Flying In A Crab

I recently received an email from a friend; this email contained a link to a compilation YouTube video of passenger jets making hair-raising landings in horrifying crosswinds in Birmingham, England.

Here’s the video. It’ll probably scare you, too.

It brought back to me many memories of my own flying days, including the most pucker-inducing phase of flight – landing in a gusting crosswind. What follows is a bit of explanation of why this is such a tense situation.

As I watched that video, I saw several aircraft touch down in a crab. No, not the arthropod crustacean. In aviation, a crab is when there is a crosswind component to the aircraft’s direction of flight, such that the plane has to fly at an angle to its ground track so as to maintain the intended path across the ground.

During a landing, for example, if the wind is blowing from 90° to the right of the runway heading, a plane pointed exactly down the runway would be blown off to the left by the wind. Therefore, to maintain a ground track down the runway, the plane has to fly a heading several degrees into the wind.

Touching down in a crab is not really good for the aircraft tires or landing gear, as they are engineered to support the aircraft rolling straight ahead. In a strong crosswind, the crab can be as much as 25° off the aircraft’s ground track. The worse the crab, the greater the landing strain on the tires and landing gear.

To prevent touching down in a crab, the procedure is typically to fly in a crab until just before touching down. When you reach the flare – the point where you pull the nose up a bit to cushion the landing – you then step on the downwind rudder pedal to align the aircraft with the runway, while you simultaneously lower the upwind wing so as to keep from drifting to the downwind side of the runway.

This results in a high-drag cross-control situation, in which the aircraft is flying in a different direction than it should. The bank (wing tilt) of the plane tells it to go one way, but the rudder tells it to go the other. (This is also called a “slip.”)

This cross-control condition destroys some of the wings’ lift, which is why it’s not usually done until you’re ready to touch down.

When the winds are strong and gusty, you have to constantly adjust the crab to keep the plane on the glide path, and the cross control inputs before touchdown get very dicey. This is a lot of what I saw in the video above.

CRABBING IN LARGE AIRCRAFT

Some aircraft have such long wings they cannot do the cross control maneuver at all, for if they did, one of the wings would strike the ground before the wheels did, and likely cause the aircraft to cartwheel and crash. This would ruin everyone’s day.

To get around this, the B-52, which has four main landing gear trucks, two forward and two aft, is able to rotate its landing gear trucks to accommodate for the crab, such that even though the aircraft itself may have a 15° crab into the wind relative to runway heading, the landing gear itself is pointed directly down the runway.

A Boeing B-52 With Landing Gear Adjusted For Crab
A Boeing B-52 With Landing Gear Adjusted For Crab

This makes for a much easier landing.

CRABS IN SUPERSONIC AIRCRAFT

The Northrup T-38 Talon
The Northrup T-38 Talon

Other aircraft, particularly the T-38 in my experience, cannot fly cross controls at low airspeeds, such as at landing time, because doing so will cause the aircraft to do a snap roll. This could make you end up flying inverted only a few feet above the runway. Not a happy situation to be in.

THE T-38 CRAB “INCIDENT”

This very thing happened in a pilot training class one or two classes ahead of mine in Laredo, Texas in 1970. I got the report from the RSU (Runway Supervisory Unit) student officer who saw it happen.

The RSU is a small, trailer-mounted windowed control center, where pilots monitor each aircraft in the landing pattern. At a pilot training base, almost all the planes in the pattern have student pilots, and the RSU officers verify that each landing aircraft’s landing gear is down and the aircraft is on a safe glide path. The RSU has a radio, and RSU officers can make calls like “Vixen 34, gear.” or “Fog 88, go around!” for an unsafe approach. They even have a flare gun for radio-out situations.

During this incident, the RSU officer said a T-38 was descending through about 25 feet altitude. when in less than the blink of an eye it was upside down. The student pilot had input cross controls and snap-rolled the aircraft.

In the T-38, the interphone between student and instructor is always live. The radio transmit button is under the left thumb on the throttles and must be depressed to say anything over the radio. In a tense situation it’s easy to press that button when you need to make a rapid throttle adjustment.

Before the RSU officer could say anything to the inverted bird, he heard over the radio, “I’VE GOT IT!” – obviously the IP (instructor pilot), as twin cones of blue flame erupted from the afterburners of the plane. The plane climbed out, inverted, to halfway down the runway, then turned back upright.

Possible disaster averted. Whew. And a good illustration of how while flying, things can go from normal to disaster in the blink of an eye.

Anyway, because the T-38 could not apply cross controls in a crosswind, the procedure was actually to touch down in a crab – much like the unintentional crab touchdowns in the above mentioned video – then step on the rudder pedal to bring the nose in line with the runway immediately on touchdown. The aircraft was so lightweight (12,000 pounds?) and the tires were so skinny, this wasn’t much of a problem, and the landing gear was designed to take that stress.

PRACTICING CROSSWIND (CRAB) LANDINGS IN LIGHT AIRCRAFT

Landing in crosswinds is not something anyone does naturally. It’s definitely an acquired skill, and like any other skill, the more you do of it, the better you get at it.

However, it’s not something you can just go out and practice, because you need some good, strong crosswinds to practice it, and the winds don’t usually blow the way you want them to.

When I was developing my light aircraft handling abilities – usually in a Cessna 152 or 172, I wanted to make the most of any crosswind days I could find. So when I got a flying day with some good crosswinds, I would fly an approach to landing, then just before touchdown I’d add some power and try to hold the aircraft one foot off the runway, flying with cross-controls.

It’s not easy, and several times I touched down, which I was trying not to do, but it was okay, since I was in landing attitude anyway. I would fly most of the way down the runway this way, then add power and climb back into the traffic pattern, come around and do it again. I vividly remember doing this at the small Pompano Beach (FL) airport.

You can only do this at an airport that’s not very busy, and if it’s an airport that has a control tower, they’ll be greatly amused at your antics. But it works. It was the best way I ever found to learn to fly an aircraft in a cross-control situation, and ever since I did that, I’ve been much better at crosswind landings.

Some day, maybe I’ll get to practice them again.

Leave a Reply

Your email address will not be published. Required fields are marked *