Monday, January 31, 2011

School of Hard Knocks

Just back from a flight, Mal Raff warned me ominously, "It's very turbulent up there." To which I replied dryly, "Good, it will keep me awake." This developed into a sort of tradition where Mal would provide me a pilot report (PIREP) for turbulence and I would play the part of the brave pilot who never admits to being bothered by turbulence. This was all an act. While I've become somewhat inured to turbulence, I'll be the first to admit that I've encountered turbulent conditions that scared the daylights out of me. Pilots and passengers universally dislike turbulence and the ones that say otherwise are either fibbing or are in need of some serious couch time. Thankfully there are several strategies for avoiding turbulence, handling it when it is unavoidable, and making PIREPs that accurately describe the character and degree of turbulence you just encountered. (Note that I won't be delving into clear air turbulence or CAT, since few GA pilots are flying at altitudes where it is a factor).

Get Briefed

Student pilots learn early on that weather reports and forecasts can provide plenty of clues about the likelihood of encountering turbulence. Getting a briefing not only fulfills the regulatory preflight action requirement, it can help you anticipate turbulence well before the PIREPs start flooding in. Sure there are pilots out there who skip the pre-flight briefing or don't examine the information carefully. Some of them even brag about it, but you're not like those pilots, right? The National Weather Service has several products that address turbulence that I'll illustrate using the Aviation Digital Data Service site, which now offers a Qualified Internet Content Provider login.

AIRMET tango (think "T" for Turbulence) is of interest primarily to pilots of smaller aircraft and should be released when conditions are ripe for moderate turbulence over a large area (3000 square miles or more). There are also AIRMETs sierra for IFR conditions and zulu for icing. Problem is that AIRMETs describe the area being affected in a sort of connect-the-dots fashion using three letter identifiers for VOR stations and distances to define the dots: Not very user-friendly. Even if you are familiar with all the VORs, assembling a mental map of the affected area can be challenging.

ADDS has a product called G-AIRMET that provides a map-like depiction of areas affected by AIRMETs and SIGMETs. You can even look at an animated representation of how the conditions will unfold over time. Graphical AIRMETs are also depicted on sites like Even ForeFight and cockpit XM weather displays will depict AIRMETs on a map, though you'll have to dig a bit to access them.

Forecast winds aloft will also give you an idea whether or not turbulence and wind shear are on today's menu. Combine strong winds aloft with rough terrain and the ride gets bumpy at lower altitudes, downwind of the terrain (an area known as the lee side). If the wind direction and velocity is forecast to change abruptly as altitude increases, expect wind shear - a sudden change is wind velocity or direction over a short distance. Wind shear can occur over a horizontal distance across the ground or over a vertical distance as you climb or descend. If you plan to fly over rough terrain or mountains, forecasts winds aloft in excess of 20 to 25 knots should give you pause. ADDs lets you access winds aloft forecasts in textual or graphical format.

PIREPs (pilot reports) are like AIRMETs in that decoding text versions requires knowledge and practice. My favorite tool for PIREPs is the ADDS Java PIREPS Tool. Launch most any browser that supports Java (this excludes the iPad), select part of the continental US, and choose the types of reports you're interested in. Symbols for turbulence will appear at locations on the map where the PIREPS were reported and you only need to mouse-over those symbols to access the details. You can access PIREPs on and other sites, but they are strangely missing from the mobile versions of ForeFlight.

While pilots of smaller aircraft are more susceptible to and tend to over-report the intensity of turbulence, pay particular attention to PIREPs from transport category aircraft since they are flown by flight crews who have a lot of experience with turbulence. More on this later.

If the weather forecast is waaay behind what is actually happening, a Center Weather Advisory or CWA can be issued. The Aeronautical Information Manual strays into neologism by calling the CWA a nowcast: "By nature of its short lead time, the CWA is not a flight planning product." To which I say, "Horse feathers!" If you get a pre-flight briefing and it contains a CWA, you most certainly can use it for pre-flight planning purposes. If you are already airborne, CWAs are available over the Hazardous In-flight Weather Advisory Service (or HIWAS) that is broadcast over the voice component of many VOR stations. Just tune in the VOR and listen.

Studying Turbulence

While much of the country has been enduring snow and cold, unseasonably warm weather settled over coastal areas of Northern California last week. This January weather gift was due to a stationary area of high pressure that kept the jet stream and the wet weather well to the North. The downside was it also generated persistent Northeasterly winds aloft that created a lot of turbulence and low-level wind shear in the San Francisco Bay Area.

Strong winds flowing over terrain is much like fast-moving water flowing over rocks in a stream: You may not be able to see the air boiling and swirling on the lee side of terrain, but rest assured it is there. You should only have to experience this once to know the lee side is not a pleasant place to be when the winds are howling, yet the lee side is exactly where several pilots reported encountering turbulence. Duh!

My student had planned a dual cross-country flight from Oakland to Columbia and a Center Weather Advisory was in effect for moderate to severe turbulence. The first part of the route would start with a climb-out on the lee side of the Oakland/Berkeley hills - the heart of the action. The surface winds were calm, but the winds at 3000 feet were forecast to be 050˚ at nearly 30 knots, so our plan was to avoid or minimize the turbulence by climbing as high as possible as quickly as possible. We discussed what we'd do if we encountered turbulence (more on that later).

Contacting the tower for takeoff, my student requested an early hand-off to Norcal so we could ask for a climb into class bravo. Reaching 800', the tower handed us off and Norcal approved our climb into bravo. The result was occasional light chop while climbing and a smooth ride all the way to Columbia. Cruising comfortably at 5,500 feet, we made a detailed PIREP to Flight Watch. Opening the VFR flight plan on the way back, the Flight Service specialist warned us of even more PIREPs of moderate to severe turbulence.

Our strategy for the descent back into Oakland was similar to what we'd used on departure: Stay above 3,500 feet for as long as possible and be prepared for turbulence during the descent. We requested a bravo clearance from Norcal, explaining it was to avoid turbulence. They were fine with us crossing Lake Chabot at about 3,500 feet, then we started our descent. We encountered continuous light chop, occasional moderate chop, and a bit of an updraft. Nothing to write home about.

I've witnessed pilots who were reluctant to make special requests for things like an early turn or a climb into bravo airspace. I'm not sure what causes this reluctance, but I encourage pilots to speak up. If you have a special request and a reason for that request, most controllers will do what they can to accommodate you. Ask and ye shall likely receive.

Back on the ground, we went online to see the PIREP we'd filed, but lo and behold, it wasn't there! Perhaps it wasn't disseminated because our PIREP for mostly light chop was at odds the dire warnings contained in the CWA. Who knows, but my student was not impressed by the results of all our PIREP efforts.

Riding the Beast

It may seem obvious, but ensure your seat belt is snug and tight when you know or suspect you're going to encounter turbulence. Beware of newer seat belts that contain airbags because they can feel like they are tight when they aren't. Old, worn seat belts may be difficult to tighten, but it's worth the effort: You don't want to bang your head on the ceiling when the action starts.

Slow down to at least the published maneuvering speed (Va) for your aircraft at your estimated weight. If you don't know your estimated weight, take a moment to appreciate the advantage of calculating weight and balance before each flight. Once you are flying at or below maneuvering speed, it's highly unlikely your aircraft will experience structural damage from turbulence.

If you have loose items in the cockpit, now is the time to secure them. If the turbulence rises to the level of moderate or greater, you'll know when your ham sandwich takes flight inside the cockpit.

I avoid using flaps in turbulence because in most GA aircraft, the addition of any flaps reduces the aircraft's maximum design load factors. For many GA aircraft, the positive load limit of 3.8G will drop to around 2G and the -1.5G negative limit may drop to 0G. Bottom line: Delay extending flaps until you are preparing to land.

Most GA aircraft are designed to be stable and they tend recover from upsets in pitch and roll without much assistance from you, so let the plane to ride the bumps. You do not need to respond on the controls to each and every bump that you encounter, but when you do use the controls, think of dampening out the variations rather than trying to overcome them. Over-controlling will increase the load on the aircraft and make the ride feel more uncomfortable for you and your passengers. Autopilots tend to over-control in turbulence and they are designed to spontaneously disconnect in moderate or greater turbulence. You may as well disconnect George and hand fly since you'll undoubtedly do a better job. If the turbulence is moderate or greater, do your best to maintain a level pitch attitude and accept variations in altitude as you just let the plane ride.

Updrafts and downdrafts can be difficult to perceive until you have more experience, but a good way to learn is to keep the airspeed indicator in your scan. Increases in indicated airspeed without changes in power or pitch mean you're in an updraft. Decreases in airspeed mean you're in a downdraft. It's not uncommon to need to make dramatic and frequent adjustments in engine power to compensate for updrafts and downdrafts.

If you expect turbulence while descending, remember that altitude is energy: You'll have to reduce the throttle to be able to descend and stay at or below maneuvering speed. If you're one of those pilots who usually descends by just pushing the nose over and trimming, you'll want to rethink that strategy in turbulence.

Your reward for reading this far is that I'm now going to reveal my secret for flying through turbulence: Be patient, stay as relaxed as possible, have a mental image of how you want the plane to be flying, then do your best to make reality match that mental image. Without a concept of how you want the aircraft to be flying, you'll simply be reacting to the conditions. Developing a piloting concept is a deep topic, perhaps for another day.

Making the Call

Do your fellow pilots a favor and make a PIREP when you've encountered turbulence. Less experienced pilots flying smaller aircraft frequently over-report the intensity of turbulence they encountered. Pilots in more rigid, composite airframe aircraft (like Diamond and Cirrus) tend to experience and report a rougher ride that their aluminum-flying brethren. In any case, you can avoid over-reporting turbulence intensity by doing your best to set aside your emotional reaction and using as objectively as possible the following criteria (Aeronautical Information Manual, section 7-1-23, emphasis added).

Light Turbulence - momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly.

Light Chopcauses slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.

Moderate Turbulence - similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed. Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged.

Moderate Chop - similar to Light Chop but of greater intensity. Rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.

Severe - Large, abrupt changes in altitude and/or attitude. Large variations in indicated airspeed. Aircraft may be momentarily out of controlOccupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about.

Extremeaircraft is violently tossed about and is practically impossible to control, structural damage to aircraft is possible.

Whenever you are startled or frightened by turbulence, stop and take a breath, then do your best to assess what has happened or is currently happening. On numerous occasions I've talked to pilots who said they encountered moderate-to-severe turbulence, but when I ask them some basic questions it seems they probably encountered light-to-moderate turbulence. One tip-off: If the head on your Barry Bonds bobblehead doll is bobbling, but the doll itself is staying put, it's probably light turbulence.

Brave vs Smart

Turbulence can become a serious risk factor; it is distracting, it adds to your workload, and it's tiring. Combine turbulence with instrument meteorological conditions and/or night time and the risks for single-pilot operations only get worse. If you find turbulence scary and uncomfortable, join the club! There's no shame in being afraid, but there's seldom any excuse for not anticipating or expecting turbulence. With some planning and a little strategy, turbulence can often be managed, but there's nothing wrong with canceling or delaying a flight to avoid turbulence. I'll take a smart and thoughtful pilot over brave and risky pilot any day of the week.

1 comment:

Blake said...

Another Great Post!

Your comment about the PIREP not appearing online echos my experience up here in Canada.

Many-a-time I've filed a pirep, only to find out later that it never got disseminated.

It's frustrating to see this type of information not being made available online sometimes.

In my personal experience, I've had about 60% of my pireps appear online for use by other pilots during planning.