Of forecasts and pireps

September 1, 1998

A trip last spring reinforced my belief that seeing is believing. Sometimes we can use others' eyes to help. Forecasts, as has been said many times, are educated guesses. Sometimes the guess is spot-on. At other times the weather service collectively shrugs and mumbles about the art rather than the science of weather prognostication. But Cray supercomputers and brilliant forecasters are not the only provisions pilots have to divine the future. The lowly pirep is one of the most powerful and reliable tools that we can use.

I was flying southbound down the East Coast in a Beech Bonanza. A strong cold front pushed through, and big high pressure was sliding in behind the front with packed isobars, clear skies, and blustery winds. I had planned to leave early, but there was always just one more delay, so it was nearly 10 a.m. before I could get away. An early morning DUAT briefing made the trip questionable, with a forecast of widespread moderate and occasional severe turbulence. Surface winds were predicted to be from the northwest at 25 knots, with gusts to 38 knots.

Many forecasts predict occasional (not widespread) moderate turbulence, and many pilots generally disregard them as just so much liability protection for the government. To quickly review, moderate turbulence means that stuff in the cabin moves. There are a few pilots who enjoy the sensation of being whacked with a two-by-four and watching passengers turn green, but most of us will enthusiastically avoid moderate turbulence. Moderate tends to get overreported because we've felt sharp tugs on the seatbelt and it is uncomfortable. I generally keep a loose object somewhere in eyesight where it won't do any damage; if it moves, then moderate turbulence is confirmed and it's not just me being squeamish.

Severe, of course, means that the aircraft is momentarily out of control, and that means the air surrounding our aircraft is not safe for continued flight. Spend much time in severe turbulence and there is a possibility of aircraft structural damage, some injury to the occupants, and a certainty that the passengers will have grave doubts about the PIC's judgment.

The bad news about leaving late is that surface winds tend to pick up as the day wears on. The good news is that there's a chance to see if the METARS match the forecast and to gather pireps. This is a golden opportunity to grade the weather gurus. Is it doing what they said, when they said it would? The first airplane to launch in the morning is a trailblazer — a lonely or brave soul who we hope will report what's happening. On this trip I was able to gather the information that made a "go" decision easy.

The actual surface winds at the reporting stations along my route weren't higher than 22 knots. Strike one against the forecast. Secondly, when there really is moderate turbulence, it tends to get reported. There were four or five reports this day, with only one reporting moderate chop. Strike two. It was from a Cessna Skyhawk at low altitude on the lee side of the mountains. My route was planned to take me farther east to minimize the exposure to mountain wave activity, and there were smooth ride reports at the cruise altitude of 7,000 feet. Strike three and time to start.

The flight proved to be uneventful and comfortable. After an hour in the air I made a point of filing a pirep to pass along the good news so that someone else could make the "go" decision. It's as important to report that bad things are not happening as it is to report when they are. However, using pireps requires some interpretation. It seems as though there are always some strings attached.

One pirep does not validate or invalidate a forecast — which is too bad when you're in a hurry to launch, and who isn't? It's a big sky and just one aircraft's having a good or bad flight in a particular piece of it shouldn't be taken as gospel. Like the FBI, to be certain, you're better off to collect a dossier of reports. Naturally, if turbulence or icing is forecast and somebody verifies it, that is a mandate to gather more information before putting the flight in a compromising situation.

The type of aircraft making the report is also a factor. If an airliner reports moderate turbulence or light icing, figure that conditions will not be suitable for a light aircraft. Highly wing-loaded aircraft ride better in turbulence, and if a Boeing is having a bouncy day, your ride will probably be terrible. Likewise, the fat wings (compared to those of a GA aircraft) of an airliner have relatively low collection efficiency for ice, so what the pilots of an airliner see as minimal ice may turn us into an iceblock. Airliners also move faster and climb better, which tends to minimize ice. One inexplicable thing I've always noticed about ice is that it is seldom as troublesome when the aircraft has full deice equipment. No deice equipment and it is just enamored with the aircraft — hmmm.

The most valuable reports, naturally, are from aircraft that are similar in size to yours and are flying along the same route. A few reports can make your decision much easier. Strong westerly flow over the ridges and pireps of moderate chop below 12,000? A no-brainer — we have either to fly higher, which is not usually an option for normally aspirated aircraft and pilots without oxygen, or to adjust the course away from the ridges. If that's the way you have to go, wait until the wind subsides or buy a ticket.

Timing is critical. The one thing that is undeniably certain is that weather will change. Landsberg's Law: Invariably, it will change faster when you want it to be slower and vice-versa. An old pirep is yesterday's news; either bad or good, it is likely to mislead.

We are still learning about the nuances of forecasting icing, and in the next few years the ability to predict and verify its location should improve significantly. A pirep on icing is a valuable commodity because ice isn't measured well from the ground. The forecasts are so general and conservative that, many times, we are forced to cancel because there just isn't enough information, and the consequences are severe for trespassing without deice equipment.

The development of Doppler weather radar makes thunderstorm verification much better than it was even a decade ago. There are so many sources for preflight thunderstorm information that there is little excuse for getting caught — in the first half-hour of flight anyway. Once you are airborne, gathering the weather is more challenging.

Pireps can add a new dimension, although they must be used very carefully around building cumulus. When clouds build at more than 2,000 feet per minute, a 10-minute-old pirep of light chop and a little rain can lead you into a place that you'll wish you hadn't gone. Here, your knowledge of the weather system's stability, the time of day, the type of front, and the characteristics of the clouds all play a part. For the VFR pilot this should never be an issue, since clear of clouds means avoiding the worst. For the IFR pilot without thunderstorm detection equipment, pireps must be carefully weighed. The tactics of thunderstorm flying are topics for another day, but my approach is that if someone says it's bad — it's bad. If the source says it's good, it may be bad when I get there. So more information is required before I'll venture into a questionable cloud.

Another area where pireps can be very helpful is on an IFR approach. Automated weather reporting through ASOS and AWOS can be a mixed bag. In stable and uniform conditions of low ceilings and visibilities, these devices can be better than human observations. However, in rapidly changing conditions or where there is a discontinuity, a pirep from someone who has just flown the approach is very helpful. It does not come with any guarantee; it's just additional information. When you break out of the clouds and land, let ATC know what your conditions were.

Make pireps and ask for them. The Air Safety Foundation is currently working with the FAA to improve the pirep dissemination system. For air traffic controllers there may be a work-load issue to disseminate pireps depending on traffic. But if everyone passed on the word about flight conditions, more flights could be made safely and fewer flights might be launched in marginal conditions.


The AOPA Air Safety Foundation would like to hear about your experience regarding pireps in decision making. E-mail asf@aopa.org or write to AOPA Air Safety Foundation, Project Pirep, 421 Aviation Way, Frederick, Maryland 21701.


See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.