BuzzBox;
From the Boeing Flight Crew Training Manual for the 777:
Yes, understand the AOM remarks, thank you - I don't want to take the thread off on a tangent because frankly I don't think there is any low-hanging fruit in the wind-shear/airspeed/energy line of thinking. This was not a significant windshear situation and there was no evidence in the METARS of convective activity at the time.
I
very much want to stay away from Airbus-Boeing comments at all times! I am just trying to clarify my understanding of the two different a/t systems.
In the 777, the a/t will:
- fly the command speed which can be either be calculated by the FMC according to weight, landing flap config etc, or it can be modified by the crew according to the standard formula for such occasions, (Vref plus half the wind, plus all the gust up to a max of 20kts, or slight variations on the theme);
- In other words, if Vref is 145kts making the approach speed Vref + 5, (command speed) 150kts, that's what the a/t's will fly, and if gusts or shear causes the aircraft to lose speed below that command speed, they will (very swiftly, the AOM indicates) add thrust to maintain the command speed of 150kts.
- Groundspeed is not sensed for this purpose, so in a 20kt headwind component, the groundspeed of the aircraft would be 130kts.
The Airbus 320/340 GSMini autothrust mode will however, command sufficient airspeed so as to maintain the groundspeed equal to the approach speed set in the FMC.
- If Vapp (Vref + 5) on a 340 is 150kts and that is the approach speed calculated by or entered into the FMC and there is a 20kt headwind component, the a/t system will fly the approach to maintain 150kts groundspeed. The resulting airspeed could be as high as 170kts, (getting close to flap limiting speed). GSMini mode is entered when final landing flap configuration is set.
The a/t system is sufficiently aggressive so as to make quick modifications to the groundspeed flown so as not to compromise the landing distances required. The GSMini can move up and down the airspeed scale quite rapidly in gusty conditions or as strong winds shear out closer to the ground.
We used the technique decades ago in the '8 when INS was first introduced and I'm sure many here will be familiar with it.
Again, I do NOT want to cause thread drift or even hint that there is an Airbus/Boeing discussion here - I know there is the potential to do so in these remarks. There is no such intent - I just wanted to understand how the 777 a/t system behaved so I could better understand aspects of the aircraft's energy level. As I say, I don't think these areas of speculation will be fruitful. Thanks for your response.
PJ2