December 8, 2016

Soft-field landings


Soft-field landings


A couple of months ago I wrote about hard landings and so this month I thought I’d write about soft landings. While the concept of making a soft-field landing is a simple one, the actual process of making the landing as soft as it should be is not always so simple.

Making a soft-field landing requires precise pitch and sideslip control. This is necessarily so since a truly good soft-field landing requires a touchdown at slow speed and must be, as the name implies, soft.

The soft-field landing is generally described as one made from a normal full-flap approach. This is true in most cases, but later I’ll discuss a landing scenario that may be closer to reality when it comes to making the approach to landing. Anyway, let’s proceed with the discussion as if we are approaching the landing from a normal approach.

Normal approach means that we are configured with full-flaps and have stabilized to final approach speed prior to reaching the runway threshold (preferably much sooner than that if your experience level is limited). The landing flare for a soft-field usually proceeds in one of two ways, and it is these two ways that deserve some explanation.

Method one for flaring to touchdown during a soft-field landing is to maintain a small amount of power throughout the flare and touchdown, rather than reducing the power to idle as would be the case for a normal landing.

The extra power during the landing phase allows for the nose to be held off and to facilitate gently settling to the soft ground.

Method two prescribes that the flare should proceed in an identical fashion to a normal landing, but just prior to touchdown, a smidgeon of power be added to cushion touchdown and help maintain a nose high attitude.

Each method has its benefits and drawbacks, so here’s a little on each. The benefit to method one is that the pilot need not precisely judge when to bring in power to reduce settling as is necessary for method two.

Frankly speaking, it’s easier to control settling to the ground with power set slightly above idle. However, according to my observations, method one’s biggest disadvantage is that most pilots use too much power in the flare and float half-way down the runway waiting for the airspeed to properly bleed off.

Any attempt to speed up the touchdown process generally leads to a premature touchdown and we all know that a touchdown with higher than normal airspeed will most likely be flatter than normal.

That kind of landing violates the two cardinal rules of soft-field landings; slow speed, and nose off the ground.

Remember that anything other than an extremely soft touchdown will drag the nose down more readily. And if the attitude is flat in the first place, it won’t take much to get a nose-wheel contact too soon in the landing roll.

Method two is my preferred method, and I see consistently better results from pilots who use this method than for those who do not. The major advantage is that the pilot approaches the landing exactly as he or she would under normal circumstances and flares normally. However, late in the flare just prior to touchdown, a small amount of power is added to cushion the touchdown.

An additional advantage to this method is if a normal idle-power flare is made, the nose will be at the right attitude for the ideal soft-field touchdown. The distance required to land is minimized when using method two since we are not prolonging a float by decreasing the aircraft deceleration rate, as with method one, but rather decelerating the sink rate just prior to touchdown.

The biggest problem with method two is that it requires the timely and precise application of power. In my opinion, familiarity with the aircraft is an absolute requirement if method two is to be employed with consistent success. This might be one reason why I see more pilots using method one when attempting the soft-field landing.

Exactly how much power is required for either method is dependent on the airplane, the weight of that airplane, and the density altitude. In general, I’ve found that no more than 100-200 RPM or 1-2 inches of power are required for either method.

So, that’s the difference between power-use methods for the soft-field landing technique, but there is more to making this kind of landing work than just power application.

Directional (or sideslip) control is also critical. Although a crabbed touchdown on a soft-field is probably not as hard on the aircraft as touching down on a hard-field would be, it will reduce the pilot’s ability to make a successful soft-field touchdown.

Any kind of side-load that is imposed at touchdown will reduce the pilot’s ability to maintain directional control, and also to hold the nose gear off the ground.

The problem that occurs during a side-loaded touchdown is that the aircraft will jerk as the longitudinal axis aligns with the flight path and this jerk makes it difficult to maintain nose attitude.

The solution here is to maintain a proper sideslip to control heading alignment with the runway and eliminate drift off centerline during touchdown. Regardless, it will be increasingly difficult to perform a “textbook” soft-field landing under high crosswind conditions, and I personally recommend not trying the procedure beyond about 2/3 of the demonstrated x-wind component of the airplane.

There is no structural problem with trying the soft-field landing in high crosswind conditions per se, but it will be difficult to maintain directional control after main gear touchdown while keeping the nose wheel off the ground.

The approach: I’ve said before that the approach to a soft-field landing should be normal; so that means three degree glide path with full flaps. However, I’ve seen any number of soft-field landing strips that also qualified as short-fields.

If the landing strip is indeed short, then you need to make a short-field approach to maximize the amount of runway available for the soft-field landing technique. Proper short-field approach technique dictates a steeper than normal approach descent.

Dragging in the final at low altitude, in the absence of obstacles, is not advisable under any circumstances. The drawback of making the proper steep descent is that the precise flare technique for making a soft-field touchdown is now a bit trickier. Practice it before being forced to do it for real!

A few words about touchdown: When making a soft-field landing, the speed at touchdown should be as slow as consistent with safety. This implies the use of full flaps, but the surface conditions may dictate if flaps are used, especially in low-wing airplanes.

Touching down at the slowest possible speed will improve controllability once on the ground. The extra power left in through touchdown helps keep the nose gear airborne during further deceleration, but when the nose can no longer be held off aerodynamically its time to smoothly lower it to the ground.

Braking is generally not necessary on a truly soft field and extra power may needed after landing roll to keep from getting stuck in the muck. In general, do not stop on a soft surface unless absolutely necessary.

If the nose or main gear sink even a little there may be insufficient power to get moving again. On most soft surfaces you can expect the landing distance to be longer than for a hard surface since maximum braking cannot be achieved. Also remember that aerodynamic braking through the use of full follow-through on the elevator or stabilator is of substantial assistance in slowing down.

There are conditions that will probably result in a shorter than normal landing distance but these are generally beyond what most manufacturers specify for “given” conditions, assuming they even provide a soft-field landing performance chart.

The absence of soft-field landing charts for a particular aircraft does not necessarily mean that its not possible. In the absence of performance charts for soft-field operations, unless the manufacture specifically warns against making such landings (or takeoffs for that matter), you can still do it but you are now a test pilot for all intense purposes.

Mastering the soft-field landing makes is possible, and safe, to venture into many airports that you might not have otherwise visited. Some of the neatest aviation sights I’ve seen have been at these small grass fields.

Learn and practice one of the methods I’ve described above so you can feel confident to experience these new places.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.