Coast somehwere.

 

I most definately wouldn't have been making much headway in the Tecnam thats for sure.

 

 

[408059]

I've been flying around and over the Snowy Mountains for the better-part of three decades now and I am still leaning. The snow capped peaks, rocky gorges, and ice covered lakes still take my breath away to this day but you must respect them. The weather patterns provide fantastic opportunities such as the time earlier this year when I flew smooth mountain wave on the North side of Lake Jindabyne climbing 1000' per minute in a Jabiru with the throttle backed to idle but such times are punctuated with reminders of how treacherous the mountains can be. On a return from Wangaratta to Cooma in April I crossed the mountains and the flight was silky smooth lulling me into false sense of security. As I approached the Eastern side of the range I started descending from 8000' and hit a wall of air sending me skyward to hit the canopy and popping open the retracted undercarriage. By all accounts the rotor should not have been there and I learnt a lesson. Barry Wrenford wrote a good article a few years ago on mountain wave. It not only covers the theory but also describes the local wave systems around the Snowy Mountains when they set up with different winds. Well worth reading if you intend venturing down this 'neck of the woods'. The article can be found on the Jindabyne aero club web site in the pilot briefing menu.

 

 

[poteroo]

I have delved back into some old textbooks of mine......very interesting.

 

In 1977, the BoM published Aviation Meteorology, Part 2. It has been reprinted several times since. On p38, 5.14 Airflow over Mountains - it covers the above discussed principles pertaining to lee side downflows. On p39, section 5.14 - it states

 

Vertical speeds may be more than 15 knots and have been measured at more than 40 knots in the Sierra Nevada, California.

 

 

A quick calculation of these numbers shows that 15 knots down = 25 fps = 1500 fpm, and 40 knots calculates out to 4000fpm - way beyond any light aircraft to climb out of! These are truly frightening numbers, and explain why so many aircraft have ended their days below the ridgeline of a mountain range when trying to fly over it from the lee side.

 

This phenomena isn't just something that occurs on major mountain ranges - it is very common in Australia, and has been the cause of many close shaves and incidents.

 

See: Aviation Safety Digest, No 88, 1974, p28 for a pilot contribution about his close shave approaching Cooma, NSW.

 

 

NB - 1974, a long, long way back, and I've no doubt other reports could be found earlier.

 

Happy days,

 

 

[dsam]

I consider myself fortunate that I had lots of meteorological training when I did my initial gliding training in southern Alberta, Canada. There was an area near Cowley where you had thermals, ridge lift, and mountain wave lift all in the local area.

 

One would look for the lenticular clouds to take advantage of the rising secondary wave, well away from the higher terrain of the initial mountain lee-side (as per poteroo’s first diagram above).

 

 

[poteroo]

Aviation Safety Digest, 88, 1974, p28

 

'I was still flying in rain and the weather conditions were no better than before, when, on my port side, I caught sight of trees at about our own level. Then, as I strained to look ahead through the reduced visibility, I glimpsed more trees in front of the aircraft. I immediately banked to starboard and, still at full power, increased the angle-of-attack still further. Although in a climb attitude at 70kts,the actual rate of climb was very slight, and the wheels actually brushed through the tops of the trees before the aircraft could clear them. But, apart from collecting some twigs and leaves in the undercarriage legs, no damage was done.

 

 

 

Once over this ridge, I managed to climb to 5500 feet and from this point onward I had no trouble maintaining altitude. The aircraft was then climbed back to 6000 feet and the approach to Cooma and subsequent landing were quite normal. Later in the day, when I submitted my flight plan for the return trip, I learned that a SIGMET had been issued, warning of severe turbulence with westerly winds exceeding 45 knots at 5000 ft. It was only then that I realised I had been flying into wind on the lee side of the mountain ridges approaching Cooma, and that I had been caught in the downdraught that exceeded the climb capability of the aircraft.

 

 

 

At the time of the incident, I had flown 700 hrs, including 100 hrs NVMC experience, and I was also undergoing instrument training for a higher rating. Although my flight to Cooma had been planned under the VFR, and it is doubtful that VMC existed at all times, at no stage did I have any difficulty in maintaining control in the reduced visibility. It was the prevailing downdraught conditions that almost caused the disaster, and not any loss of visual reference, as might have been supposed.......'

 

 

Talk about rationalisation! If he'd read the forecast in more depth, if he'd planned via a lower route, if he'd actually read up on mountain waves in the meteorology texts, books of the day.........If he'd really been VMC, then he'd have seen the ridgeline 3nm away, and also seen that it was 'rising' in the windscreen. A check of the VSI would have confirmed that the aircraft was actually losing altitude, and the turn away initiated minutes earlier than reported. (On climb in mountainous terrain, watch your VSI and ALT - because you might not actually be 'climbing' at all. This pilot must surely have won Lotto next week!

 

Incidentally, several DC3's survived with branches in one engine after trying to outclimb some PNG ridgelines as well. A few of the legends of PNG aviation involved too.

 

happy days,

 

 

[pmccarthy]

If you have access to the old crash comics I would love to read an article from about 1972. Pilot was flying with pax toward Sydney and got disorientated in cloud. As I recall the rubbish on the floor fell onto the cabin roof etc. When they landed there were branches caught in the undercarriage. Scariest article I ever read.

 

 

[GraemeK]

If you have access to the old crash comics I would love to read an article from about 1972.

The full archive is here.

 

 

[Bruce Tuncks]

Quite a few gliders have come to grief in Nevada. If you get below the ridge tops, you have to thermal back up by flying very close to the mountain, preferably in a gully because that's where the lift is best.

 

The valley floor can be 5,000ft and the ridge-tops 12,000ft, so getting below the ridge-top still gives you a lot of height. On good soaring days though, there are cloud streets over the ridge-tops at 15,000 ft or more, and you can fly fast and straight with 15,000 ft on the clock but only 3,000 ft above the ridge.

 

There can be two weather systems in play, because the mountain can be higher than a low-level inversion. For example, an inversion between 3,000ft and 4,000 ft above the valley floor would confine a glider to flying low, but the ridges projecting above the inversion can send thermals up much higher.

 

Anabatic winds or thermals overcome the inversion so the mountainside is the way up past the low inversion.

 

 

[pmccarthy]

The full archive is here.

That is amazing. Thanks.

 

 

[pmccarthy]

Found that story, July 1971. It is just as scary as I remember. Pilot who deliberately flew into cloud and survived to tell what happened.

 

 

[Steve L]

You can also get a Mountain breaking wave which occurs about twice the height of the terrain, although very rare. I experienced just that returning to Gawler from Aldinga. Weather was marginal with 40-45 kt gusting winds, I still had more than the required visibility. Briefed my passenger before departure and tracked close to Murray Bridge in case we had to divert. Due to the strong winds I climbed to 4200 to cross the range (1900) on approach to Lyndoch, once clear I reduced power for a long decent to Gawler, as I reduced power with one hand still on the throttle the Bulldog flipped upside down. immediately applied full power and righted the plane. I wasn't quite sure what happend so I studied to get a better understanding of the weather. Found that even airliners have been lost to this phenomena.