Mountain Wave for the ATC Specialist

                        Mountain Wave
                           for the
                       ATC Specialist
                              
                     Steven H. Philipson
                         May 4, 1996
                              
                      * * * DRAFT * * *


Abstract

   Mountain wave is an atmospheric phenomenon in which air
currents form a standing wave pattern of up and down vertical
flow.  This flow can present a flight hazard to all aircraft, but
the hazard is particularly severe for aircraft of limited climb
performance including virtually all light aircraft.  Mountain
wave is cited by the NTSB as a cause or factor in several fatal
and non-fatal accidents each year.  Pilot error plays a part in
these accidents, but it is common for air traffic controllers to
fail to issue appropriate pre-flight warnings to pilots and to
issue in-flight instructions that contribute to these accidents.
This article is intended to present information on mountain wave
for air traffic control specialists so that they may be able to
both predict and detect wave conditions, and help airplane pilots
deal with this hazard.  Effective assistance to aircraft in
mountain wave downdrafts can make the difference between a flight
with an unusual event versus one that ends in a fatal crash.


Background

   Mountain wave is an atmospheric phenomenon in which an
obstruction (typically a mountain ridge) excites air currents
into a standing wave pattern of up and down vertical flow.  It
can be visualized as ripples downstream of a stone in water.  The
vertical rates of flows in mountain waves have been reported to
8,000 feet per minute, with rates of several thousand feet per
minute being common.  These rates greatly exceed the climb
performance of most light airplanes, particularly at the high
elevations at which they are encountered.

   In the western United States wave is generally encountered in
mountainous areas at altitudes above 7,000 feet.  Typical
aircraft climb performance in still air at these altitudes is in
the range of 300 to 500 feet per minute.  These aircraft are thus
incapable of climbing or even maintaining altitude while flying
in a strong  mountain wave downdraft.

   Wave effects can extend from the surface downwind of the
obstruction to extremely high altitudes.   Persons on the ground
in wave areas have reported both hearing and feeling rotors
associated with wave and have observed property damage from these
winds.  U2 pilots have reported detecting (and having operational
difficulties from) wave at altitudes in excess of 60,000 feet.
Thus wave is an operational hazard for all aircraft, no matter
how low or high they fly.

   Most airplane pilots react to downdrafts by attempting to
climb.  Pilots are trained to maintain altitude during cruise
flight, thus their first inclination upon noticing a loss of
altitude is to try to regain the original cruise altitude by
climbing.  This tendency is reinforced in the IFR environment as
there is a regulatory requirement to maintain the assigned
altitude.

   Attempts to climb are effective as long as the intensity of
the downdraft is not too severe.  If the downdraft is moderate
the airplane may be able to maintain altitude.  Once the
intensity of the downdraft exceeds the airplane's climb
capability the aircraft will begin to descend, but flight at
climb speed will produce a relatively shallow descent as long as
the resulting rate of descent is fairly low.  As the intensity of
the downdraft increases, attempts to climb become counter-
productive and actually cause a steeper descent angle than flying
at cruise airspeed.  Most airplane pilots are unaware of this.
Aircraft typically do not have angle of descent indicators, thus
many pilots try to minimize altitude loss by minimizing rate of
descent.  Many have maintained climb speed during descent in a
downdraft all the way to ground impact.

   Well-intentioned instructions from air traffic controllers to
"try to maintain altitude" contribute to the problem as it
reinforces the pilot's tendency to try to climb.  In addition,
controllers often try to vector aircraft in distress via the
shortest route toward lower terrain along the original route of
flight.  This usually prolongs the time that the aircraft flies
in the downdraft and increases the loss of altitude.  Pilot
compliance with these instructions can make ground impact
unavoidable.

  Controller awareness of mountain wave conditions and effective
escape techniques for aircraft can help the controller to predict
and recognize critical flight conditions, issue appropriate
advisories, and issue instructions that will aid the pilot rather
than exacerbating an already critical situation.


Prediction of Wave Conditions

   There are four primary indicators of mountain wave. Waves
typically begin to form when :
    1) winds across mountain peaks exceed 25 knots,
    2)  the wind direction is within 30 degrees of
    perpendicular to the line of the mountain ridge,
    3) wind velocity increases uniformly with altitude, and
    4) wind direction is relatively constant with altitude.

   In the western United States, a winds aloft forecast (FD)
showing winds of greater than 30 knots at 9000 feet, relatively
constant direction through 38,000 feet, and winds at the 34,000
foot level in excess of 75 knots virtually guarantees that there
will be wave somewhere.

   It is important for controllers to recognize the importance of
the winds aloft profile from low altitudes through high altitudes
for prediction of wave conditions.  Most pilots of non-turbo
charged aircraft will not ask for winds higher than 12,000 fee.
Briefers have been known to be reluctant to provide winds aloft
data for altitudes that are clearly above the normal flight
altitudes and even the service ceiling of such aircraft.
Nevertheless, knowledge of these high altitude winds are critical
to the prediction of wave.

   Neither SIGMETS or AIRMETS are currently issued for mountain
wave conditions.  However, advisories for "strong up and
downdrafts in the vicinity of mountainous terrain" and "moderate
to severe turbulence over rough terrain" often (but not always)
are issued when weather conditions support the formation of
mountain wave.

   ATC specialists performing pilot weather briefing services
should look for these conditions and advisories as predictors of
mountain wave conditions.  Some or all of these conditions may be
present without waves forming, but when the four factors above
are present wave formation is likely.


Location of Waves

   As noted above, wave formation requires the wind direction to
be within plus or minus 30 degrees of perpendicular to the line
of a ridge or other obstruction.  Controllers should survey WAC
and Sectional charts within their area of responsibility to
identify areas where ridges are oriented in a perpendicular
orientation to common wind flow directions.  Particular attention
should be given to low altitude Victor airways that cross such
ridges as these tend to be repeat sites for mountain wave
accidents.  Two of the more infamous sites in California are
Victor 459 north of the Lake Hughes VORTAC (in the Gorman Pass)
and Victor 458 just east of the Julian VORTAC.  Additional
information on these sites is included below.


Techniques for Detecting and Countering Mountain Wave

   A previously published paper examines the problem of flight in
mountain wave downdrafts from the perspective of aircraft
performance optimization and pilot technique.  Both the theory
and technique are extensions of basic speed-to-fly theory for
gliders.  Speed-to-fly theory describes the principles and
procedures used by glider pilots to produce the shallowest
descent angle while flying through areas of lift and sink. The
techniques as applicable to light airplanes can be summarized in
several short rules as follows:

   The most critical case in dealing with a mountain wave
downdraft is when flying directly into the wind.  Ground speed is
greatly reduced thus increasing the length of time the aircraft
spends in the downdraft, the altitude lost in the downdraft, the
forward progress of the aircraft and hence the angle of descent.
Thus if only one rule is to be remembered, it is this:

       If you're at Vy, and YOU ARE GOING DOWN FASTER THAN YOU
       SHOULD BE GOING UP, SPEED UP TO CRUISE SPEED.

   A refinement of this rule takes into account wind direction
and velocity versus direction of flight.  This results in three
rules, plus an observation....

       If flying into a headwind and you're at Vy, and YOU ARE
       GOING DOWN FASTER THAN YOU SHOULD BE GOING UP, SPEED UP
       TO CRUISE SPEED.
       
       If flying with a tailwind and you're at Vy, AND YOU ARE
       GOING DOWN THREE TIMES AS FAST AS YOU SHOULD BE GOING UP,
       SPEED UP TO CRUISE SPEED.
       
       THE GREATER THE HEAD WIND, THE SOONER YOU SHOULD INCREASE
       TO CRUISE SPEED.
       
       All else being equal, IT'S BETTER TO FLY DOWNWIND AND
       FAST.

   These rules provide some insight for the air traffic
controller on how to detect potential and actual wave conditions
based on observed aircraft performance, and on how to provide
effective assistance to an aircraft in distress.

   First,  watch for aircraft with very slow ground speeds.  A
typical 150-180 h.p. four seat fixed gear light aircraft will
cruise at 115 to 135 knots.  When wind speeds exceed 30 knots the
ground speed of such aircraft will be at or less than 100 knots.
If the aircraft slows to climb speed, ground speeds will decrease
to 50 knots or less.  Ground speed and heading readouts may
become unreliable as there may not be enough target motion for
the radar system to accurately process.  Large (greater than 60
knot) differences in groundspeed of aircraft of similar types
headed in opposite directions are another indicator of strong
winds that support wave.  Large crab angles (in excess of 15
degrees as reported as differences between heading and ground
track) indicate strong winds across the current route of flight.

   Next, be alert for pilots reporting that they are unable to
maintain altitude.  It does not take much of a downdraft to
exceed the climb performance of these aircraft.  Slow
groundspeeds and initial reports of altitude loss are the first
indications that an area of strong mountain wave downdrafts has
been entered or is about to be entered.  Given the small
performance range of these aircraft, you should assume that a
descent rate of greater than 500 feet per minute with a ground
speed below 60 knots is an indication that the aircraft is flying
against a headwind and is in a strong downdraft, which are the
most critical conditions for mountain wave accidents.  These
numbers are also appropriate for higher performance 4-6 seat
retractable gear aircraft.

   Aircraft in such conditions which continue to try to climb
instead descend rather steeply.   Several accidents have occurred
in the past when controllers attempted to vector aircraft over
higher terrain ahead in the hope of reaching lower terrain
beyond.  The problem here is that these aircraft were losing more
than 1000 feet per nautical mile and ran out of altitude before
clearing the higher terrain.  An aircraft that is encountering
downdrafts due to wave is almost certainly downwind of the ridge
or obstruction that is exciting the wave. Vectoring the aircraft
upwind thus is vectoring the aircraft toward higher terrain.

   If terrain is present downwind that is of lower altitude than
the aircraft's present altitude, the best approach is to vector
the aircraft directly downwind toward that lower terrain.  The
increased ground speed cuts the descent angle tremendously, and
the aircraft usually will be immediately moving away from the
closest, highest terrain, namely the ridge that is exciting the
wave.  In addition, it often will be helpful for the pilot to
increase speed above climb speed in accordance with the above
rules.  However, it may not be allowable to instruct the pilot to
accelerate to cruise speed as ATC is generally not supposed to
tell pilots how to fly their aircraft.   Controllers may need to
use their own judgment as to when to "recommend" acceleration to
cruise speed.  At the minimum, it would be least damaging to
avoid admonitions to "try to climb" or to "try to maintain
altitude" if the aircraft is descending at a high rate.
Immediate clearances for lower altitudes or block altitudes can
help the pilot to maintain increased airspeed.

  A common factor in mountain wave accidents is that the aircraft
involved encountered wave downdrafts at a relatively low above-
ground altitude.   Several accident aircraft were either flying
at the minimum enroute altitude (MEA) (2000 feet above the
highest local terrain in mountainous areas) or 1000 feet higher.
The MEA is often predicated on the height of the ridge that forms
the wave, thus these aircraft are flying at altitudes at or below
3000 feet AGL.  A rule of thumb for pilot's flying in strong
winds is to maintain 1000 feet AGL for each 10 knots of wind
velocity.  Some accident aircraft filed for and were maintaining
an altitude consistent with this rule, but winds increased above
forecast values, or ATC directed the aircraft to maintain a lower
altitude to resolve traffic conflicts.

   It would be prudent for controllers to be aware of the limited
climb performance of light aircraft  and "keep 'em high" whenever
practical.  It is much easier for a commuter turboprop with a
2000+ fpm rate of climb capability to negotiate wave downdrafts
while flying downwind than it is for a Cessna 172 or like
aircraft with a 300 fpm climb capability to do so while flying
upwind.  Early assignment of a higher altitude may be a critical
factor in avoiding an accident following a subsequent encounter
with downdrafts.

   Aircraft which encounter wave downdrafts often initially
encounter wave updrafts.  Standard glider technique is to take
advantage of these updrafts and climb in them to accumulate an
altitude reserve which will be expended shortly thereafter when
the glider enters the adjacent area of downdrafts.  Airplane
pilots may be hesitant to take advantage of updrafts,
particularly if they are at an assigned altitude.  Controllers
can make the pilot's job considerably easier and safer by
offering a block altitude, thus allowing the airplane to climb
and descend with the wave flow as needed.

   When an airplane enters an area of strong downdraft and is
losing altitude rapidly, a quick decision must be made as to
whether the airplane will be able to penetrate through the
downdraft and exit the other side with safe terrain clearance
remaining.  An aircraft flying at the MEA/MOCA which encounters a
downdraft resulting in a descent rate of 1500 feet per minute
will have about one minute an 20 seconds before it descends to
the obstruction altitude.  Given the slow groundspeed of aircraft
in these conditions, the aircraft will likely not exit the
downdraft nor clear the high terrain exciting the wave if it is
more than a mile downwind of that terrain.

   These factors indicate that an aircraft at low altitude which
encounter wave downdrafts have a poor chance of clearing the
terrain if they try to continue flight in an upwind direction.
The fastest and safest escape for these aircraft is for them to
make an early turn directly downwind, thus expeditiously flying
out of the downdraft and toward lower terrain.  If a controller
intends to use this escape, it must be done early and with
instructions to expedite the turn;  a 180 degree turn takes one
minute at standard rate, and the aircraft will likely be
descending rapidly for this entire time.
If the aircraft is in visual conditions, then a rapid response
from the pilot and high rate of turn is more likely than if the
flight is in instrument conditions.  This is simply a reminder
that the situation is more critical in instrument conditions.

   When the aircraft has flown downwind and out of the area of
downdrafts, the aircraft may safely be instructed to climb again.
If another attempt to cross the area of downdrafts is desired,
the aircraft should first be allowed to gain sufficient altitude
to allow a large altitude loss in the next attempt before it is
begun.   Use of a different flight path to cross the area of high
terrain may allow avoidance of the wave.  In some cases, the only
viable approach is to abandon attempts to cross the high terrain
until conditions improve.


Critical Wave Sites

  Mountain wave accidents tend to be a regular occurrence at
several sites.  This is due to the alignment of prevailing winds
with prominent ridges which repeatedly generate waves in the same
places.  When those places are along commonly traveled airways
the result is multiple accidents at the same site.

   Two of these sites are located in southern California.  The
first is in the Gorman Pass, between Burbank and Bakersfield.
Although the entire pass can be treacherous during wave weather
conditions, a particularly bad spot is located 14 nautical miles
north west of the Lake Hughes VORTAC on Victor 165-459.  At that
point a prominent ridge crosses the airway at a near
perpendicular angle.  The ridge is also aligned nearly
perpendicular to the prevailing winds that form mountain waves.
The elevation of the ridge is 6800 feet MSL, while the MEA of the
airway is 9000 feet, just 2200 feet above the elevation of the
ridge.  The Gorman Pass tends to funnel and thus intensify winds
aloft, thus increasing the intensity of downdrafts and reducing
the forward speed of aircraft. These effects combine to increase
the chances that a north-bound aircraft will be sustain a
critical loss of altitude.  Wave accidents have been so numerous
there over the last 40 years that the local search and rescue
personnel have been known to call it "Aluminum Ridge" for all the
aircraft that have cashed there.

   There is lower terrain both to the northwest and southeast of
this ridge.  This tends to produce strong waves, but it also
provides escape routes for aircraft encountering wave downdrafts.
A typical accident scenario here begins with aircraft being
assigned an altitude at or close to the MEA in preparation for
descent into the central valley.  Once at the MEA these aircraft
may find it impossible to maintain altitude against downdrafts
encountered there.  Air traffic controllers have attempted to
assist by vectoring aircraft directly towards Bakersfield in an
attempt to get them to have the shortest path toward lower
terrain ahead.  Unfortunately such vectors are also often
accompanied by requests to "try to maintain altitude" which leads
the pilots to maintain a slow climb airspeed and thus descend
more steeply than if a higher airspeed were used.

   It seems that the best approach for dealing with aircraft
encountering a downdraft here is to assign a heading that is
nearly directly downwind, or to the south east toward Lancaster.
Terrain drops away rapidly in this direction, and ground speeds
will increase dramatically thus minimizing the descent angle.  A
course reversal along the airway is less desirable as it leads
back to the higher terrain on which the Lake Hughes VORTAC is
located.



   Perhaps the worst site that the author is aware of is at the
Julian VORTAC, located to the east of the city of San Diego in
southern California.  Every year or two there is at least one
crash in wave conditions within a few miles of the VORTAC.  In
some years there are multiple wave accidents.  Many of these
accidents include fatalities.

  The VORTAC is located on a prominent north-south ridge.  It
forms the Victor 458 airway, which approaches the ridge at a
shallow angle.  The ridge sits nearly perpendicular to prevailing
winds, thus when wave conditions arise with westerly winds, a
wave forms just east of the VORTAC.  Aircraft flying on the
airway approaching the VORTAC from the southeast spend a large
amount of time in the downdraft area -- they are effectively
soaring the downdraft.

   The elevation of the ridge is 5719 feet MSL, with an
obstruction shown on the San Diego terminal area chart at 5798
feet.  The MEA on this airway is 7700 feet -- only 2000 feet
above the terrain.  Aircraft are frequently assigned to fly at
the MEA.  There is thus precious little altitude that can be lost
before terrain clearance becomes zero.  An even more critical
situation exists for aircraft flying instrument approaches to the
Ramona airport, as the altitude for the initial approach fix and
holding pattern is 7000 feet, a mere 1280 feet above the terrain.

   Aircraft flying northwest on the airway at the MEA which
encounter a wave downdraft typically have less than two minutes
of flight time remaining before they descend below the level of
the ridge.  Unfortunately, due to the shallow angle at which the
aircraft approaches the ridge, this may be longer than the time
it takes for the aircraft to transition through the area of the
downdraft.  An aircraft flying at 130 knots true airspeed will
take from 1.2 to 1.7 minutes to fly through a typical wave
downdraft here.  If the aircraft slows to best rate or best angle
of climb speed, it will take from 2.5 to 6 minutes to cross
through the downdraft, or even longer as the velocity of the wind
increases.

   Controllers who observe an aircraft descending along this
route who desire to provide assistance to the pilot should either
vector the aircraft directly upwind (course approximately 240
degrees magnetic) or directly downwind (heading 060 degrees
magnetic).  The upwind vector has the advantage of losing less
altitude in the course change, while the downwind vector offers
lower terrain ahead.  A course reversal is counter productive as
more altitude is lost in the longer turn, and the aircraft will
fly directly through the area of downdraft again. Use of best
forward speed should be encouraged if possible.  At the very
least, admonitions to "maintain altitude" or to "try to climb"
should be avoided.

   A better approach to this problem is to avoid the critical
situation altogether. Controllers should consider assigning a
significantly higher altitude to aircraft flying upwind along
this route, using the terrain clearance recommendations discussed
above as a guideline.  Another approach is the vector the
aircraft west before reaching the VOR.  The route of Victor 66 is
also much less likely to encounter wave downdrafts during these
conditions.

    Numerous other critical sites exist around the United States
and the rest of the world.  A search of accident reports may not
show all of them as many reports do not include wave as a factor.
Each ATC facility should track incidents and accidents within its
area of jurisdiction so as to be aware of the critical sites and
develop strategies to effectively assist aircraft encountering
mountain waves.


Summary:

   Mountain waves can present a serious hazard to all aircraft,
but low-performance light aircraft are particularly vulnerable.

   The primary requirements for wave are as follows:
    1) winds across mountain peaks exceed 25 knots,
    2)  the wind direction is within 30 degrees of
    perpendicular to the line of a mountain ridge,
    3) wind velocity increases uniformly with altitude, and
    4) wind direction is relatively constant with altitude.

   Watch for indications of high wind velocity and strong
vertical flow.

   Keep aircraft altitude high.

   Assign block altitudes so aircraft may gain and use altitude
when needed.

   Keep aircraft speed up -- do not encourage flight at climb
airspeed when the aircraft is descending in a downdraft.

   Turn aircraft downwind / toward lower terrain early, in time
to complete the turn with altitude to spare.

   When the aircraft exits the area of downdrafts, allow the
aircraft to increase altitude such that there will be sufficient
reserve for altitude loss in making another crossing attempt
before commencing that attempt.

   Track locations of wave incidents and accidents so that
strategies may be developed to help aircraft avoid and escape
from mountain wave downdrafts.



Comments:


   Comments on the draft of this article are welcome and
encouraged.   Please use any of the following to contact the
author:

U.S. Mail:     Steven H. Philipson
               936 Erica Drive
               Sunnyvale, CA 94086-8211

E-mail:        steve@flightquest.net
               (Please remove the  in the e-mail address)

phone:         (408) 530-9584

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