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B377-800 rotation

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B377-800 rotation

Old 4th Dec 2019, 17:28
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B377-800 rotation

Just in the middle of full flight for the type endorsement and would ask of someone who has experience on the actual type:

Does the rotation effort change markedly at a precise point in the take-off? In this sim, the rotation is easily initiated up to about 7 degrees or so whereupon the pitch rates stagnates and a substantial heave is required to get it going again? Realistic? If so, what could possibly cause such a thing?
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Old 4th Dec 2019, 18:09
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I'll use different terms than "markedly" and "substantial heave". The reason is that the tail strike attitude is 9.5 degrees if I remember correctly. Target pitch limit is 8 degrees on the HUD. That takeoff tailstrike reference line limit at 8 degrees is present until 10' AGL and which point it continues at 3 (?) degrees per second. So the natural tendency for the plane to hesitate, for lack of a better word, at 7-8 degrees isn't a bad thing. A slight pause in your rotation and the plane typically flies off shortly afterwards. It's very similar to the 727-200 if you've flown that. Rotate, it sits at x degrees of pitch while it thinks about it for a second, and then it flies off.

For the 737-800 I'm talking about Flaps 1 with derate. More flaps, or max power, and it's less obvious.

I don't know the exact fidelity of the sim, in comparison to the real aircraft, that you're in but I wouldn't use "substantial" and "markedly" to describe the pause. A lot of guys don't notice it so it's not some huge event. It's there, somewhat minor, but it exists. So I wouldn't jump in the airplane and get to 7 degrees of pitch and make a "markedly" and "substantial heave" to your target climb out attitude (17 degrees is a good initial SWAG). That's how tail strikes occur. Rotate to 7.5/8 degrees, pause, get to 10' AGL (obvious ground departure) follow the HUD guidance, and you'll never get a tail strike.

If you don't have the HUD to assist you the 7.5 degree mark is on the pitch attitude indicator, pause, get obvious ground departure via peripheral vision or other cues/instruments, and then continue a normal rotation towards 17 degrees (adjust as necessary - a la Top Gun - do that pilot sh*t).

Last edited by misd-agin; 14th Dec 2019 at 15:40.
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Old 4th Dec 2019, 20:01
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It's realistic. I don't have a slightest idea what causes it. Normal liftoff pitch for 738 at flaps 5 is 8 while tailstrike occurs at 11. Welcome to the beast, I hope you'll like it.
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Old 4th Dec 2019, 20:14
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It's caused by ground effect on the horizontal stab.
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Old 4th Dec 2019, 20:23
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Originally Posted by misd-agin View Post
If you don't have the HUD to assist you the 7.5 degree mark is on the pitch attitude indicator, pause, get obvious ground departure via peripheral vision or other cues/instruments, and then continue a normal rotation towards 17 degrees (adjust as necessary - a la Top Gun - do that pilot sh*t).
That's a very poor advice and not even remotely in line with the FCTM, which suggests smooth continuous rotation towards target pitch attitude is the way to do it.

Stopping rotation at deadband has at least two negative effects. First, your obstacle clearance could be less than optimum if a donkey quits. Second, on tyre speed limited departure you can easily achieve GS in excess of tyre speed limit, since the aircraft continues acceleration on the ground instead of rotating and lifting off.

I say stick to what instructors and FCTM say, but that's just me.
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Old 4th Dec 2019, 20:56
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Originally Posted by FlyingStone View Post
That's a very poor advice and not even remotely in line with the FCTM, which suggests smooth continuous rotation towards target pitch attitude is the way to do it.

Stopping rotation at deadband has at least two negative effects. First, your obstacle clearance could be less than optimum if a donkey quits. Second, on tyre speed limited departure you can easily achieve GS in excess of tyre speed limit, since the aircraft continues acceleration on the ground instead of rotating and lifting off.

I say stick to what instructors and FCTM say, but that's just me.

Agree 100% with this. The "deadband" is caused by the horizontal stab/elevator passing through the disturbed air behind the wing at about 7 to 8 degrees, just about liftoff pitch attitude. The elevator loses a little effectiveness as it rotates downward through this band. The 757 and 767 do it also but not as pronounced. It seems to be more noticeable in the -800 (I've flown the 200, 300, 400 and 700). Ask your instructor (if he's allowed due to syllabus) to let you do a slow flight exercise - level flight, t/o flaps and Vr speed - you will feel it.

As Flying Stone says, the FTCM recommends a smooth continuous rotation to target pitch attitude. You will find in the actual airplane that lots of factors will affect your rotation "feel" - t/o weight, trim, flap setting, etc. - sometimes you'll need the extra tug, sometimes not.
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Old 4th Dec 2019, 22:55
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I agree that following the FCTM guidance isn't really optional.

Regarding your initial query, simulator fidelity varies enough that there are subtle differences in things like dead band feel. My employer has three 737-800 sims (from different manufacturers)(!) and none of them feel the same, or quite the same as the aircraft. It seems like aerodynamic damping effects are never modelled entirely accurately.
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Old 6th Dec 2019, 18:37
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So far in my experience if you pull enough through the dead and to give a continuous rate of rotation it pitches up uncomfortably quickly after the dead band.
So key is continuous rotation as opposed to maintaining the rotation at exactly the same rate throughout. When you get to the deadband pull a bit more but not loads to help it through. Be prepared for pitch rate to increase soon!
I was told that if you are at white bug speedish after rotation you've done good. Higher speed rotation was too slow and vice versa.
We use Flap 5-25 for takeoff.
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Old 7th Dec 2019, 19:06
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Initially during rotation, the aircraft is rotating around the main gear, which is like having a really aft cg. The yoke input feels like an pitch acceleration command. Once the aircraft unsticks, it rotates about the center of gravity and the yoke commands angle of attack, which feels stiffer for pitch response.
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Old 10th Dec 2019, 02:16
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With engine failure at V1 (737-300) and decision made to continue the takeoff run, the interrupted feeling that is evident around 10 degrees body angle is further exacerbated if one engine is inoperative while still on the runway.
The nose down change of trim that is apparent from the original trim setting in Units as part of the before takeoff drill, means the aircraft needs a significantly increased pull force to get to the initial 12-13 degrees body angle

In the simulator it is common to see pilots caught by surprise at the strong pull force needed to lift off. Often the main wheels touch the ground several times before the aircraft starts flying. Judicious use of stabilizer trim is needed to alleviate the stick force once the aircraft is safely clear from the runway surface.
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Old 10th Dec 2019, 23:59
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Type “Wet runway take off blue air” into You Tube to get a graphic illustration of what is happening to the airflow over the tail during rotation, causing the reduction in elevator effectiveness. The tail plane is in “clean air” until midway through the rotation and then enters disturbed air until clear of the runway when the pitch can be seen to increase again. All simulators I have used replicate the aircraft behaviour in this regard quite well.
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Old 11th Dec 2019, 01:01
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So we have 3 theories posted above:

1. Elevator enters ground effect
2. Elevator enters down wash from wing
3. Transition of rotation from wheels to C of G

I too have heard all 3 theories over the years, but no concrete evidence.

So, which is it?
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Old 11th Dec 2019, 10:04
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Perhaps it’s a mix of all 3 ......
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Old 12th Dec 2019, 22:35
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Mostly ground effect.

T tails that I have flown don't have this phenomenon at all.
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Old 14th Dec 2019, 15:59
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Originally Posted by FlyingStone View Post
That's a very poor advice and not even remotely in line with the FCTM, which suggests smooth continuous rotation towards target pitch attitude is the way to do it.

Stopping rotation at deadband has at least two negative effects. First, your obstacle clearance could be less than optimum if a donkey quits. Second, on tyre speed limited departure you can easily achieve GS in excess of tyre speed limit, since the aircraft continues acceleration on the ground instead of rotating and lifting off.

I say stick to what instructors and FCTM say, but that's just me.
You're correct. The manual uses 10 degrees of target pitch for the TO/GA line. Correct my statement from "8 degrees" and "7.5 mark on the ADI" to 10 degrees for the TO/GA and pitch mark. It's been 8-10 years since I flew the 737/800. The fact is trying to 'make it fly' if your pitch is at the TO/GA mark (10 degrees) continuing to rotate while the aircraft is still on the ground is an invitation for a tail strike.

Here's what the manual says - Rotate at 2 to 3 per second (approximately 4 seconds to reach TO/GA line in HUD or 10 pitch line in PFD). At VR. Rotate to put the aircraft reference symbol (boresight) in the TO/GA line gap. Caution If pitch exceeds the TO/GA line, the chance of a tailstrike is increased. The TO/GA line remains at +10 until 10 ft RA, and then rates up at 2 per second to match the PFD FD. Keeping the aircraft reference symbol in the TO/GA line gap is identical to flying the PFD FD.

And - Rotate at 2 to 3 per second (approximately 4 seconds to reach TO/GA line in HUD or 10 pitch line in PFD).

It's not a great feeling having to get into the charts and comparing 'tail strike struts compressed' vs 'tail strike struts extended' data. Been there done, done that. New FO in type and in his previous jet it was almost impossible to get a tail strike (<12 degrees??). His rotation rate was faster than recommended in longer fuselage aircraft. Who's tracking 2.5 vs 3 degrees per second? No one. Based on that I'm a fan of pausing at specific pitch attitudes if you're still on the ground. Longer jets get down to 9.5, 10, or 11 degrees for a tail strike risk and the risk of a tail strike is higher. Personally I'm in favor of a pause if the plane won't fly at the recommended pitch attitude.
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Old 14th Dec 2019, 21:26
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Sorry, but could you please tell us which manual is this from? Because it surely isn't Boeing's FCTM for 737, which says (my formatting):

When a smooth continuous rotation is initiated at VR, tail clearance margin is assured because computed takeoff speeds depicted in the PI Chapter of the FCOM, airport analysis, or FMC, are developed to provide adequate tail clearance.
Above 80 knots, relax the forward control column pressure to the neutral position. For optimum takeoff and initial climb performance, initiate a smooth continuous rotation at VR toward 15 of pitch attitude. However, takeoffs at low thrust setting (low excess energy) will result in a lower initial pitch attitude target to achieve the desired climb speed.
The use of stabilizer trim during rotation is not recommended. After liftoff, use the attitude indicator, or indications on the PFD or HUD (HUD equipped airplanes), as the primary pitch reference. The flight director, in conjunction with indicated airspeed and other flight instruments is used to maintain the proper vertical flight path.
Note: The flight director pitch command is not used for rotation.
Who's tracking 2.5 vs 3 degrees per second? No one.
Surely PM and they should speak up if they think rotation rate is excessive.
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