Cloud Base and Visibility
You should be thinking about visible moisture all the time……
Some important questions about cloud base and visibility:
- Do I have reliable cloud base information?
- If I have to fly lower to remain clear of clouds, will terrain be a factor?
- If I do, how much ground clearance will I have?
- How much airspace do I have between the cloud base and the terrain along my route? Do I need to change my planned altitude?
- Will I be over mountainous terrain, where the weather can change rapidly?
- What visibility can I expect? Will it be affected by haze, showers, rain etc?
- Given the speed of the aircraft, expected light conditions, terrain, cloud base and alternates available, are the reported and forecast visibility conditions sufficient for this trip?
- Are there conditions that could reduce visibility during the planned flight? Look for a small and/or decreasing temperature/dew point spread.
- Are cloud base and visibility values above my personal or club minimums?
Review wind conditions for departure aerodrome, the cruise, and destination. You will also need a mental picture of vertical wind profiles, so you can select the best altitude(s) for cruise flight, the best flight path to mitigate turbulence, and to determine whether windshear is present.
Consider the magnetic wind direction and the effect terrain will have on the wind’s behaviour. A small angular change in the direction the wind is striking terrain can make a significant difference to turbulence, making it greater or lesser. Are the winds at departure and destination likely to be affected by a sea or land breeze?
Also take into account surface heating leading to convective turbulence. This will be particularly noticeable on warmer days.
Some important questions about turbulence:
- Are the wind conditions at departure and destination aerodromes within the gust and crosswind capabilities of both you and your aircraft?
- What is the manoeuvring speed (VA) for your aircraft at the expected weight? Remember that VA is lower if you are flying at less than maximum gross weight.
Always have some escape options. Know where you can find good weather that is within your aircraft’s range and endurance capability. Where is it? Which way do you turn to get there? How long will it take to get there?
Knowing where to find good weather doesn’t do you any good unless you have enough fuel to get there. Taking only minimum legal reserve fuel could significantly limit your options. More fuel means access to more alternatives.
Having plenty of fuel also spares you the worry (and distraction) of fearing fuel exhaustion when weather has already increased your cockpit workload.
The GAP booklet, Fuel Management, has a lot of good information about fuel planning and management, and a Time in Your Tanks planning card is also available.
Know how low you can go without encountering terrain and/or obstacles. Consider a terrain avoidance plan for any flight that involves:
- weather at or below the range tops or ridge lines
- a temperature-dew point spread of 4 degrees C or less
- any expected precipitation
- operating at night.
All visual navigation charts have maximum elevation figures (MEFs) in each quadrangle. The MEF is determined by locating the highest obstacle (natural or man-made) in each quadrangle, and rounding up to the nearest 100 feet.
Ensure your training has included terrain awareness or mountain flying if considering operating in the mountains. See the Mountain Flying GAP booklet.
In addition, many GPSs include a feature showing the minimum safe altitude, enroute safe altitude, or minimum enroute altitude, relative to the aircraft’s position. If you have access to such equipment, be sure you understand how to access and interpret the information about safe altitudes.
There is a real danger in focussing on the gain of reaching your destination compared with the losses associated with not going, or turning back. For example, extra costs, missed appointments, disappointed passengers, etc. Don’t fall into this trap – look for the gains from the alternative action – being alive and safe with an intact aircraft (with probably very relieved passengers), having avoided the potential major loss (and cost) of bent metal, injuries, or worse.
For this reason, your weather planning should include briefing your passengers (and anyone waiting at your destination).
If you jointly plan for weather contingencies, and brief your passengers before you board the aircraft, you will be less vulnerable to the pressure to continue in deteriorating weather conditions.
Discuss the vagaries of light aircraft trips with your passengers, in particular:
- Departure and arrival times cannot be guaranteed – the weather may have other plans.
- The weight of baggage they can bring is limited – anything over the limit will be left behind.
- Turning back, taking an alternative route, or diverting, is always a possibility.
What your contingency plans will be if you are delayed, diverted or have to cancel.