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VFR flight into IMC, NASA

IMC for VFR Pilots

When VFR only rated pilots continue flight into instrument meteorological conditions the usual results are spatial disorientation of the pilot and loss of control of the aircraft from which recovery is almost impossible. This is a serious hazard for pilots without a current instrument rating.

Non instrument rated pilots have only a few hours of training under the hood in their logbooks, usually enough to qualify for their PPL or CPL license but in most cases that experience was obtained was years ago. Relying on ancient experience can be deadly within minutes of entering IMC conditions.

Low ceilings and reduced visibilities kill more pilots than thunderstorms, icing and turbulence (source: AOPA). This is due to the low experience level in IMC by VFR only pilots. Without visual clues in IMC, these pilots begin to rely on body motion and gravity sensors. The result is a confusing struggle between believing the instruments and what the body is telling the pilot (seat of the pants). With a deadly outcome, usually.

The EASA also conducted a research into weather related accidents: "Aircraft operations have always been influenced by the weather. Whilst modern aircraft design and the availability of weather observations and forecasts contribute to a predominantly very safe flying environment, there remain occasions where severe weather events have been identified as being a contributing factor in the causal chain of accidents and incidents." Continue reading in the document: Weather Information to Pilots Strategy Paper.



Loss of Control Factors

When obtaining a weather report pay attention to cloud bases, temperature, wind and relative humidity (or dew point) this will give you an idea where visibility might be reduced. Modern applications like ForeFlight and Garmin Pilot provide these clues in their weather section.

Winter

Flying in IMC during the winter season or above freezing levels could mean aircraft icing. You will need an aircraft with de- or anti-icing equipment for known icing conditions. Wing profiles (lift) will be disturbed by ice and stall speeds and aircraft weight will increase. A recipe for disaster.

Limits

VFR flight is usually allowed with visibilities down to one statute mile (1,5 km). But you as the PIC has to decide if its a wise decision to go on a cross country in these conditions especially if low ceilings are forecast. When a weather briefer or information indicates that VFR flights are not recommended we can always take a look and file a PIREP (Pilot Report) when conditions are better or worse. But pressing on in adverse conditions is just plain stupid.

See the next table for a reminder of visibilities and cloud ceilings.

Flight CategoryColorCeiling (AGL)Visibility
Visual Flight RulesVFRmore than 3000 feetmore than 5 sm
Marginal Visual Flight RulesMVFR1000 to 3000 feet3 sm to 5 sm
Instrument Flight RulesIFR500 to 1000 feet1 sm to 3 sm
Low Instrument Flight RulesLIFRbelow 500 feetless than 1 sm

Horizon

With reduced visibility and flying near large bodies of water the natural horizon can almost become indiscernible. Continuing flight in such conditions maybe legally VFR but without good outside references VFR pilots will have difficulty maintaining correct aircraft attitude, altitude and speed. The flight has literally become IFR in VMC.

Equipment

Most aircraft have the familiar basic six instruments or a sophisticated EFIS needed for flying IFR. It is imperative to understand that VFR pilots flying such equipped aircraft use more external visual cues than they might realize. Because of that they may over estimate their skills in using these instruments after flying for many hours in VMC conditions. The point is that they need more training with a qualified instructor in real IMC to be proficient to use these instruments under most if not all circumstances.

Instrument rating

Avoid flying in (near) IMC conditions if you are not instrument rated and current. Divert to the nearest airport in VMC and remain clear from clouds should you encounter these circumstances.

Homebuilt aircraft

Most countries do not allow homebuilt experimental aircraft to fly IFR or under IMC conditions unless strict rules were obeyed and inspections performed when these aircraft were under construction.

Flight into IMC?

Should you inadvertently find yourself in almost IMC conditions then keeping your aircraft under control is very important. For this you will need to trim the aircraft straight and level at its cruise speed and power. This makes sure that you can control the aircraft with minimum effort (fingertips) and it will remain stable in pitch, roll and yaw.

Aircraft control

Resist over controlling the aircraft and make all movements positively smooth, small and slow. If the aircraft has an autopilot then use its wing leveler or heading function, so you can concentrate on navigation and communication.

Auto Pilot

Control altitude with power changes and use no more pitch change than one bar on the attitude indicator. Any turns should be shallow, 10 degrees bank or less, so that altitude loss is minimized.

Try not to combine maneuvers: descend or climb first then turn the aircraft. The pressure the pilot is under is more than stress enough even without complex maneuvers which would raise the risk factor enormously.

Remember: Aviate, Navigate and Communicate. Easy, first of all: keep the aircraft flying in the direction you need to go and then talk to ATC when you need assistance in these conditions.

Written by EAI.




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