Modern airliners are capable of handling many weather situations. The airplanes have advancing avionics and systems that allow ever lowering minimums for takeoff and landing.

The rules and regulations governing air carriers has specific requirements for approaches, for landing, for takeoff, and for operations in adverse weather. The factors effecting a go/no-go decision are far different than what the traveling public believes.

Landing and Takeoff Minimums

If the weather looks hairy at our destination, my employer will ‘weather warn’ our passengers. Certain policies then kick in that are spelled out in our contract to carry that passengers agreed to when buying tickets.

Just a couple days ago, we had to weather warn our passengers. We were leaving Los Angeles flying first to Redding, CA and then to Arcata, CA. Redding was nice, Arcata was not. Arcata often has coastal fog hangs around. The forecast said the weather would improve enough for us to get in. But when it comes to fog, especially in Arcata, the forecasts are iffy.

When we arrived at Redding our passengers continuing to Arcata decided to call their contacts in Arcata to get weather reports, and the contact’s opinion on whether the weather was good enough for us to land. The know-it-all passengers determined it was just fine because the people in Arcata, many of whom were several miles from the airport, said we could get in.

What us pilots and our dispatcher were looking at is the fog (at the airport) was intermittently thinning enough for us to attempt the approach. When we made our go decision, the weather had temporarily lifted enough for us to legally make the attempt.

To fly an approach and land we need two things. The reported surface visibility needs to be above minimums required for that specific approach. We can’t even start the approach unless the visibility is good enough. Then, when we fly the approach, we fly to a decision point. If we don’t see the runway by that point we must fly a missed approach.

The approach that gave us the best chance of getting into Arcata required 1/2 mile visibility and our decision point was 200 feet above the ground. The reports over the last few hours had the visibility well below 1/2 mile and the ceiling height (lowest layer of clouds obscuring the sky) was 100 feet. When we made our decision, the weather had actually lifted to 1 mile visibility with the clouds still being called at 100 feet.

We could legally attempt the approach, but we didn’t think the chances were very good. However, fog is tricky. Even when it appears to be constant, the visibility and lowest layer of solid clouds changes almost continuously. We were lucky on this approach. The weather lifted a little bit prior to our attempt. The low cloud layer was not continuous over the approach end of the runway, so we were able to see it and land.

The minimum requirements for an approach vary considerably, but they are always printed on the approach chart. The minimums change according to the type of approach flown (ILS, LOC, SDF, VOR, ADF, RNAV, RNAV/VNAV, LPV, WAAS, RNP). A whole alphabet soup of acronyms. The instrument landing system (ILS) typically offers the lowest minimums. There are multiple levels available for an ILS, each depending on ground facilities, airplane equipment, and pilot qualifications. The lowest ILS allows planes to land with 0/0, not being able to see. Most airlines are not authorized to land 0/0. But many airlines are able to use 600 feet visibility with the decision point 50 feet above touchdown, at approved airports.

The next best for low visibility would be Required Navigation Performance (RNP) systems. RNP is flown like a GPS approach, but is based on computers tying all available navigation sources together for the most accurate navigation signal. I fly for one of the few airlines having airplanes equipped for RNP approaches. Though, under the FAA’s odd system, after we upgraded our computers we actually lost certification to fly RNP approaches. We are working towards getting our blessing back, and are actively training for the RNP approaches in the meantime.

RNAV, RNAV/VNAV, LPV, and WAAS approaches are all variations of GPS approaches that have varying minimums. The minimums for these approaches is generally better than for the other approach types in the alphabet soup above.

For takeoff, a specific visibility is always required. Sometimes a cloud ceiling height is required. But we can usually ignore the ceiling height because we always meet climb performance that usually allows it to be ignored. Sometimes, not meeting the climb performance means raising the required visibility too. Our takeoff minimums do vary a great deal. At my airline we are able to go down to 300 feet visibility at some airports.

Getting Off the Ground

We cannot legally fly to a destination unless, through a combination of weather reports and forecasts, we can deduce the weather will be at or above minimums to start the approach. Some airlines do have weird allowances in their FAA approved operation specifications that allows them to go anyway. But they have other rules then governing if they do so. Usually, those rules involve going to the intended destination; but actually filing and planning for another airport with weather above minimums. It just so happens they are flying over the original destination. While over it, they will check the weather and divert if able. Screwy, huh?

Sometimes, the minimum weather to takeoff from an airport is lower than what is allowed by instrument approaches to get back into that airport. So, an airplane might be able to takeoff without the option of returning. The FAA does allow this, but a takeoff alternate is required. The takeoff alternate must be within an hour flight time, with an engine failed. The airplane has to have enough fuel to fly to that alternate and then have required reserves when they get there.

There is also a requirement for destination alternates. If the destination weather is forecast to be below a set minimum which is above the weather required to fly an approach, then the flight will need to have an alternate filed in its paperwork. The actual weather at the destination that would require an alternate varies between airlines. But it is usually close to the part 91 requirements of looking at the weather between one hour before arrival to one hour after arrival. If the forecasts say the cloud ceiling would be less than 2,000 feet and the visibility less than 3 statute miles then the alternate is required.

When choosing an airport as an alternate, the weather there must also be factored into the decision and flight planning. There is minimum weather requirements to use an airport as an alternate too. After figuring out all the alternate requirements, the dispatcher must then determine required fuel. The minimum for domestic operations says the airplane must fly to the destination, then to the most distant alternate, then after that for another 45 minutes.

I threw out the odd term, most distant alternate, in the preceding paragraph. In some cases, a second alternate airport will also be required.

Usually, when dispatchers fuel plan they will use the regulations and their experience. For instance, when the weather is icky traffic get backed up and air traffic control starts messing with airplanes. Airplanes will then burn through more fuel in holding patterns, delaying vectors, and not-planned altitudes. Or if thunderstorms are present airplanes will need to fly off their planned routing to stay safe. The dispatchers will put additional fuel requirements for this stuff. Even if they don’t, most captains will also use their experience and throw on extra fuel.

Hazardous Weather

There is some weather we always try to avoid. Thunderstorms are a bad idea. As is strong wind shear during takeoff and landing. Prolonged moderate or severe icing also makes pilots nervous.

Thunderstorms are bad for airplanes for many reasons. Hail damages airplanes. Lightening strikes can screw things up. Turbulence can be of the extreme variety in the big storms, which breaks airplanes. And airplanes can accumulate lots of ice during even a short time in a thunderstorm.

Airliners do have good ways of avoiding storms. All airliners operate in the Air Traffic Control system. Controllers actually have weather radar as part of the information they have available. Workload permitting, controllers will vector airplanes around the storms they see. Airliners also have onboard weather radars. Pilots will use these to see and avoid storms. During the day, we can visually see thunderstorms if they are not embedded in other clouds.

My airline is slowly equipping our airplanes with Electronic Flight Bags. A nifty feature of these is maps with ground-based Doppler radar overlays. This information complements the limitations of the onboard radar. We aren’t fully able to use this functionality just yet though as we don’t have a pictorial display of our plane location on the EFB map. But we are told it is coming; eventually.

Wind shear during takeoff and landing is dangerous for airplanes. Wind shear makes our airspeed erratic. It can instantly decrease our airspeed to an unsafe point. If the wind shear is too bad, we will just wait until it gets better. With lesser wind shear reports, we will go; but will use higher airspeeds that give us better safety margins.

Microbursts are short lived events having really bad wind shear that is also accompanied by downdrafts. Microbursts will often exceed the performance capabilities of airliners. If they are present, we will not attempt a landing or takeoff.

Icing is our other major concern. Airframe icing in-flight adds weight to the airplane. But more insidiously, it also changes the shape of the airfoil; adding drag and reducing lift. In-flight icing can accumulate in visible moisture (precipitation and clouds) when the temperature is below freezing on the airplane surface, usually corresponding to the ambient air being below freezing. In-flight icing is unpredictable and isn’t present much of the time in the mentioned conditions.

Airliners have ice protection systems that remove accumulated ice and can prevent further ice from accumulating. The ice protection equipment typically covers the front edge of the wing, the horizontal stabilizer, and the vertical stabilizer. It also covers engine inlets and engine intakes. Certain probes are protected as are the flight deck windows.

Pilots cannot use the ice protection equipment to loiter in icing conditions. Rather, pilots use the ice protection equipment as a means of getting out of the ice. A major plus to flying airliners is they typically cruise at altitudes where icing doesn’t occur, except during rare circumstances. So, most turbojet airliners are only in icing conditions during brief moments during climb and descent/approach. Turboprop cruise altitudes often stick them in prolonged icing conditions. But the pilots of these airplanes do learn tricks to stay safe.

Ground De-Icing

The ice protection equipment covers the leading edge of surfaces, but not the tops. When icing conditions are present on the ground, the upper surfaces get contaminated and must be cleaned off and protected for takeoff. Ground icing occurs from precipitation, freezing fog, or frost.

We use a two step process with ground de-icing. First, we clean off the contamination. Second, we put on a protective coating that keeps the airplane clear until takeoff. During light icing conditions, this can often be done with the same fluid application, as one step. If ground icing conditions are actively occurring, we will have a holdover time. The holdover time says how long our current protective fluid application keep our airplane clear. It is dependant on type of icing conditions as well as outside air temperature. We will also check designated surfaces just before takeoff to make sure the fluid is holding.

There are two types of fluid used. Type 1 fluid is red/pinkish in color. It is mixed with hot water and is what we use to get the existing ice cleared from the airplane. Type 1 fluid also provides protection until takeoff. However, its holdover times aren’t very long. Type 4 fluid is green in color. It cannot remove existing ice. However, it has much longer holdover times. If we need type 4 we will first get the airplane clean with type 1.

When a large winter storm cripples airline operations; it is for a couple reasons. One, snow reduces visibility. It will often lower the weather well below required minimums for takeoff or landing. Two, the current air traffic control system is antiquated. It can only handle existing traffic when major storms aren’t around. When major storms come, airplane controlling gets bogged down. And three, airlines staff and provide de-ice equipment for average conditions. When worse than average storms come, airlines do not have the capacity to get all their planes de-iced in a timely manner.

Next time your flight is delayed from weather, please realize there are many factors effecting the decision to screw with your schedule. Please realize your friend at your destination does not have appropriate knowledge or information to comment on the go/no-go decision.