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747 Classic Autopilot And Autothrottle  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (3 years 7 months 4 days 17 hours ago) and read 5483 times:

Hello,

Hoping someone can help me with some questions on the 747 classic AP and AT. Was just watching a cockpit DVD recently and on the MCP panel, both sides had what looked like a VS roll dial but it said Flight Director pitch control. Is this a form of CWS or was it literally to set the pitch bar on the FD, if so what was the reason behind this?

In regards to the A/T system, did not all the classics have it? On one of the AC in the DVD they had an approach AT only, am I safe in thinking that it was for auto land, if so is this a requirement for auto land? On the throttle quadrant they had TOGA switches, in the auto land situation would they actually give you GA thrust? and in the manual mode would they just position the FD?.

Any info would be greatly appreciated.

Many Thanks in advance
Steve

8 replies: All unread, jump to last
 
User currently offlinejetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 1, posted (3 years 7 months 4 days 13 hours ago) and read 5426 times:

Steve,

What looks like a VSI wheel positions the FD pitch bar. Each pilot can do this independently. It allows the pilot to set a pitch attitude reference for manual flight if no other pitch mode is selected on the MCP. You could use it as a pitch target for takeoff rotation, for example.

I thnk most if not all 747 Classics had A/T but the older ones were for approach use only. These were not very sophisticated and held airspeed to the value set on the MCP. From memory the highest speed which could be selected with such a system was 250 knots. Later 747 classics had FFRATS (full flight regime autothrottle system), and this could be used for takeoff, climb, cruise, approach and landing. A separate computer calculated thrust limits according to ambient conditions and selected thrust mode. Some 747s had a PMS (performance management system) which computed the thrust and speed targets in a more integrated way, rather like the present VNAV systems.

There are no TOGA switches as such, but there are go-around switches on the rear of throttles 2 and 3. If a go around switch is operated while on approach, the FD will demand a pitch angle of 5 deg nose up (Flaps > 23) or 13.9 deg otherwise. If the aircraft has FFRATS engaged the thrust levers will drive to GA thrust.

For takeoff using FFRATS, the crew would first select TO thrust on the pedestal thrust mode select panel, along with any derate. The engines would be spooled up and when stable the A/T selected ON at the MCP. Thrust levers then advance to the rated takeoff thrust. The F/E would always manually follow up on the levers in case one didn't advance with the others. As the 747 only has one A/T motor driving the levers through friction clutches this was a distinct possibility.

[Edited 2011-05-20 10:34:44]


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently onlineMax Q From United States of America, joined May 2001, 4779 posts, RR: 19
Reply 2, posted (3 years 7 months 4 days 10 hours ago) and read 5360 times:

I didn't ask the question of course but thanks for a great and informative reply.


The best contribution to safety is a competent Pilot.
User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 3, posted (3 years 7 months 4 days 3 hours ago) and read 5266 times:

As usual jetlagged post is very accurate.

On the classics there are two types of A/T: Approach A/T System & Full Flight Regime A/T System.
Approach A/T moves the thrust levers to acquire/maintain the preselected airspeed, only during approach & landing phases. Whenever more than one autopilot channel is engaged it will also retard the levers at 30 ft AGL

Quoting jetlagged (Reply 1):
These were not very sophisticated and held airspeed to the value set on the MCP.

..the selection range is 101 to 259 knots. When the selected speed is more than 259 the first two digits are covered and only the third one remains. However the Command Airspeed Bug in the ASI will continue to indicate until the upper stop (395 kt)
No protection whatsoever is provided by this system. Engine/airframe limitations have to be carefully monitored.

On airframes equipped with FFRATS things are a bit different.
The A/T acquire/maintain a computed max EPR, selected airspeed or a selected Mach number.
In this case the selected range of speeds is 100 to 400 knots and the system continuously calculates the minimum safe airspeed based on the existing flap position.
On the ground, selection of Mach & speed mode is inhibited and only EPR mode is available. prior to engagement, EPR Limit for take off should be selected from dry or wet rating (on engines -7Q wet rating is not available) and then derated if necessary.
The system drives all thrust levers forward until the engine with the highest EPR reaches the calculated limit, throttle hold activated at 80 knots and from that point on the A/T remains engaged but doesn't move the levers. (Which is why the F/E always retains positive control of the engines as he still have to trim/set TO thrust on the other three engines, before reaching the 80 knots call out)
At the climb thrust call, A/T is set to climb and drive the levers to the climb EPR limit based on TAT.
Reaching cruise altitude, cruise mode is selected until the airplane accelerates to the mach number calculated. In cruise the A/T can be used in Mach or Airspeed mode.
Speed mode is normally used for descent, approach/landing & holding.
Once G/S is captured and flaps are in beyond 23 degrees G/A mode (EPR limit) is armed and the system will automatically retard the levers at 30 ft during the flare (Flare Mode) and disengage when the thrust reversers are deployed.
Switches in levers 1 & 4 are for disengage the A/T and in levers 2 & 3 to activate the G/A Mode (if it was armed)

On airplanes with PMS, the A/T + PMS + A/P combo provides pitch/throttle control with improved fuel efficiency for the operation. (too bad we can't use PMS vertical mode anymore)
A truly remarkable design although very outdated by today standards. Nevertheless, a joy to use. (When it works properly!)

Regards,
B747FE.



"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
User currently onlineMax Q From United States of America, joined May 2001, 4779 posts, RR: 19
Reply 4, posted (3 years 7 months 4 days 3 hours ago) and read 5255 times:

Very interesting B747FE,


I assume the reason you cant use PMS vertical mode anymore is RVSM not allowing any deviation from assigned
altitude ?



The best contribution to safety is a competent Pilot.
User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 5, posted (3 years 7 months 4 days 3 hours ago) and read 5250 times:

Quoting Max Q (Reply 4):
I assume the reason you cant use PMS vertical mode anymore is RVSM not allowing any deviation from assigned
altitude ?

Absolutely correct.
In PMS cruise mode the A/P will hold altitude plus-minus 120 ft which basically allows the airplane to slowly climb/descent within that tolerance to smooth out speed changes.
Less throttle activity = less fuel consumption

Regards,
B747FE.



"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
User currently onlineMax Q From United States of America, joined May 2001, 4779 posts, RR: 19
Reply 6, posted (3 years 7 months 4 days 3 hours ago) and read 5249 times:

Quoting B747FE (Reply 5):

Absolutely correct.
In PMS cruise mode the A/P will hold altitude plus-minus 120 ft which basically allows the airplane to slowly climb/descent within that tolerance to smooth out speed changes.
Less throttle activity = less fuel consumption

Thanks, always enjoy your posts.



The best contribution to safety is a competent Pilot.
User currently offlineThrottleHold From South Africa, joined Jul 2006, 659 posts, RR: 1
Reply 7, posted (3 years 7 months 3 days 22 hours ago) and read 5184 times:

Quoting B747FE (Reply 5):

Quoting Max Q (Reply 4):
I assume the reason you cant use PMS vertical mode anymore is RVSM not allowing any deviation from assigned
altitude ?

Absolutely correct.
In PMS cruise mode the A/P will hold altitude plus-minus 120 ft which basically allows the airplane to slowly climb/descent within that tolerance to smooth out speed changes.
Less throttle activity = less fuel consumption

Regards,
B747FE.

As far as I recall, there's another reason too. The VNAV function of the PMS is designed to disengage at low altitude, i.e when the Rad Alt comes alive at 2500 ft. If in cruise in RVSM, another aircraft 2000 ft below on the same airway would cause the rad alt to show 2000 ft and therefore disconnect the PMS VNAV.


User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 8, posted (3 years 7 months 3 days 22 hours ago) and read 5170 times:

Quoting ThrottleHold (Reply 7):
As far as I recall, there's another reason too.

Yes, on climb at or above 2,500 ft PMS control can be engaged and descent can be continued under PMS control to 2,500 ft as well.
However, the radio altimeter disconnect signal is inhibited by the A/T computer above 16,000 ft.
This feature was incorporated by a SB modification, perhaps in response to the issue you mentioned, on earlier FFRATS.



Quoting Max Q (Reply 6):
Thanks, always enjoy your posts.

Likewise Max Q

Regards,
B747FE.



"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
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