Wx Watch: Ops on Top

There are a number of reasons for wanting to fly above clouds or haze, but the desire to avoid adverse weather tops the list. It's often smoother above a lower layer of clouds; the visibility is often better; and upcoming rainshowers, thunderstorms, and other cloud build-ups are more easily seen.

But on-top operations can go bad — very, very bad if you have no instrument rating, little time on the gauges, or limited weather flying experience. The Federal Aviation Regulations let us fly VFR on top of a layer of clouds as long as we can make cloud-free climbs and descents to cruise altitude. Sometimes it doesn't work out that way.

Here are a few common weather scenarios that illustrate the point.

• In warmer weather, or convective situations, cloud tops rise as the day progresses. You want to continue flying on top, so the temptation is to climb to stay in the clear. In a non-turbocharged single or twin, you can go from cruising on top at 6,000 feet, to climbing and weaving amongst build-ups at 10,000 feet, to thunderstorm penetration in a few minutes.
• If the air is moist enough, a scattered layer can go from broken to overcast in a hurry. This would be typical of conditions preceding warm fronts or slow-moving cold fronts and occlusions. It's also true in mountainous areas, where upslope winds are conducive to dense cloud formations. Scattered clouds over valleys (especially river valleys) and coastal areas are also prone to quickly go broken or overcast. This is especially true in the early morning and late evening, when temperatures can drop to the dew-point level. Along the West Coast, a semipermanent slab of low-lying clouds — the marine layer — can persist for days. This can make VFR climbs to VFR on top nearly impossible for those flying out of coastal airports.
• Topping a haze layer may not work, either. In humid and stagnant air, a haze layer can reach 10,000 to 12,000 feet. This is typical of the eastern half of the country and near large cities. Trying to leave marginal visibilities behind, some pilots attempt to climb out of the muck. Passing through 8,000 feet or so, they notice several things: Climb performance begins eroding (again, assuming a non-turbocharged airplane), visibilities get no better, and headwinds may intensify. With no improvements in sight (no pun intended), pilots may end up accepting a VFR-in-IMC cruise in lieu of either descending and landing or taking the extra time to actually reach on-top conditions.
• Within several hundred miles of a front, clouds can form in layers. These layers often merge. Pilots who once found it easy to fly on top can find themselves caught in a squeeze as the layers come closer together. The squeeze play isn't always obvious. One moment, it looks like you're flying through a little fog between layers; the next, you're in full-blown IMC.
• If your flight is a long cross country, expect cloud changes. If the trip is to a destination a hundred or more miles away, it's likely that the weather will change — sometimes subtly, sometimes not — during the time you're in the air. Sure, a big high can invalidate this little rule, but only if it contains a relatively dry air mass with no convective potential. Towering cumulus clouds and air-mass thunderstorms often crop up in high pressure when there's enough moisture and heat.

How can you avoid being trapped on top or being unwittingly forced into flying in IMC? Avoidance begins with a good preflight weather briefing. Pay close attention to the terminal forecasts for mention of any cloud layers, as well as their nature. A route-long forecast of 3,000 scattered, 10,000 broken sounds like a good deal; 3,000 scattered, 6,000 broken, 10,000 overcast sounds a lot like a front's nearby.

At the very least, the VFR-only pilot should make sure that surface reports and forecasts indicate good VFR below the lowest cloud layers and that those layers never extend to the magic, 1,000-foot VFR ceiling minimum. If you have to make an emergency descent on instruments, you certainly want to make sure you can break out into VFR conditions below.

Area forecasts will also mention cloud layers, as well as a number of other critical forecast elements. For example, ahead of an August warm front, the area forecast for Illinois mentioned "AGL 40 SCT-BKN LYRD 150." Decoded, this means that a scattered to broken cloud base of 4,000 feet agl is forecast, with cloud layers to 15,000 feet msl.

VFR-only pilots, how does that sound to you? It should be a warning that something is cooking. Trying to top that scattered to broken layer doesn't sound like a particularly good idea, for the reasons mentioned earlier. Flight below the clouds sounds in order, terrain and obstacles permitting.

Instrument-rated pilots, would you fly into such a forecast? Maybe. Maybe not, if you read the rest of the forecast. It goes: "OVR W CNTRL IL WDLY SCT RW/TRW PSBLY SVR SPRDG OVER RMDR STATE BY 18Z. CB TOPS ABV 450. OTLK...MVFR CIG RW/TRW NRN IL...VFR TRW SRN IL." With plenty of weather flying experience, radar, lightning detection equipment, turbine power, helpful controllers, and a few escape paths, the instrument-rated pilot may be able to escape Illinois by artfully dodging build-ups and storms. But VFR on top? Don't count on it.

Pireps, if they are current, are perhaps the most valuable source of information on cloud bases and tops. Too bad pilots don't give more of them.

Naturally, you should be talking to Flight Watch on 122.0 MHz for weather updates. The more marginal the weather and the denser the lower layer of clouds, the more often you should be checking for late-breaking surface observations, terminal forecast amendments, and pireps. The moment you sense that lower layer becoming an undercast, start asking Flight Watch for the nearest airports with ceilings above your current cruising altitude.

What's the best weather setup for hassle-free VFR-on-top flying? As we all know, there are no guarantees when it comes to forecasting, but here are four fairly conservative situations that should keep you out of trouble:

1. Post-cold front: This is a time when high pressure is typically building, and cloud layers remain in a scattered, low-altitude layer. That's during the summer months. Around the Appalachians, a solid deck of stratus or stratocumulus often follows a cold front, and flying on top of them can be bad business. The clouds are loaded with ice and can top out in the flight levels. Get in there, and it'll be a frosty descent.
   
   As long as you're able to remain over a scattered layer, expect a nice, smooth ride. But below the clouds, you'd be rocking and rolling in the turbulence that nearly always attends a cold frontal passage.
2. The front side of a big high-pressure system: This would be the eastern half of a high, and it's characterized by subsiding air, good visibilities, low tops, scattered clouds, and little chance of convection. Trouble can come when flying in the "back side" of a high. Here, there's a moist, southerly flow that makes rainshowers and thunderstorms. Chances are good that a warm front is on the way, too. In this situation, safe on-top flying may be limited to the early morning hours (if there's no ground fog to impede visibility during the takeoff and climb).
3. Florida and the Caribbean: Here, you can nearly always safely top a low, scattered layer. If rainshowers or thunderstorms are around, you'll be able to avoid them visually, as well as find a spot to descend and land visually somewhere within a few minutes' flying time. Nota bene: This rule's invalid when the occasional front passes through or when tropical depressions, storms, or hurricanes move in.
4. Deserts: It's dry, right? There are not many clouds, right? Right — usually. Lows and air-mass thunderstorms can move in, but it's usually possible to top any low clouds (if there are any) and cruise all day. Don't expect a reprieve from turbulence, though. Rotors, waves, and other mountain-induced turbulence are very often the order of the day when flying in or near the West's mountains.

Let's say the worst has happened. You are a VFR-only pilot, you've been on top, but now you're in a bind. You're suddenly faced with, or in, clouds. What to do?

Clear-cut answers are difficult. If it's a convective situation, the terrain allows, and the cloud bases are high enough, a descent makes sense. As a general rule, don't allow yourself to be forced into a climb. Sooner or later, you'll have to descend, perhaps back down through a thunderstorm.

Reversing course is often a good option, followed by a diversion to an airport having VFR conditions. If you're lucky enough to have holes large enough in the cloud layer below, then by all means spiral on down — if terrain, obstacles, and weather will let you. In other words, you don't want to descend to an altitude closer to the surface if you don't have safe visibility. Descending for its own sake is out. On or off instruments, altitude is nearly always your friend.

Of course, all of this should be done with the help of an air traffic control facility. VFR into IFR is a serious situation for the non- instrument rated and qualifies as an emergency. Don't be afraid to climb, confess, and comply with ATC's suggestions for getting out of the mess.

By now, the value of an instrument rating should be obvious. Yes, private and commercial pilot certification standards require you to demonstrate proficiency in flying on instruments. But these are minimum standards, and most curricula give you only three to six hours under the hood. If you stay "current" on instruments, you may be able to perform escape maneuvers (the standards emphasize 180-degree turns, climbs, and descents on instruments) successfully. If rusty, well, you're definitely taking your chances.

On-top operations do have their advantages, and they can be done safely. Others don't share the United States' optimism on this point. VFR- on-top flying is prohibited in Canada, Europe, and most other foreign locales.

We have the privilege of giving it a shot, whether VFR or IFR. But good, common sense dictates a sound knowledge of weather basics, a healthy skepticism about forecasts, and a tactical approach to dealing with changeable weather. You'll need plenty of contingency plans if the tops rise, the layers merge, or fog or clouds close up beneath. And these plans should be fleshed out before you take off.