I think that most airlines recommend not to decrab completely upon touchdown.Bert Van Bulck

Thanks Bert.I have just read on the net actually, that not de-crabbing at all is an acceptable crosswind technique for the 737 NG.As long as it's within the cross wind limit.I guess i usually de-crab to a degree, to reduce the strain on the gear and tyres, but not completely. Nice to know I have it right.

Guys, the plane was certified to land up to certain limits. Those limits are published, but they're not a "how to" on landing. If you land in a crab, you're going to go through tires a lot more quickly. The crab only or partial de-crab (is that French?) is good on slick runways. There's not much worry about tire wear, and the crosswind and crab combined with the slippery conditions work well together.De-crab in flare on dry runways is preferred by airlines because there's less wear and tear.

I know. It's in the manuals. I should have added "wet".It's also not recommended to land crabbed, with a dry runway, with strong crosswinds, because when the nose wheel touches down, there's more of a tendency to head off the runway, due to the increased traction.Not such an issue on a wet runway due to the decreased traction.

Can someone (other than me - because I don't know) respond to this post original question on how to post a photo - help.

First you have to upload your Photo to a website. (Image Shack for Example). After that use the image button above and paste in the url for your image.

Actually, Ive just tested again.The 900 on a 3 degree glidesope is zero degrees.The chart in the Flight Crew Training Manual, section 6 tells us it should be 0.9 degrees up at flap 40.It's not much, most may not be bothered, but the OP is right, it is a small amount out.Which would make sense if the unreliable airspeed charts have been used, which refer to VREF+10 rather than 5. One degree equates to 5 knots.

I know. It's in the manuals. I should have added "wet".It's also not recommended to land crabbed, with a dry runway, with strong crosswinds, because when the nose wheel touches down, there's more of a tendency to head off the runway, due to the increased traction.Not such an issue on a wet runway due to the decreased traction.

Firstly before the nosewheel touches down the main gears will have effectively decrabbed the aircraft. Secondly a tricycle undercarriage is directionally stable, wet or dry. If crabbed it will tend to turn straight whether the nosewheel is down or not. Reduced friction in wet conditions affects all tyres equally.Matt's point about extra tyre wear on dry runways is valid of course.Kevin Hall

Firstly before the nose wheel touches down the main gears will have effectively decrabbed the aircraft.

Of course not! It's in the manual. If you don't de-crab, the aircraft will be pointing off the runway. You then have to de-crab after touchdown before the loss of speed and inertia takes you in the direction the wheels are oriented... off the runway.You must have experienced this in the sim!Wheels revolve around a central axis, they like to direct the vehicle in the direction they revolve. If they are scuffing sideways, then their is a lateral force, wanting to guide you off the runway.This is why the manual states that a fully crabbed landing is more acceptable in wet weather, simply because the loss of traction allows the wheels to continue to slide in the direction of the runway heading.On a wet runway, there is less traction, and more of a tendency for the aircraft to continue ahead, while still crabbed.I know about tyre wear, see my comment earlier in this thread. Not to mention the entire gear under more mechanical stress.The manual gives a very good rundown of the three landing techniques, it's worth a read.

tricycle undercarriage is directionally stable, wet or dry. If crabbed it will tend to turn straight whether the nose wheel is down or not.

It's not a tricycle undercarriage when the nose wheel isn't down, and no aircraft de-crabs itselfHave a read of the manuals PMDG provide.

Landing in CrabThe airplane can land using crab only (zero side slip) up to the landing crosswind guideline speeds.On dry runways, upon touchdown the airplane tracks towards the upwind edge of the runway while de-crabbing to align with the runway. Immediate upwind aileron is needed to ensure the wings remain level whilerudder is needed to track center line. The greater the amount of crab at touchdown, the larger the lateral deviation from the point of touchdown. For this reason, touchdown in a crab only condition is not recommended when landing on a dry runway.On very slippery runways, landing the airplane using crab only reduces drift towards the downwind side of a touchdown, and may reduce pilot workload since the airplane does not have to be de-crabbed before touchdown. However, proper rudder and upwind aileron must be applied after touchdown to ensure directional control is maintained.

http://en.wikipedia....osswind_landing

Of course not! It's in the manual. If you don't de-crab, the aircraft will be pointing off the runway. You then have to de-crab after touchdown before the loss of speed and inertia takes you in the direction the wheels are oriented... off the runway.You must have experienced this in the sim!Wheels revolve around a central axis, they like to direct the vehicle in the direction they revolve. If they are scuffing sideways, then their is a lateral force, wanting to guide you off the runway.This is why the manual states that a fully crabbed landing is more acceptable in wet weather, simply because the loss of traction allows the wheels to continue to slide in the direction of the runway heading.On a wet runway, there is less traction, and more of a tendency for the aircraft to continue ahead, while still crabbed.I know about tyre wear, see my comment earlier in this thread. Not to mention the entire gear under more mechanical stress.The manual gives a very good rundown of the three landing techniques, it's worth a read.

Martin, if you land in a crabbed condition on a dry runway the sideforce on the main wheels (behind the CG remember) will decrab you very quickly. There may be some lateral deviation but the aircraft has no lateral momentum before touchdown and its inertia will tend to keep the deviation small. If you watch those extreme crosswind landing test videos online where the aircraft touch down in full crab you'll see what I mean.Initially the main wheels are not rolling and can only provide a friction force opposing the aircraft's motion (which is along the runway). As well as creating drag, this also creates a turning moment which de-crabs the aircraft (the contact points are behind the CG and offset to the side due to the yaw angle). The wheels spin up quickly, but by the time they start to generate steering forces the aircraft will be pointing down the runway.My main reason for replying to your post was that you said the nosewheel would tend to turn the aircraft away from the centreline when it touches:

It's also not recommended to land crabbed, with a dry runway, with strong crosswinds, because when the nose wheel touches down, there's more of a tendency to head off the runway, due to the increased traction

The nosewheel would create a turning moment opposing that of the main wheels but because the mains create much greater friction forces the total ground reaction turning moment would still be towards the centreline. If that was not the case tricycle undercarriage aircraft would ground loop like tail-draggers. The relative forces and moments between the mains and the nose are the same regardless of the friction level. Finally, the word is "friction", not "traction" (which is to do with driven wheels).

It's not a tricycle undercarriage when the nose wheel isn't down, and no aircraft de-crabs itselfHave a read of the manuals PMDG provide.

Your own wiki link mentions the aircraft decrabbing itself, you even highlighted the text. Do you imagine the aircraft will immediately start moving in the direction its nose is pointed when it touches down, and stay moving in that direction?As for your tricycle undercarrriage comment see above.Kevin Hall

Martin, if you land in a crabbed condition on a dry runway the sideforce on the main wheels (behind the CG remember) will decrab you very quickly. There may be some lateral deviation but the aircraft has no lateral momentum before touchdown and its inertia will tend to keep the deviation small.

Did you not read the following above…The greater the amount of crab at touchdown, the larger the lateral deviation from the point of touchdownAnd...On dry runways, upon touchdown the airplane tracks towards the upwind edge of the runwayAnd...Immediate upwind aileron is needed to ensure the wings remain level while rudder is needed to track center line

, if you land in a crabbed condition on a dry runway the sideforce on the main wheels (behind the CG remember) will decrab you very quickly.

I dispute that as a significant phenomenon! It matters not that the main gear is behind the centre of gravity. The only feasible way that could transpire is if the nose wheel, above the ground at this point, was not decelatring as fast as the main gear. It actually is, as it’s connected to the same structure, the aircraft itself. Both the nose wheel and main gear are subject to the same deceleration forces, imposed by the friction on the main gear.

If you watch those extreme crosswind landing test videos online where the aircraft touch down in full crab you'll see what I mean.

The pilots in question are landing crabbed, but using rudder to align with the runway, after the main gear touches down. Just as the Boeing manuals advocate.Again... read the Boeing manual that came with the NGX. It’s clearly stated, that landing in a crabbed configuration, requires the pilot to apply rudder to prevent the aircraft from tracking toward the edge of the runway. Boeing doesn’t advise this for fun, they obviously feel it’s necessary.

My main reason for replying to your post was that you said the nosewheel would tend to turn the aircraft away from the centreline when it touches:

The Boeing manual, states that, when landing in-crab, the pilot should de-crab using rudder, as the main gear touches down, implying, before the nose wheel touches down. I’m sure you can devise your own theory as to why you think this is the case.

Your own wiki link mentions the aircraft decrabbing itself, you even highlighted the text. Do you imagine the aircraft will immediately start moving in the direction its nose is pointed when it touches down, and stay moving in that direction?

Actually, it doesn’t specifically say the aircraft de-crabs itself; it could be referring to pilot input.And if you read the next sentence, which refers to the moment following main gear touchdown, it specifically tells you this is the case...Immediate upwind aileron is needed to ensure the wings remain level while rudder is needed to track center lineNotice the use of the word "immediate" as in, after the main gear touches down and before the nose wheel touches down.Fairly obvious that even if the aircraft does have a tendency to de-crab itself to a degree, it’s not enough, and rudder input is required by the pilot. As stated above. As advocated by Boeing.

To be honest Kevin, I think you may be getting confused with a light aircraft, that when it lands with any degree of crab still present, touches down on one wheel, thus automatically turning in the direction of the runway.This doesn't apply to a large passenger jet, as the technique is to land with wings level, both wheels simultaneously, due to the danger of impacting the large jet engines hanging beneath each wing.

Ok, let's now go back to topic please! Just did a flight from KPDX to KSEA, in the 737-900WL from United Airlines. I used the default payload, and used 9000 lbs of fuel. My VREF was 134 so my FAS (Final Approach Speed) was 139 with no major winds or something, weather was fairly normal. But now, on final approach, the nose was even pointed down a little! Around 1 degree! Even though I was perfectly maintaining 139 KTS.

Sorry Arjen, didn't mean to side track your thread.Yes, as I said above, I see the same discrepency at VREF, at 40 flap. I see O.9 degrees too low because I'm compensating for the SFP, so yes, more like 1.9 degrees too low at VREF+5.The manual states that the aircraft should be pitced up at 0.9 degrees, at flap 40. At VREF+5.So yes, after several flights in the 900, I can confirm that the pitch is 1.9 degrees too low at flap 40.It's not a lot, but it is present.PMDG don't seem too happy that we are bringing this up again, so I'm not sure if they will change it in the next patch. If you wish to tweak that yourself with a lift-scalar alteration it's very easy. A minor adjustment.Flight Crew Training Manual section 6.7.3 degree glideslope. Flap 40+5.-600 = 2 degrees up-700 = 2 degrees up-800 = 1.4/2.7 degrees up [After the brackets is the SFP]-900 = 0.9 degrees up-900ER = 2.5 degrees up [With SFP as standard equipment]

Guys, since ER has SFP as standart, I feel they will have to correct the behaviour, one way or another...

Not sure what you mean there Peter, the PMDG 900 we have at present is the 900, not the ER.Pitch should be 0.9 up, we see one degree down.The ER is 2.5 degrees up, because it has SFP. We haven't got an ER yet. :(