PIREPs: Word from Above

I was in the right, front seat of a 1999 A36 Bonanza as we launched on an IFR training mission in northern California. A gloomy overcast had rolled inland off San Francisco Bay and we heard traffic holding overhead when Center gave us our clearance to go. My student, new to the Bonanza, did a superb job of holding attitude as he arced over the now-unseen hills while we turned inland; the holding pilot, now cleared for his approach, asked the Center controller about the weather at our departure airport.

‘Bonanza 329PT, what were the conditions on departure?’ the controller asked us. My student immediately keyed the mike, replying only ‘It’s solid clag.’

‘The departing aircraft says ‘it’s solid clag,” the controller echoed to the arriving pilot. We burst though the cloud tops into the blue.

I intervened. Hitting my push-to-talk switch I reported: ‘Center, this is Nine Papa Tango, bases of the overcast are at 1800, tops at 2400, high cirrus above.’ The arriving pilot acknowledged this amended PIREP (pilot report) as we intercepted the airway east toward our practice approaches.

What’s a PIREP Say?
Pilot reports fill the gaps between weather reporting stations, and are the only means of providing information about conditions above the bases of the lowest cloud layers. In the case of the aircraft shooting an approach into the airport my student and I were departing it was the only way he could learn the flight conditions below the clouds he was currently within-his clue as to whether he was likely to break out in time to land, or if he should break off the approach and go to his alternate.

Why should you file accurate pilot reports? Ask yourself: if you were receiving a pilot report, what information would you want to hear? Not to fault my student (whose local CFI had obviously not taught him the art of PIREP-ing), but I imagine you’d like to hear something closer to what I’d reported, and not a vague report of ‘clag.’ Put another way: report in-flight weather conditions as you would like others to report conditions unto you.

Parts of a PIREP
The best pilot reports contain specific elements. They include:
1. Aircraft type
2. Time, location and altitude
3. Sky condition (i.e., cloud cover)
4. Visibility
5. Turbulence report
6. Outside air temperature
7. Wind vector, if known
8. Icing, as appropriate
9. any other information potentially helpful for other pilots

Let’s briefly look at the importance of each:

Aircraft type: Different weather phenomena will present themselves differently to different-sized airplanes. For instance, turbulence felt and reported as ‘light’ to by the pilot of a Cessna 172 is likely much less intense than ‘light’ turbulence to the captain of a Boeing 777. You can see where type-specificity becomes important-the most applicable pilot reports are those coming from airplanes similar in size and weight to yours. There’s a secondary benefit of knowing the airplane type in a PIREP as well. Although it’s far from universally correct, PIREPs from larger, more complex airplanes often come from more experienced pilots, who may be more likely to accurately describe the weather and its effects. So knowing the aircraft type in a PIREP provides an indication of how the weather described is likely to affect your airplane.

Time, location and altitude: PIREPs are a snapshot picture of weather as it exists at a four-dimensional point-a time, a location (longitude and latitude, or a known spot in space) and an altitude. You don’t need to report the time when calling in a PIREP…the specialist receiving the report will record it for you. You do need to accurately describe your location (over a point, or commonly a radial and distance from a VOR) and your altitude.

Sky condition: One of the most valuable bits of information obtained from a PIREP is sky condition, i.e., the height and extent of cloud cover. Use ‘book’ terms like ‘overcast,’ ‘broken’ or ‘scattered,’ and give altitudes (or estimates, if above or below clouds). Again, think about what you’d like to know when you file a report. That pilot arriving after my student and I departed couldn’t use ‘in the clag’ to make a decision, but ‘bases of the overcast 1800, tops 2400, high cirrus above’ told him precisely what he needed to know to plan his approach.

Visibility: Similarly, a good PIREP includes a description of flight visibility and, if applicable, the cause of any reduction in visibility. ‘Flight visibility estimated at three miles in light rain’ is far more useable to a pilot than ‘visibility limited’ or ‘I can’t see much’ (terms I’ve heard from pilots over the radio).

Turbulence: Turbulence reports are often very subjective, but the FAA givens us fairly precise definitions we can use to make more accurate reports. ‘Light’ turbulence, for instance, produces only minor ‘bumps’ and slightly displaces objects in the aircraft. ‘Moderate’ turbulence causes strain against the seat belts, excursions in aircraft bank and pitch, and large movement of objects in the cabin-think ‘head hitting the overhead’ if you’re not strapped in tightly. ‘Severe’ turbulence includes momentary out-of-control pitch and bank excursions as you try to recover from the bumps. ‘Extreme’ turbulence means the aircraft is virtually out of control, at least momentarily. ‘Chop’ is a rhythmic, repetitive occurrence of turbulence. This is where knowing the type of airplane filing a PIREP is handy; what makes a Skylane bounce around in pitch and bank won’t do the same to a Citation or a 737. Call it what it is-don’t report ‘moderate turbulence,’ for instance, unless you’re really bouncing around up there.

Outside air temperature: OAT information in a PIREP is extremely important, especially in or near clouds when the temperature is near, at or below freezing. Air temperature aloft also provides clues to atmospheric stability, which in turn helps savvy pilots anticipate turbulence, thunderstorm development or mountain wave turbulence. OAT is almost universally reported in degrees Celsius, but if you must report it in Fahrenheit make it obvious that you’re doing so (‘Outside air temperature at 4000 feet is thirty-three degrees Fahrenheit’).

Wind vector: Most modern GPSs have the ability to compute the wind vector (direction and speed) if they can also reference the airplane’s heading, track and true airspeed information. If you can compute the wind vector (with GPS or old-fashioned navigation), include it in your PIREP. This will help others plan fuel requirements and cruising altitudes. The experienced weather-pilot will use ‘actual’ wind vector information to verify or refute an early weather forecast, by comparing the wind vector to forecast winds aloft, and using that information to plot the direction and speed of weather phenomena.

Icing: Airframe ice is the great unknown of winter weather flying. There are aviation forecast products that warn of the probability of ice, but PIREPs are the only current method of determining if ice does or does not exist in a point in space at a particular time. Like turbulence, icing reports should be made using standard terminology. ‘Light’ ice is a very slight accumulation that would likely be hazardous is allowed to continue (the old term of ‘trace’ ice has been eliminated, with a ‘trace’ being reported as ‘light’ icing). ‘Moderate’ ice is enough accumulation that, by definition, requires ‘continuous use of deice equipment’-given that most sources still recommend activating deice boots only after ½ to ¾ inch of ice forms, this means a rapid enough accumulation of ice that, were you flying a deice boot-equipped airplane, you’d be continually getting ½ to ¾ inch ice build-ups immediately after ‘popping the boots.’ A ‘moderate’ accumulation, by the way, is considered dangerous even for ‘known ice’ airplanes if the pilot does not immediately escape icing conditions. ‘Severe’ ice is that in which even fully de-iced airplanes cannot keep the wings and tail clean. Like turbulence, report ice as it truly is.

Any other information potentially helpful for other pilots: This is the ‘miscellaneous’ category. Include anything you think you would lie to know if you were planning a flight. Examples include ground fog, mountain obscurement, distant towering cumulous, dust devils on the surface, and other hazards that don’t quite fit into the standard format of a PIREP.

Take a look at a couple of actual PIREPs and see how their format fits this model:


Decoded: a Cessna 208 (Caravan), reporting from a point 16 miles from PSM on the 330 degree radial at 8000 at 1947Z, reports IMC conditions in light rain with occasional light ‘chop’ turbulence.


Decoded: Filing an ‘urgent’ pilot report (indicative of highly dangerous conditions), an Air Force KC-10 tanker at 400 feet on final approach to Runway 6 at KWRI, reports low level wind shear that caused airspeed to drop by 10 knots, at 2003Z.

HWV UA /OV CCC180006/TM 2050/FL100/TP C208/SK BKN085 TOP087/TA 03/WV 330010/TB NEG

Decoded: Another Cessna Caravan, this one at 10,000 feet at a point six miles from CCC on the 180 radial (due south), reports the bases of a broken cloud layer at 8500 feet, with tops at 8700 feet. The temperature aloft at that altitude was +3C and winds were from 330 at 10 knots. There was no turbulence at the time of this report, 2050Z.

BOTTOM LINE: PIREPs are the one of the only source of weather information above the bases of the lowest cloud layer, and help fill the gaps between reporting points. Filed correctly, PIREPs are your best source of weather data that will affect your flight. For the system to work we all need to file frequent, and accurate, pilot reports.