Is airspeed control in B777 fundamentally different than in a glider?

In article ,
Craig Funston wrote:


I also suspect having 4 pilots involved ended up being less safe than just
two.

Craig

No doubt!

Once watched a Grob 103 run off the end of a runway into the weeds with
two very experienced and individually competent pilots on board. Each
said he thought the other was PIC from the time they entered the landing
pattern.


"None of us are as stupid as all of us".

"Is there something fundamentally different about airspeed control in large jet aircraft? My experience is that pitch controls airspeed and power controls climb/descent, especially in the approach to landing situation. "

It depends on what level of automation is being used and the automation modes that are active. For a coupled (autoplilot engaged) ILS approach on almost all large transport aircraft, pitch is used to null glideslope error and thrust is used to null speed error.

Similarly, in an altitude hold mode, pitch is used to null altitude error and thrust to control speed error.

However, there is another set of control modes in which speed is controlled by pitch and climb/descent rate is controlled by thrust (thrust typically being idle in descent and current thrust limit for climb)

That's a simplified picture and the modes, and the names of those modes, are not well standardized. In the end, no matter what mode you use, pitch and thrust combine to control performance.

On a hand flown ILS approach experienced pilots will argue about whether thrust controls path or speed. However you can't escape from the fact that they interact and, if you change one, you most likely also need to change the other.

Direct lift control has also been used for glidepath control and it has the advantage of eliminating the pitch/thrust coupling seen using the more conventional method.


Andy

On Wednesday, July 10, 2013 7:13:56 AM UTC-7, wrote:
The news about the B777 accident at SFO is getting me confused. The news stories are constantly pointing to the Autothrottle not maintaining airspeed (and question if the 3 pilots in the cockpit were monitoring airspeed)but in all the aircraft I have flown airspeed is controlled primarily with pitch. Is there something fundamentally different about airspeed control in large jet aircraft? My experience is that pitch controls airspeed and power controls climb/descent, especially in the approach to landing situation.



Guy Acheson "DDS"

Hi Guy,

I fly gliders for fun(except when digging out of a hole which is work), and B767/757 for work(which is fun most of the time). Pitch plus power=performance. The debate over what controls airspeed/altitude is very old and obviously still alive. The FAA used to say "when power is variable and available, pitch controls airspeed and power controls altitude." That was the easiest way to teach students how to fly at minimum controllable airspeed. Of course its not black and white. There is the old joke about a pilot sitting at the end of the runway flapping his elevators, trying to get enough airspeed for takeoff! Personally I fly the jet just like I did light GA aircraft and turboprops-pitch for airspeed and power for altitude. Whatever technique works is fine for each pilot. There is talk of AOA on this thread but most civilian aircraft don't have them.

There are many truths in aviation. One is that the spot in the windshield that isn't moving is where you will land(glider or 777). If the desired spot is moving up in the windshield you are low, if its moving down in the windshield you are getting high.

All the talk in the press about autoflight modes makes me dizzy. I have 20+ years and 15,000+ hours in "glass cockpits"(B737,757,767) and know how to use autoflight but can't give you a dissertation on it. (sorry I am a pilot not an engineer). Luckily I still remember what the sight picture looks like. And if I get low I push up the throttles NOW if nothing is happening.. And in our world if a pilot get 5(five) knots slow the other will say something.....

The "extra" pilots were because it was an augmented crew for an 8+ hour flight. Two pilots in the cockpit at cruise while the others rest. All 3 or 4 in cockpit for takeoff and landing. In theory to look for traffic, and monitor the two pilots at the controls. Its SOP for domestic and foreign carriers and should enhance safety.

Fly safe and happy soaring.

Dean "GO"

A significant factor appears to be that the PIC had most of his experience in Airbus aircraft where the auto-throttle operates in different modes than those used by Boeing. The overall problem is the interface between humans and computers and deciding who/what has control and when. Why they continued to descend into the water when the pilot in the third seat kept shouting "glide slope!" is another question.

Yes, we glider drivers don't have engines, but we do have similar interface problems.

Mike

On Wednesday, 10 July 2013 22:13:56 UTC+8, wrote:
The news about the B777 accident at SFO....Is there something fundamentally different about airspeed control in large jet aircraft?

I learnt to fly GA aeroplanes where Attitude for Airspeed, Power for Rate of Descent was taught for the approach. This same school taught Pitch for Path, Power for Airspeed once you got into their high performance aircraft. So even within the one establishment, variation raged as Dean suggests below. The felt that the former was safer for ab-initio students. In my airline career the latter has been taught and whilst I personally prescribe to the latter, I honestly don't think it matters too much due to the inter-relationship between the two. In a powered aircraft, if you are low on approach and pitch the nose up to correct, you're going to have to add power to maintain airspeed. Alternatively, if you are low on approach and add power to correct, you're going to have to pitch the nose up to maintain airspeed.

In a transport category jet at flight idle for descent, speed is controlled by pitch/attitude. When I was flying Embraer 170's they went as far as to call this "Speed on Elevator" (Where the alternative, speed controlled by engine thrust, was called "Speed on Thrust"). This is where a jet aircraft behaves exactly like a glider. Things change however, on an approach such as an ILS. Notwithstanding deceleration for configuration (gear and flap extension) a correctly flown ILS will be flown with pitch/attitude controlling the flightpath and thrust controlling airspeed. This is accomplished by the Autopilot and auto-throttle in concert. The auto-throttle is like a smart version of the cruise control in your car. It will command the required thrust to achieve the target speed which in the latter part of the approach was Vref, the target approach speed.

In a Boeing, the throttles are backdriven by the auto-throttle. This means that unlike an Airbus, electric motors cause them to move in response to commanded changes in thrust. If a pilot wants to overpower this movement they can. They're designed to! Just like you might override the cruise control in your car by pressing a little harder than it wants when approaching a hill, a pilot can physically override the auto throttles.

From the information released by the NTSB to date, no effort was made to overpower the auto-throttles during the approach. The first reported effort made to increase thrust occurred at 1.5 seconds prior to impact when the decision to go-round was made. Far, far too late.

CJ