Yet another X-wind question--Old Smokey??

[galaxy flyer]

Does anyone have a formula (suitable for basic math) to determine the maximum crosswind that can be corrected for a given bank angle in a "side slip" (i.e. wing low only) landing technique? I know in the C-5, Lockheed stated that at the flight manual-derived maximum crosswind limit, holding wing low alone at the geometric limits of the airframe, drift could not be stopped. In other words, a combination of wing low and de-crabbing or just de-crabbing was required to land.

I ask this because my present plane has, fleet-wide, had some scraps and I contend that the geometric limits may require de-crabbing (which is taught) to land in strong crosswinds.

GF

[bookworm]

Don't think there's a formula that's as simple as you'd like.

The relationship between the bank angle and the sideslip angle will depend on the coeffiicents of directional and lateral stability of the aircraft, i.e. an aircraft with high directional stability and will require much more bank than one with low directional stability. You might find a rule of thumb based on assumption about the ratio, but there's nothing simply geometrical.

[Contract Con]

Gday GF,

Boeing state recommended x-wind limits for sideslip only landings in the B73.

eg. 20kts for F30 and 23kts for F40.

De-crab technique they give 40kts as the max.

Maybe you can approach your types manufacturer for guidance, or get everyone into using their big hoof on the rudder pedal at the appropriate moment.

Cheers,

Con:ok:

[galaxy flyer]

Thanks Contract Con, I have one other question or comment. It has been mentioned to me that Boeing builds "free play" into the landing gear to compensate for landing in a crab due to crosswinds. Is this true? I know about castering in the 74 and 77, but I did not think Boeing built a crosswind facility into the gear. I would like to know about this free play.

GF

[Contract Con]

Gday GF,

I can't answer that one definatively unfortunately. Like you I am aware of the movement of the gear in the 74 and 77, however none of the current Boeing Systems manuals I have describe it. Maybe someone with a maint. manual can answer??

I have noticed that even a 73 will taxi a little crossed up into wind, so I believe that there must be a some play in the gear assembly in that axis, intentional for X-wind purposes or not.

Anyone??

GF, I imagine that the C-5 is't overly comfortable with too much crab angle at touchdown?

Cheers,

Con:ok:

[BOAC]

Contract Con - beware! I have different figures from the Boeing 737 Training Manual!

For both 'Classic' and NG, (Pub date Oct 2002), 16 kts and 18 kts respectively (2 kts less for winglets)

[Contract Con]

Gday BOAC,

Thanks, but the manual I have here, Oct 31 2005, has the above figures.

And you are correct, reduce by 2kts for winglets.

Doesn't bother me too much as I like to kick it straight in the flare:eek:

Cheers,

Con:ok:

[BOAC]

I'm obviously out-of-date! In the kick club:ok:

[Brian Abraham]

I have noticed that even a 73 will taxi a little crossed up into wind, so I believe that there must be a some play in the gear assembly in that axis, intentional for X-wind purposes or not.
From b737.org.uk

Unfortunately the 737-700 was particularly prone to a dramatic shudder from the main landing gear if you tried to land smoothly. Fortunately Boeing started fitting shimmy dampers to this series from L/N 406 (Nov 1999) and a retrofit was made available.

One of the peculiarities of the 737 is that it invariably appears to crab when taxying. Theories for this include: A slightly castoring main gear to increase the crosswind capability; Play in the scissor link pins; Weather-cocking into any crosswind impinging on the fin; Torque reaction from the anti-collision light !!! Engineers will tell you that is due to the main gear having a couple of degrees of play due to the shimmy dampers.

[galaxy flyer]

With regards to the 737, I find it hard to believe a few degrees of castor would markedly improve crosswind capability.

CC: Yes, an otherwise smooth landing would be a real rattler, if touchdown was crabbed. But much worse, if drifting any. An afternoon teaching LTs how to handle a near limiting crosswind was character-building. We had a normal zone, which was restrictive for wet and touch-goes and caution zone for dry full-stops. Dry the limit was about 32.5 knots direct, wet was about 25.5. It decreased some at lighter weights. Was the determinant in the limit, I know not. In the sim, decrabbing I could do the low 40s, but trying. Engineers kept us honest in the plane, but I was close on several times in Lajes and Mildenhall.

GF

[Contract Con]

All sounds like good clean fun GF!:}

Cheers,

Con

[Old Smokey]

galaxy flyer,

Thank you for mentioning my name in the original post, but I have to agree with previous posters in saying that there's no generic formula for all operations, it is entirely dependant upon the airplane type and it's own individual characteristics of control limitations and geometry.

On a secondary point, I am a strong advocate of the 'yaw during flare with appropriate roll control to keep wings level' technique. In gusty crosswind conditions, a little wing down subjectively applied in combination with the 'yaw' technique nicely compensates for transient variations.

When I was training on the B747, I would ask trainees "Why did Boeing equip the aircraft with 4 sets of main gear?". My answer was "Because it needs them, don't put all of the landing load on one set of gear when the aircraft needs 4".

Of course, on the B747, a pod scrape was a distinct possibility, now, on the B777, you can just about land it any damned way you like!

Regards,

Old Smokey

[Brian Abraham]

This not you practicing Old Smokey? :p
http://video.google.com/videoplay?docid=-2498234148335857479
Amazing that they can with stand this sort of treatment. Wouldn't want to try it on the line though, talk about spilling the drinks.

[Old Smokey]

Not Guilty Brian! Agreed, it is amazing that aircraft can take this type of treatment, but, truthfully, I do know of a number of EXPERIENCED pilots who do practice landing with no correction for drift whatsoever, either the 'wing down' or the 'de-crab' technique. I've observed it from the jump seat, and it feels horrendous. Every component of the aircraft seems to shudder in revolt.

Interestingly, the Auto-Land (at least on the B777) seems to use a mix of both of the "good" techniques.

Of course the aircraft can take it, but there is such a thing as cumulative stress!:eek:

Regards,

Old Smokey

[galaxy flyer]

Old Smokey:

Your welcome, I have found your posts to be informative and sensible. I reviewed the videos and two thoughts:

One, it is incredible what abuse a Boeing will take.

Two, in every case, the downwind wing dipped down to some extent as the plane straightened. In my experience, it always happens, but much worse if the pilot allows any downwind drift to occur before touchdown. Better to land with a bit of crab, but not any drift. I taught-"squeeze out the crab during the flare and think about some aileron into the wind." "Squeeze" entered my speech after one Lt applied "kick" as in "kick out the crab", an exciting view out the windscreen and a go-around from 20 feet. If you think aileron, you will put enough to keep the plane laterally level as the plane touches down.

Back to my original question-does Boeing build some facility into there airplanes that makes them especially capable of landing in a crab? I don't think so and, short of a full-blown crosswind gear (NO, please no, I have tried it and, believe me, you don't need it), I cannot imagine a gear designed to assist in crabbed landings.

One of the solutions to our wingtip scrapes is to increase approach speeds, which seems to me to only increase the possibility of a scrap due to floating, followed by some rapid control movements to stop drift and overcontrolling the bank. While some think 6 degrees is a lot a bank, I don't think so. Anything
Again, thanks for the answers everyone.

GF

[Old Smokey]

galaxy flyer,

I stand ready for correction on this point - flak jacket ON, but.....

Boeing (and others including Airbus) do build the aircraft to be capable of landing without any drift correction whatsoever.

Having said that, they then describe various recommended techniques for crosswind landings which are much less stressful for the airframe.

My own technique, as I described earlier, is to hold the drift to the flare point, and then simultaneously, flare, straighten the aircraft with the runway centre-line with rudder, and keep the wings level with roll control as required. The problem is in the timing, if one flares a little too soon, drift off the centre-line occurs, a little wing down will correct this, although undesirable.

If I recall my "best" landings, they've all been in significant crosswind conditions. I figure that this is because a lot more effort is put into the manoeuvre, as opposed to the non-crosswind Flare, Chop, Yawn, Hope for nice contact.

In terms of airframe stress, in order of least to most stressful to the airframe -

(1) De-crab during flare with rudder, keep wings level, and land 'straight' with wings level, putting the landing forces equally on all main gears and minimal side load,:ok:

(2) Use the 'one wing down' technique, putting all of the landing forces on one set of gear, but still with only a fairly small side load,:)

(3) Land with no drift correction whatsoever, which imposes maximum side load, but at least, the landing forces are equally absorbed by all of the main gear. The aircraft are built to withstand this, but the manufacturers don't recommend it. (One Airbus instructor at Toulouse was famous for recommending it). Don't forget cumulative stress!!!!:eek:

Regards,

Old Smokey

[Brian Abraham]

For those interested in trivia, or how the designers try to look after us. From FAR 25
25.473 Landing load conditions and assumptions.
(a) For the landing conditions specified in §25.479 to §25.485 the airplane is assumed to contact the ground—
(1) In the attitudes defined in §25.479 and §25.481;
(2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and
(3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).
(4) The prescribed descent velocities may be modified if it is shown that the airplane has design features that make it impossible to develop these velocities.
(b) Airplane lift, not exceeding airplane weight, may be assumed unless the presence of systems or procedures significantly affects the lift.
(c) The method of analysis of airplane and landing gear loads must take into account at least the following elements:
(1) Landing gear dynamic characteristics.
(2) Spin-up and springback.
(3) Rigid body response.
(4) Structural dynamic response of the airframe, if significant.
(d) The landing gear dynamic characteristics must be validated by tests as defined in §25.723(a).
(e) The coefficient of friction between the tires and the ground may be established by considering the effects of skidding velocity and tire pressure. However, this coefficient of friction need not be more than 0.8.
25.479 Level landing conditions.
(a) In the level attitude, the airplane is assumed to contact the ground at forward velocity components, ranging from VL1 to 1.25 VL2 parallel to the ground under the conditions prescribed in §25.473 with—
(1) VL1 equal to VS0 (TAS) at the appropriate landing weight and in standard sea level conditions; and
(2) VL2 equal to VS0 (TAS) at the appropriate landing weight and altitudes in a hot day temperature of 41 degrees F. above standard.
(3) The effects of increased contact speed must be investigated if approval of downwind landings exceeding 10 knots is requested.
(b) For the level landing attitude for airplanes with tail wheels, the conditions specified in this section must be investigated with the airplane horizontal reference line horizontal in accordance with Figure 2 of Appendix A of this part.
(c) For the level landing attitude for airplanes with nose wheels, shown in Figure 2 of Appendix A of this part, the conditions specified in this section must be investigated assuming the following attitudes:
(1) An attitude in which the main wheels are assumed to contact the ground with the nose wheel just clear of the ground; and
(2) If reasonably attainable at the specified descent and forward velocities, an attitude in which the nose and main wheels are assumed to contact the ground simultaneously.
(d) In addition to the loading conditions prescribed in paragraph (a) of this section, but with maximum vertical ground reactions calculated from paragraph (a), the following apply:
(1) The landing gear and directly affected attaching structure must be designed for the maximum vertical ground reaction combined with an aft acting drag component of not less than 25% of this maximum vertical ground reaction.
(2) The most severe combination of loads that are likely to arise during a lateral drift landing must be taken into account. In absence of a more rational analysis of this condition, the following must be investigated:
(i) A vertical load equal to 75% of the maximum ground reaction of §25.473 must be considered in combination with a drag and side load of 40% and 25% respectively of that vertical load.
(ii) The shock absorber and tire deflections must be assumed to be 75% of the deflection corresponding to the maximum ground reaction of §25.473(a)(2). This load case need not be considered in combination with flat tires.
(3) The combination of vertical and drag components is considered to be acting at the wheel axle centerline.

25.485 Side load conditions.
In addition to §25.479(d)(2) the following conditions must be considered:
(a) For the side load condition, the airplane is assumed to be in the level attitude with only the main wheels contacting the ground, in accordance with figure 5 of appendix A.
(b) Side loads of 0.8 of the vertical reaction (on one side) acting inward and 0.6 of the vertical reaction (on the other side) acting outward must be combined with one-half of the maximum vertical ground reactions obtained in the level landing conditions. These loads are assumed to be applied at the ground contact point and to be resisted by the inertia of the airplane. The drag loads may be assumed to be zero.
25.723 Shock absorption tests.
(a) The analytical representation of the landing gear dynamic characteristics that is used in determining the landing loads must be validated by energy absorption tests. A range of tests must be conducted to ensure that the analytical representation is valid for the design conditions specified in §25.473.
(1) The configurations subjected to energy absorption tests at limit design conditions must include at least the design landing weight or the design takeoff weight, whichever produces the greater value of landing impact energy.
(2) The test attitude of the landing gear unit and the application of appropriate drag loads during the test must simulate the airplane landing conditions in a manner consistent with the development of rational or conservative limit loads.
(b) The landing gear may not fail in a test, demonstrating its reserve energy absorption capacity, simulating a descent velocity of 12 f.p.s. at design landing weight, assuming airplane lift not greater than airplane weight acting during the landing impact.
(c) In lieu of the tests prescribed in this section, changes in previously approved design weights and minor changes in design may be substantiated by analyses based on previous tests conducted on the same basic landing gear system that has similar energy absorption characteristics.