1. Weather: The Interrelated Web of Air
The ATIS is a snapshot, but a pilot’s go/no-go decision requires connecting that snapshot to the broader weather picture.
- Temperature & Density Altitude: This is a crucial relationship. An ATIS reporting a high temperature doesn’t just mean a hot day. It means a high density altitude, which directly and significantly degrades aircraft performance.
- Concern: The pilot must assess if the runway length is sufficient for the increased takeoff roll, and if the climb gradient will clear obstacles at the end of the runway. A “good” runway on a standard day can become dangerously short on a hot day.
- Thumb Rule: Takeoff distance can increase by 10% for every 1,000 feet of density altitude. Rate of climb can decrease by a similar or even larger percentage.
- Compounding Conditions: The effects of high density altitude are compounded by a fully-loaded aircraft, an engine that is derated due to age or wear (which can reduce power output by 15% or more), and high humidity. High humidity adds water vapor to the air, which is less dense than the nitrogen and oxygen it displaces, effectively increasing density altitude. Further compounding this is a dirty air filter, which restricts airflow to the engine, reducing power. The pilot must also consider the friction of a soft-field or wet runway, which increases takeoff roll. All of these factors multiply the performance degradation, potentially turning a legally permissible takeoff into a high-risk event.
- Wind Speed & Direction: This information from the ATIS must be paired with the runway’s orientation.
- Concern: The primary concern is the crosswind component, which impacts a pilot’s ability to maintain control during takeoff and landing. The ATIS may report a wind speed that seems low, but if it’s coming from 90 degrees to the runway, the crosswind component is 100% of that speed.
- Thumb Rule: The “clock face” rule is a quick way to estimate crosswind. For every 15 degrees the wind is off the runway heading, the crosswind component is roughly 25% of the total wind speed.
- Crosswind = sin(degrees between wind and runway)
- 2 1/2 min is 15 degrees and crosswind is about 25% of total wind reported
- 5 min is 30 degrees and crosswind is about 50%
- 7 1/2 min is 45 degrees is about 75%
- 10 min is 60 degrees or more and is 100%
- Compounding Conditions: A moderate crosswind becomes much more difficult to handle with a pilot who has low recency of experience, especially in a gusty wind condition. The combination of a high crosswind component, gusts that exceed the steady wind by more than 10 knots, and a wet or contaminated runway dramatically increases the risk of a runway excursion. This is further compounded by a tired or distracted pilot, or one with limited proficiency in the specific make and model of aircraft being flown.
- Many pilots have a personal minimum crosswind limit well below the aircraft’s POH limit.
- Temperature-Dew Point Spread: This relationship is a key indicator of the risk of fog.
- Concern: If the ATIS reports a small spread (e.g., less than 5° F or 2° C) and calm winds, it indicates that the air is nearing saturation. Any cooling could lead to the formation of fog or low-lying clouds, which can severely reduce visibility and ceiling. This is particularly important for night flights or flights into airports near bodies of water.
- Thumb Rule: A pilot might set a rule like, “No flights into this airport at night if the temperature-dew point spread is less than 3 ° Celcius.”
- Standard Lapse Rate: Temperature decreases about 2 ° Celcius per 1000 feet.
- Cloud Base (in feet AGL): ([Temperature (°C)−Dew Point (°C)] / 2.5 ) × 1000
- Freezing Level: A rough estimate for the freezing level is the altitude where the temperature drops to 0°C. This is crucial for anticipating icing conditions.
- Compounding Conditions: A small temperature-dew point spread is most dangerous when combined with a low ceiling and a non-instrument rated pilot. In such conditions, a simple VFR flight could quickly become an inadvertent IMC (Instrument Meteorological Conditions) situation, where the pilot loses visual reference with the ground. This risk is amplified if the pilot is fatigued, the terrain is unfamiliar, or the flight is at night, when the visual cues needed to maintain situational awareness are already limited.
2. Aircraft: Performance and Condition
A pilot must assess how the aircraft’s capabilities intersect with the environmental conditions.
- Payload & Performance: The pilot must consider the combined weight of passengers and baggage relative to the aircraft’s maximum gross weight. This directly relates to the density altitude concern.
- Concern: A fully-loaded aircraft will require more runway and have a lower climb rate, making it more susceptible to the effects of high density altitude. A pilot might be comfortable with a short runway in ideal conditions, but the added weight and density altitude could make the go decision unsafe.
- Relationship: A 10% increase in aircraft weight can lead to a 20% or more increase in takeoff and landing distance.
- Takeoff Distance vs. Landing Distance: A rule of thumb is that landing requires about half the runway distance of takeoff. However, this varies by aircraft type and conditions.
- Headwind/Tailwind: A headwind increases climb performance and reduces takeoff/landing roll. A tailwind does the opposite, and counters effectiveness of the rudder, often requiring a “no-go” decision.
- During a crosswind landing with a tailwind component, the wind acting on the vertical stabilizer (the tail) increases pressure on the tail the further off centerline the tail is pushed. Also the tailwind on the horizontal stabilizer pushes the tail of the airplane downward, causing the nose to want to rise further and turn into the wind. This is the opposite of the stabilizing effect you’d want on final approach and touchdown. Potentially forcing a ground loop even in tricycle gear aircraft, and promoting porpoising as a pilot attempts to regain control.
- While taking off with a tail wind may be required to avoid high obstacles at the end of a runway; a transition of a few seconds in relation to relative wind and the high power propellor wash providing control surface influence. Landing with a tail wind requires considerably more skill; and is generally avoided.
- Compounding Conditions: High payload, high density altitude, and a short runway can create a critical safety hazard. This is exacerbated by a derated engine, a dirty air filter, and moisture on the wings, which disrupts the smooth flow of air and reduces lift. A pilot can mitigate some of this by taking off with less fuel and planning to refuel at a more suitable airport (e.g., one at a lower elevation with a longer runway) to reduce takeoff weight. However, this strategy is only effective if there are no other compounding factors that make the flight to the alternate airport risky.
- Engine & POH Limits: The pilot must ensure the engine is operating within safe parameters for the conditions.
- Concern: A pilot might find that due to high temperature or high density altitude, the engine’s oil temperature or cylinder head temperature (CHT) is nearing the top of the green arc during a pre-takeoff check. While legally within limits, this could be a personal “no-go” item to prevent engine stress.
- Coversely, delay takeoff until the engine has warmed up to POH recommendations.
- Go/No-Go: “If the CHT exceeds [specific number] on the ground, I’m not flying.”
- Compounding Conditions: An older engine with over one thousand hours on it may not produce the power listed in the POH, especially on a hot day. This is compounded by high density altitude and a heavy aircraft, leading to significantly degraded performance that may not be accounted for by the POH numbers. This is further worsened if the engine has minor mechanical issues like a dirty carburator, a dirty air filter, or is running on a fuel grade that is not optimal for the conditions. Fouled plugs from running too rich. Engine valve damage from running the engine too lean. The propellor if set for cruise will reduce thrust for takeoff. A dirty or worn propellor. A leaking engine head gasket and worn cylinders. All these factors combine to reduce the engine’s power output, making a marginal takeoff or climb potentially dangerous.
- Concern: A pilot might find that due to high temperature or high density altitude, the engine’s oil temperature or cylinder head temperature (CHT) is nearing the top of the green arc during a pre-takeoff check. While legally within limits, this could be a personal “no-go” item to prevent engine stress.
3. Pilot: Skills and External Pressures
A pilot’s own abilities and state of mind are as important as the weather and aircraft.
- Pilot Fatigue: Mental incapacity makes flying safely impossible.
- Concern: Tired pilot control movements are often more twitchy, less precise, and uncoordinated. Causing over control, pilot induced oscillations, stall incursions, ground loops. The tired pilot is responsible for missed communications, missed use of checklists, missed visual references, radio babble, responding contrary to ATC instructions …
- Recency of Experience: The legal minimums are often insufficient to maintain proficiency.
- Concern: A pilot who is legally current for a cross-country flight may not have flown in actual crosswind or turbulent conditions for a long time. The reported winds on the ATIS might be legally flyable, but the pilot’s lack of recent practice makes it a personal “no-go.”
- Relationship: Combine the ATIS report with your own flight log. “I haven’t practiced a full-flap landing in a 12-knot crosswind in over six months; therefore, I will not attempt this flight today.”
- Compounding Conditions: A pilot with low proficiency is far more susceptible to the negative effects of crosswinds, gusty conditions, and a short, wet runway. This risk is compounded when coupled with fatigue, as reaction times and decision-making abilities are degraded. Other factors like being late for a meeting, flying with inexperienced passengers, or trying to impress others add to the risk by encouraging the pilot to push their personal minimums.
- External Pressure: This is a subtle but potent factor.
- Concern: The desire to get to a family event, avoid hotel costs, or impress a passenger can override a pilot’s judgment. The ATIS report might be marginal, but the pilot feels pressured to go.
- Go/No-Go: This is a personal decision to be made before leaving the house. The pilot must have a mental “abort plan” to avoid caving to pressure. “If the ATIS reports a crosswind over 10 knots, I will drive instead of fly.”
- Compounding Conditions: External pressure is most dangerous when combined with a pilot who has low recency of experience or who is fatigued. The combination of these factors can lead a pilot to push their personal minimums and make a go decision in marginal weather or with a known aircraft discrepancy. This is particularly insidious because it can cause a pilot to rationalize away the risks from a wet runway, a derated engine, or a high-density altitude.
4. Logistics and Route
The pilot must also assess the entire flight profile, not just the departure airport.
- Alternate Airports & En Route Weather: The ATIS is just for the departure airport.
- Concern: What are the conditions at the destination? What about the alternate airport? Is the en route weather deteriorating? A pilot might have a personal rule to not fly if the weather at the destination is forecast to be below their personal minimums.
- Relationship: The ATIS at the destination airport (or the TAF, Terminal Aerodrome Forecast) must be assessed. A “go” from the departure airport could turn into a “no-go” if the destination is reporting low ceilings and visibility close to personal minimums.
- Compounding Conditions: The risk of flying to a destination with marginal weather is compounded if there are no suitable alternates within a reasonable distance or if the terrain en route is not suitable for an emergency landing. This is especially true at night, when off-field landings are far riskier. The risk is further heightened by the limited fuel carried to reduce takeoff weight, as it reduces the pilot’s ability to divert and find a more suitable airport.
- Time of Day: Light conditions are a critical factor.
- Concern: The pilot must consider the complexity of night flying and how it combines with other factors. A 20-knot crosswind in the daytime might be manageable, but at an unfamiliar airport at night, it could be a personal “no-go.”
- Relationship: A pilot’s personal minimums for crosswind, runway length, and visibility should be higher for night flying than for daytime flying.
- Compounding Conditions: The risks of night flying are compounded when combined with a non-familiar airport, a short or unlit runway, and a forecast for a small temperature-dew point spread. This combination can lead to a dangerous situation where the pilot is unable to see the runway or a suitable landing spot due to fog or low visibility. It is also worsened by the presence of moisture on the wings, which can be difficult to detect at night and can compromise lift.