B777 airspeed in turbulence

[changer]

Can somebody explain to me why the 777 indicated airspeed increases when entering chop?

[Aileron Drag]

Been retired for several years now, but I remember the IAS and Mach increasing in chop, and having to pull some power off. I reached the point where, as soon as we hit a little chop, I'd wind the speed back a little, in order to anticipate the increase. But I never knew the reason for this phenomenon.

The other little 777 'nasty' was its habit of pitching down quite severely if the speedbrakes were deployed without finesse.

Nice piece of kit, though. Be interested to see if there are any aerodynamicists out there with an answer.

[oz in dxb]

Turbulent air penetration speed is used when you encounter Severe Turbulence as stated in the 777 FCOM. "Chop" is not normally severe.

Turbulence penetration for the climb is 270kts below FL 250 and 280kts at and above FL 250. Speed should not reduce below min manouevre if below 0.82MN.

Normal climb speed is +300kts for average weights, so you encounter a IAS reduction.

In the cruise you would reduce to .82MN or 15kts above min manouvre speed.

Turbulent penetration speed increases your max and min buffet margins.

Oz

[changer]

Right. Got all that. But that's not what I was asking.

Indicated airspeed has tendency to increase all by itself when entering chop or turbulence. I've only noticed this on the 777, and not on any other airplane I've flown.

I'm hoping someone could explain to me aerodynamically why it happens.

[Cubbie]

The logical assumption is that the chop or turbulence is caused by vws with up and down drafts, changes in SAT, or changes in the direction of the wind. Any updraft will cause the aircraft to climb but as the autopilot is maintaining altitude it lowers the nose to maintain it and since the autothrottle is in a lower activity cruise mode it allows the speed to increase. Likewise if the wind shifts,- increases on the tail, or reduces on the nose,more often than not there's a speed increase. Since this is not as noticeable on the 744 it most probably is due to the wing design being so 'slippy' hense be very careful when close to the barber pole, the auto throttle doesnt always get more aggressive in it's response. However there are occasions when the speed does drop first before entering turbulent conditions, which can be also noted by a change in SAT.

[Aileron Drag]

Still doesn't answer changer's question, Cubbie. No other aircraft I've flown exhibited this characteristic. At the slightest hint of chop, the speed would increase with no alteration in pitch attitiude. The 777 was a bit of an enigma in many ways. One sometimes wondered what exactly was going on.

[changer]

Cubbie,
your explanation would infer an airspeed variance either up or down when entering chop or turbulence. However I've only noticed an increase in airspeed, and have not noticed any tendency for airspeed to decrease in turbulence.

[MrBernoulli]

Indicated airspeed has tendency to increase all by itself when entering chop or turbulence.Not something I have noticed on the 777! Sometimes it goes up, sometimes it goes down, but I have never seen it only go up. Sorry.

[777AV8R]

For my two bits worth, I've been on it since '98. The only reason that I can think of is that you could have been 'shearing to a headwind', which would give you an increase in indicated airspeed. If the SAT was decreasing, you could also expect that you could have an increase in indicated airspeed. The aircraft has a tremendous amount of energy and anything that is 'out there' that tries to disturb it, always shows up in the dynamics until the energy state gets sorted out.

As for turbulence penetration speed, the difference from a cruise of 0.834 to get down to 0.82, is so minuscule. By the time one could get the aircraft slowed down, you'd probably be out of the turbulence. Its always amusing to watch someone immediately reach up to the MCP and dial the speed back to .82 at the least 'ripple'. Relax...the airplane is built rock solid.;)

[Aileron Drag]

I was one of those who pulled the speed back. It wasn't for fear of the wings coming off (I think the 777 is as strong as its ancestor, the B17), but fear of a nasty phone call from the SESMA rep or Flight Manager.:)

[FCeng84]

Aileron_Drag,

I'm quite curious about your comment regarding 777 speedbrakes and pitch response. I was part of the design team for the 777 control system and this is the first that I have heard of the response during speedbrake deployment being a 'nasty'. I would appreciate further details so that I can look into it and see if there is something there that we should avoid on future designs. For what flight conditions was the characteristic that you found objectionable most pronounced?

Regards,

FCEng84

[Aileron Drag]

I was warned about this characteristic during my conversion line-flying. Having been 'cleared' on the line, I did on one occasion deploy the speedbrakes with some haste, and the aircraft pitched down quite rapidly. The speed was on the barber's pole in a very short time, and I had to disconnect the A/P and gently pitch up to contain the speed.

Some months later (this was in 1999), I was in the right-hand-seat training an SFO on his command course. We were heading for the Gulf, but had (most unusually) been routed over Iran. We were kept up high, late. I had warned the other guy about this characteristic, but when finally cleared to descend we were very high, and he deployed the speedbrake very rapidly. He had to subsequently disconnect the A/P to avoid busting the 'pole'. QED.

I'm sorry to say that he went to the same destination, some days later, and experienced the same set of circumstances, but this time with a manager, and on his final command check. He had to disconnect the A/P and recover. This time, however, a cabin-crew member was injured in the recovery, and the proverbial hit the fan.

Surely, every 777 pilot knows about this characteristic? It was not a problem at all if the brakes were deployed with finess. But the 777 A/P is the only one I've known to be so incredibly slow to react to such a speed increase.

[777AV8R]

Actually, the FCTM is quite specific and brings attention to flight crew in regards to use of the speedbrakes while the autopilot is engaged. If the autopilot has captured the selected MCP altitude (which can be 2000 feet above depending on descent rate), the autopilot calculates a fixed descent profile to intercept the altitude. If the speedbrakes are are 'stowed' during the maneuver, the autopilot will pitch down to compensate for the drag loss, resulting in an overspeed. With the resulting overspeed the autopilot reacts to envelope protection and will level off.

The proper method of retracting speedbrakes during this process is to intervene with an MCP speed of about 20 knots less than what was being indicated, prior to speedbrakes stowage. The aircraft will begin to slow down and when the speedbrakes are stowed, the autopilot will command a smooth transition to adjust for the speed/pitch. Works like a charm. It is not a design fault but a characteristic of the autopilot system and the low drag profile of the airplane. If being held 'high and hot' is a problem, inserting a speed/altitude on the legs page, down track will provide a good 'how-goes-it' with regards to a vertical profile during the descent.

[INLAK]

The 747-200 also had a nasty habit of excessive nose down pitching if the speedbrakes were deployed rapidly.

[FCeng84]

Aileron Drag and 777AV8R,

Thanks for your explanations. The 777 control laws are designed to compensate for the lift change associated with speedbrake extension or retraction. As the speedbrakes are extended, pitch attitude increases. As the speedbrakes are retracted, pitch attitude decreases. The intent is to avoid any significant change in load factor during speedbrake transitions. The design focussed on the airplane response during manual flight. Until now, I have not been aware that this feature leads to undersired response with the autopilot engaged. I'm curious as to which autopilot mode(s) are active when the problem occurs.

Another 777 speedbrake feature is that the control system rate limits deployment and retraction when in the air so that there is no difference in response between stepping the lever from one stop to the other and ramping it from stop to stop over three seconds.

[Hahn]

The speed goes up because turbulent air has more energy than still air. This happens to every aircraft, even to a bird!