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Orange future
17th May 2023, 20:25

The investigation noted that a 10 knot tail wind increased the chance of a tail strike.

Why would this be the case?

MechEngr
17th May 2023, 21:37
There are a few things to consider.

The first is that to take off a plane needs to have an airspeed matching the aircraft pitch. The lower the takeoff speed, the higher the required pitch.
The pitch is limited by the amount of rotation before the tail strikes the runway.
When there is a tail wind it takes longer to accelerate the plane to that airspeed.
The longer it takes to accelerate the more runway is required.

If the pilot has screwed up then they will run out of runway before they get to the minimum airspeed where the tail doesn't strike.
To make the screw up greater, the pilot who sees the runway running out will pull the plane to a higher pitch to take off at a lower airspeed - but that higher pitch is too high and the plane tail strikes the runway.

Look at a couple of limit cases. If there is a headwind equal to the no-tail-strike take-off speed the plane won't need any runway, just pull back on the controls and when the correct pitch is reached the plane takes off, with no runway roll at all. This trick has been done in light planes; maybe 30-40 knots. Alternatively, on a runway that is miles and miles long, with a sufficient tailwind that the airspeed is never positive, the airspeed required to take off is not reached because the tires cannot roll that fast.

Orange future
18th May 2023, 01:23
The accident report did not indicate that runway was insufficient (although it may have been).

Lets assume a theoretical case where runway length was not limiting and the pilot did not rotate early or too much.

A simple steady state tailwind and a perfectly normal rotation rate and pitch attitude.

It still leaves me puzzled as to why the report stated that tailwind was a cause in this incident?

MechEngr
18th May 2023, 03:30
I presume you mean : https://www.mot.gov.sg/docs/default-source/default-document-library/9v-mgl-tail-strike-ktm-6-may-22_final-report.pdf about B737-800, 9V-MGL
Tail Strike Event at Kathmandu Airport, Nepal?

I don't know why the report is written that way. They are very concerned with the overall mishandling of the plane and the post-strike response by the crew.

They do note "The aircraft was operating close to its maximum take-off performance limit and the take-off configuration used was Flap 1 without using engine bleeds" which would mean a tailwind would have an adverse effect on the ability to leave the runway. They also mention it is surrounded by unfavorable terrain so leaving the runway is just one consideration that requires a healthy vertical velocity component.

I'd ask, would a headwind of 10 knots have allowed the plane to lift off before getting beyond the pitch limit? If so, then a tailwind of the same magnitude would be detrimental in a notable amount. The report indicates a general local wind of 8 knots tending to be a headwind was reported. That calculated tailwind is -18knots off of that.

Fursty Ferret
18th May 2023, 16:08
Different wind at each end of the runway. Accelerate with calm wind, rotate, go into tailwind, and the pitch attitude will increase while lift is lost, degrading margins to tail strike.

vilas
19th May 2023, 07:37
Don't know aerodynamics of lift during rotation but there's a general feel about the rotation like the ground speed and rate of change of attitude. In TW the GS is higher which could create illusion that rate of rotation is slower and make pilot inadvertently increase rate of rotation. There's a tail strike pitch for each aircraft that needs to be monitored during rotation and if approaching that without lift off then you need to freeze rotation.

punkalouver
20th May 2023, 03:44
There are a few things to consider.
When there is a tail wind it takes longer to accelerate the plane to that airspeed.
The longer it takes to accelerate the more runway is required.

If the pilot has screwed up then they will run out of runway before they get to the minimum airspeed where the tail doesn't strike.
To make the screw up greater, the pilot who sees the runway running out will pull the plane to a higher pitch to take off at a lower airspeed - but that higher pitch is too high and the plane tail strikes the runway.

Look at a couple of limit cases. If there is a headwind equal to the no-tail-strike take-off speed the plane won't need any runway, just pull back on the controls and when the correct pitch is reached the plane takes off, with no runway roll at all. This trick has been done in light planes; maybe 30-40 knots. Alternatively, on a runway that is miles and miles long, with a sufficient tailwind that the airspeed is never positive, the airspeed required to take off is not reached because the tires cannot roll that fast.

I don't believe that theory. Otherwise max derate/reduced thrust would be just as likely to create a tailstrike hazard.

Don't know aerodynamics of lift during rotation but there's a general feel about the rotation like the ground speed and rate of change of attitude. In TW the GS is higher which could create illusion that rate of rotation is slower and make pilot inadvertently increase rate of rotation. There's a tail strike pitch for each aircraft that needs to be monitored during rotation and if approaching that without lift off then you need to freeze rotation.

I don't believe that theory either. Otherwise, heavy weight takeoffs would have more tailstrikes due to increased Vr speed.

MechEngr
20th May 2023, 06:12
I don't believe that theory. Otherwise max derate/reduced thrust would be just as likely to create a tailstrike hazard.

Which theory and why?

Here's another incident:
The tailstrike was the result of an excessive rate of rotation during the takeoff. This was exacerbated by a variable headwind component which contributed to a lift off speed that was lower than intended by the manufacturer and compounded the loss of tail clearance. 2005, a Thomsonfly 757, https://www.fss.aero/accident-reports/look.php?report_key=230

dream747
20th May 2023, 19:06
Vr speed is based on IAS. Assuming in both headwind and tailwind situations the Vr is the same, the aircraft should behave exactly manner aerodynamically (time to lift off, pitch at lift off.. etc) if both takeoffs are done precisely the same way. The only difference here is the resultant groundspeed - in a tailwind more ground covered. I do not think there is an increase risk of tailstrike as well in a tailwind situation. Unless we are talking about a sudden lost of headwind component (sudden reduction in HW or increase in TW) during rotation, resulting in the IAS and thus lift to decrease at this moment especially with the aircraft at its take-off performance limits and the pilot keeps pulling back.

megan
21st May 2023, 03:29
Unless we are talking about a sudden lost of headwind component (sudden reduction in HW or increase in TW) during rotation, resulting in the IAS and thus lift to decrease at this moment especially with the aircraft at its take-off performance limits and the pilot keeps pulling back.The sudden loss of ground cushion at the tailplane with the reduction of IAS be an influence towards tailstrike?

dream747
21st May 2023, 12:03
The sudden loss of ground cushion at the tailplane with the reduction of IAS be an influence towards tailstrike?

An instantaneous loss of headwind component or increase in tailwind component will result in the IAS below calculated Vr. If your performance limited, it is very likely that the Vr would not have been optimised. The lift required in lifting the aircraft off in a "normal" rotation technique would not have been sufficient and the aircraft remains on the ground due to the lack of lift. If the pilot keeps pulling back towards climb pitch attitude that's probably how a tailstrike can occur. We are talking about an aircraft being at its performance limits for that particular day. On most other days when V speeds are optimised (Airbus speak), the calculated Vr would typically have margins.

Just to be clear, taking off in a tailwind does not increase the risk of a tailstrike.

vilas
21st May 2023, 12:40
It's clearly concluded that Tail strike happened due to faster rate of rotation and reaching an attitude beyond tail clearance attitude. The discussion is about did prevailing tail wind as suggested facilitate that?

hans brinker
21st May 2023, 18:29
It's clearly concluded that Tail strike happened due to faster rate of rotation and reaching an attitude beyond tail clearance attitude. The discussion is about did prevailing tail wind as suggested facilitate that?

Just stating the obvious: Steady state tailwinds won't change anything about the rotation from a physic's point, and changing wind conditions apply the same to HW as they do to TW.

The thing I can imagine happening with a strongish TW vs the more normal HW in combination with a heavy AC is the ground speed will be much higher, and that sight picture might have led the pilot to make an inappropriately large ANU input.

john_tullamarine
21st May 2023, 23:24
Don't folks rotate jets with reference to the A/H these days ? ... especially on Types which have a liking for tail strikes. I was brought up on the Boeing three holer and the A/H reigned supreme during the lift off rotation sequence.

fdr
22nd May 2023, 00:12
Just stating the obvious: Steady state tailwinds won't change anything about the rotation from a physic's point, and changing wind conditions apply the same to HW as they do to TW.

The thing I can imagine happening with a strongish TW vs the more normal HW in combination with a heavy AC is the ground speed will be much higher, and that sight picture might have led the pilot to make an inappropriately large ANU input.

...except that, steady state at the measurement height does not equate to no vertical velocity gradient. From the turbulent layer of the ground there will a lower velocity near the surface than higher up, and the direction alters slightly in most cases as well. That will give a very small undershoot shear condition in a tailwind case, but it is in the noise level.

Historically, tail strikes are over-represented in crosswinds and tailwinds, due to pilot input, and not the conditions themselves. An increase in tailwind component (TWC) will result in a higher pitch attitude required to fly off the ground, and as most often the crew are not scanning the airspeed indicator, (ASI, AS, or MASI), then an increased TWC at the point of rotate will reduce tail-ground clearance at liftoff. Most cases but not all tail strikes I have investigated occur immediately after the wheels come off the ground, although the total events were pretty evenly distributed between takeoff and landing cases. The control inputs are usually found to be more abrupt and larger displacement than a zero wind or simple headwind case. Humans are curious creatures. In almost all transport category aircraft, a rotate commenced at the correct speed and rate will result in V2 being achieved immediately before the wheels come off the ground, when operating a multiengine aircraft without a power loss, yet we removed the call of achieving V2 around 1990... and loss of tail clearance events increased.

oicur12.again
24th May 2023, 18:57
There are several documents out there worth reading, the main ones being:

“Preventing Tail Strikes at Takeoff” by Airbus

“Tail Strike Avoidance” by Boeing

“Tail Strikes: Prevention” by the Chief Pilot of Boeings Flight Ops Safety Department

Between Airbus and Boeing the causes of tail strikes include:

Early Rotation

Incorrect Rotation Technique

Incorrect configuration

Incorrect CG calculation or stab setting

Oleo under inflation

Improper use of flight director

Mis handling during gusty winds and crosswinds

No where is it suggested that tail wind is a contributor.

The most significant is gusty wind, wind shear and crosswind.

A headwind gust can contribute to a tail strike in the same way that a tail wind gust can.

punkalouver
26th May 2023, 13:09
Bottom line..,,,rotate at the proper rate for your aircraft and if anything, do it at a slightly lower rate on gusty days. Assuming no major mistakes such as mis-configuration, speed, wind calculation, or thrust setting errors, things will be fine.

I suspect the tail strikes that happen due to early rotation speeds are where the tail strike pitch attitude is approached with wheels still on the ground but the rotation is continued. If one were to stop the rotation early because they had sensed that the aircraft was not getting airborne, they could prevent the tailstrike and wait for the aircraft to accelerate to a safer speed which is a good reason to be aware of the tailstrike pitch attitude on your aircraft type.

Perhaps the above is easier said than done. I have never tried early rotation in the sim, but it might be an interesting exercise.