Safe VFR Flight Over the Top of Clouds

Flight above clouds

Our Zenith 601HDS plane, N314LB, is not equipped for flight under instrument flight rules (IFR). Nor do we have the appropriate rating for that kind of flight. Hence, we are restricted to visual flight rules (VFR). They demand that we cannot fly into clouds, let alone fly while embedded in clouds.

Want to learn more about flying?

  

My book Lessons from Piloting for 45 Years covers close shaves over more than four decades involving thunderstorms, mountain weather, faulty runways, equipment failures, and more.

Learn key strategies for coping with these dangerous situations. They include two tools called Sully's Rule and Forecast Evaluation.

VFR Cloud Conditions

Below 10,000 ft MSL, VFR flight must be at least 500 ft below clouds or 1,000 ft above clouds, and horizontally must be at least 2,000 ft away from clouds.

Above 10,000 ft MSL, two of the conditions are stricter: The 500 ft condition for flight below clouds is increased to 1,000 ft, and the horizontal distance condition becomes 1 mile.

The conditions are relaxed close to the ground: Up to 1,200 ft above ground level (AGL), VFR flight only needs to stay clear of clouds. Near airports, indicated by magenta shading on maps, that limit is lowered to 700 ft AGL. 

The above summary is incomplete since it ignores conditions for night flight, which we never dare to do, and since it skips over some subtleties as well as certain visibility conditions. But the summary suffices for the present discussion.

We also need to introduce the concept of cloud types and ceiling.

Cloud Types and Ceiling

Clouds are characterized as follows: There are few clouds, or the clouds are scattered, or the cloud cover is broken, or there is an overcast. 

Percentage figures for cloud coverage characterize these types of cloud covers. In flight these numbers are not so useful. For a detailed discussion, see METAR-Wikipedia. A practical guide is that a scattered layer consists of individual clouds that are separated by large holes allowing easy penetration under VFR conditions. When this is no longer possible but holes are still present, we have a broken layer. When there are no holes at all, we have an overcast.

Cloud layers, of whatever type, are always given with altitude of the bottom of the layer, measured in feet AGL.

Scattered layer, Rocky Mountain National Park

The last two cases, broken and overcast, describe a cloud layer that may not be penetrated in VFR flight. The bottom of such a layer constitutes a ceiling for VFR flight.

Broken layer, Rocky Mountain National Park

Consequences of the Cloud Rules

Suppose we have the following situation. We take off under VFR conditions and later encounter an extensive area of clouds so low that we cannot fly below them. We have two choices: We land and wait for the clouds to lift, or we climb above the clouds and continue. 

If we go for the first choice, we may have to wait for several hours, and thus may not be able to make it to the planned destination the same day. 

In case of the second choice, the cloud deck may extend beyond our destination. Thus, when we reach that point, there is no safe way to descend. Worse yet, while we are flying above the cloud deck, the clouds may rise higher and higher. Accordingly, we must climb to stay 1,000 ft above the clouds. In the nastiest case, we get to the point where the plane cannot outclimb the clouds or where we reach the limit of 14,000 ft MSL for flight without supplemental oxygen. Terrifying situations indeed. 

Here is another situation where flying above a cloud layer seems attractive but ultimately is the wrong choice: We could fly below a cloud deck, but the ride would be bumpy or would encounter a strong headwind. In contrast, above the clouds, the flight would be completely smooth with a strong tailwind. Lured by these attractive flying conditions, we begin flight above the cloud deck, but later run into one of the problems cited above.

Flying on top of a cloud deck under VFR conditions is called "VFR over-the-top." How can we safely conduct VFR over-the-top and avoid terrifying scenarios?

VFR Over-the-Top

VFR over-the-top can be simple. For example, near the coast, there often is a low-level cloud bank. We rise above the cloud bank, cross it, and descend again once we are over open water. Similar situations can occur when we cross a mountain range, or when we are facing a well-defined cluster of clouds near a lake. 

Cloud bank, Grand Teton National Park

The situation is much more complicated when the cloud cover extends beyond the visible horizon. This post describes how we can safely conduct VFR over-the-top in that setting. 

We begin with a discussion of equipment that, though not legally mandatory, in our opinion is a must.

Equipment

We consider VFR over-the-top for extended distances only when we have an autopilot, electronic map display, and an ADS-B In receiver on board. In our case, the map and ADS-B connection are supplied by a Garmin GDL 39 receiver linked to the Garmin Pilot system, with display on an iPad. 

The autopilot not only keeps us reliably on the intended route, but also frees us for the extensive checking described in a moment.

The ADS-B information provides timely weather data for all airports with weather reporting equipment. The data include Terminal Area Forecasts (TAFs) for major airports. The forecasts give us weather predictions well beyond the duration of our flight. 

ADS-B In also supplies a radar image of rain and thunderstorms overlaid on the electronic map.

Needless to say, we assume here that the plane has performed flawlessly for an extended period of time before VFR over-the-top is deliberated. Think of VFR over-the-top as equivalent to flying over open water to develop the proper respect for the operation.

There are two decision processes for VFR over-the-top. First, should we begin such flight? Second, how and where shall we descend from such a flight?

Decision to Start VFR Over-the-Top

First, we must have clear sky or a scattered layer to climb through for VFR over-the-top. This is easily ascertained.

Second, we must be sure that the broken or overcast layer does not rise beyond the capability of the airplane or the legal limit for flight without supplemental oxygen. We impose a much more stringent condition: We must be sure that the layer rises at most by a trivial amount. To ascertain this, we look at all reported weather data of airports along or near the planned route. This includes checking of all available TAF data applying during the entire flight.

Third, we must be sure that prior to the destination, we have clear sky or at most a scattered layer to descend through. TAF data are crucial here. All airports near the destination with TAF data must confirm this condition.

Decision to Continue VFR Over-the-Top

While proceeding above the clouds, keep on looking up the weather conditions for the airports enroute and at the destination. This includes checking TAF data: They are unlikely to change, but checking them time and again is a good idea. We do this at least every 10-15 minutes.

Special Aspects

The airports along the route and at the destination may report a very high layer, say at 25,000 ft AGL, that may be broken or overcast. You can safely ignore that layer. 

But if such a layer is not far above your intended altitude, you effectively would be sandwiched between them. Do NOT attempt any such flight since you may be squeezed from the bottom and top to the point where VFR is no longer possible.

Never attempt VFR over-the-top when the broken or solid cloud layer below starts at several thousand ft AGL and is also rather thick. Such clouds can easily puff up and trap you on top. The preferred cloud layer below is not more than 2,000 ft AGL and is rather thin and uniform, say not more than 1,000 ft thick.

Never attempt VFR over-the-top when significant rain is predicted or occurring, or when thunderstorms are developing or are in progress. In both cases, cloud development is unpredictable and thus can be very dangerous.

Fata Morgana for Pilots

When flying several thousand feet above an overcast, dark areas may appear in the distant cloud cover. They seem to indicate that holes are opening up due to the warming effect of the sun.

Apparent holes near horizon

The dark areas seem to signal that descent will soon be possible. But don't be fooled. A scattered layer of clouds far above, say at 25,000 ft, can produce shaded areas in the overcast below you and thus gives the wrong impression that descent will soon be possible. If you fly towards such supposed holes, they always disappear as you get closer. 

So to decide if holes are really opening up in the distance, look at the weather reports of airports in that area.


Example: Andrews, TX (E11) to Aero Country, TX (T31)

It was a late morning flight. Clouds at Andrews in West Texas had broken up to just a few clouds, while solid clouds prevailed along much of  the eastbound route. The screen shot below was taken while we were at 9,500 ft MSL, about 4,000 ft above a broken layer. The earlier photos showing the view from the cockpit were taken at that time.

Route Andrews, TX to Aero Country, TX

Note the airports with MVFR label along the route. They had clouds with bottom just above 1,200 ft. Thus, in principle VFR flight was allowed below the clouds. But at that altitude, there was considerable headwind and a bumpy ride. In contrast, above the clouds the air was smooth and provided a 10 kt tailwind. 

Near the destination, all airports were reporting clear sky or a scattered layer, and airports with TAF data predicted that this weather would continue for the next 24 hours. 

Here is the TAF of the DFW airport in Dallas. The time period from Fri 15:00 Z (= Greenwich time) to Sat 17:00 Z was relevant for us. In terms of CDT, it was the period from 10 am Friday to 12 pm Saturday and thus bracketed our flight time from 11 am - 2 pm on Friday.

TAF data for DFW airport

The layer with few clouds at 4,000 ft mentioned in the TAF was at the same altitude as the clouds below us at the present position. This confirmed that the clouds below us would not rise. The scattered layer at 25,000 ft was irrelevant. Even if it had been broken, it could have been safely ignored.

Conclusion

VFR over-the-top must be carefully managed to be a safe operation. At times, it can be the only option. For example, in a recent flight from Dallas to Albuquerque, it was the only way to transition across Northwest Texas and Eastern New Mexico, an area that during spring and fall loves overcasts at 500 ft AGL that sometimes last all day.

Have any questions or feedback about this post? Please share your thoughts in the comments.

Want to learn more about flying?

  

My book Lessons from Piloting for 45 Years covers close shaves over more than four decades involving thunderstorms, mountain weather, faulty runways, equipment failures, and more.

Learn key strategies for coping with these dangerous situations. They include two tools called Sully's Rule and Forecast Evaluation.