what is a flapswitch?
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From what I can remember on the engineer's course, Boeing use flap position as an indicator of air-speed. I think it's something along the lines of 'Flaps Not Up - Turbo fans in packs run'. These fans cool the heat exchangers, i.e. something that flight airflow would normally do.
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B737-300
Pneumatic Control Panel (overhead panel)
and
Ram Air Inlet (left and right wing roots)
How does the pack logic know that the flaps are up or that the flaps are not retracted?
The flapswitch senses flap position and provides a "flaps up" signal to the pack logic.
Pneumatic Control Panel (overhead panel)
Air Conditioning PACK Switch :
OFF - pack signalled to OFF.
AUTO -
OFF - pack signalled to OFF.
AUTO -
- with both packs operating, each pack regulates to low flow
- with one pack operating, operating pack regulates to high flow in flight with flaps up
- when operating one pack from APU (both engine BLEED air switches OFF), regulates to high flow.
- pack regulates to high flow
- provides maximum flow rate on ground with APU BLEED air switch ON.
Ram Air Inlet (left and right wing roots)
Ram Air System
The ram air system provides cooling air for the heat exchangers. Operation of the system is automatically controlled by the packs through operation of ram air inlet doors.
On the ground, or during slow flight with the flaps not fully retracted, the ram air inlet doors move to the full open position for maximum cooling. The RAM DOOR FULL OPEN light illuminates whenever the ram door is fully open.
A turbofan is located in in each ram air exit duct just upstream of the exit louvres. It augments the ram airflow on the ground or during slow flight (flaps not retracted).
The ram air system provides cooling air for the heat exchangers. Operation of the system is automatically controlled by the packs through operation of ram air inlet doors.
On the ground, or during slow flight with the flaps not fully retracted, the ram air inlet doors move to the full open position for maximum cooling. The RAM DOOR FULL OPEN light illuminates whenever the ram door is fully open.
A turbofan is located in in each ram air exit duct just upstream of the exit louvres. It augments the ram airflow on the ground or during slow flight (flaps not retracted).
The flapswitch senses flap position and provides a "flaps up" signal to the pack logic.
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Mr(s) Yeager a most excellent explanation. Pictures help greatly for someone like me who has a vague idea about much of the mechanical stuff, instantly recognises the lay out of the various panels, but doesn't really know anything. Thank you for taking the time.
Follow up question, hoping you will indulge my curiosity.......
A flapswitch malfunctions. As per your explanation I now understand how this can cause ram air inlet doors to be not fully open (they can be fully closed as well right?) and thus no inflow of cold air into the heat exchangers. How does this cause a dual pack tripoff, and what is it please?
Follow up question, hoping you will indulge my curiosity.......
A flapswitch malfunctions. As per your explanation I now understand how this can cause ram air inlet doors to be not fully open (they can be fully closed as well right?) and thus no inflow of cold air into the heat exchangers. How does this cause a dual pack tripoff, and what is it please?
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Flaps - I cannot provide the technical reason for a double pack trip in this situation, except that a few years ago at top of climb out of BRU to LGW I, (RHS) observed to the boss, (LHS) - "look, both 'ram doors full open lights are on' - that's unusual"
Checklist out and at the same time the left pack tripped. "ooh , we said"
30 seconds later, right pack tripped. Double "ooh" and down we went. Both packs reset almost immediately in the drills. As CY says, it could well have beeen the flap sensor switch.
Pack trip, as I understand it, is caused by excessive pack activity, and in this case, again as I understand it, the 'Ram door full open' in the cruise means excessive pre-cooling for the 'cold' air going into the pack. I cannot reconcile the logic of the double trip, perhaps one of our systems experts can? We never saw the results of the tech follow-up on that one, but, yes, it did happen to me!
Edited to avoid confusing 'flaps' about her 'switch' due to silly mistake in my post (100 Mea Culpas!) thanks CY for that.
Checklist out and at the same time the left pack tripped. "ooh , we said"
30 seconds later, right pack tripped. Double "ooh" and down we went. Both packs reset almost immediately in the drills. As CY says, it could well have beeen the flap sensor switch.Pack trip, as I understand it, is caused by excessive pack activity, and in this case, again as I understand it, the 'Ram door full open' in the cruise means excessive pre-cooling for the 'cold' air going into the pack. I cannot reconcile the logic of the double trip, perhaps one of our systems experts can? We never saw the results of the tech follow-up on that one, but, yes, it did happen to me!
Edited to avoid confusing 'flaps' about her 'switch' due to silly mistake in my post (100 Mea Culpas!) thanks CY for that.
Last edited by BOAC; 20th Jul 2004 at 08:32.
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B737-300
Q: dual pack tripoff - what is it please?
A PACK TRIP OFF is when the temperature inside the air conditioning pack has exceeded a temperature limit.
This usually occurs when the pack is "working hard" - ie trying to reduce the temperature of the air a lot.
A PACK TRIP OFF could happen for a number of reasons - most usually on a very warm day with a cold cabin temperature selected :
--> very warm air into pack + very cold air out of pack = high temperature inside the pack = PACK TRIP OFF <--
It could also happen due to a reduced ram airflow for cooling - eg because the RAM AIR DOORS are not FULL OPEN when they should be FULL OPEN, or because the turbofans are not working when they should be working. A faulty flap position sensing switch could cause these kinds of problems.
The problem with a PACK TRIP OFF is that the affected airconditioning pack is no longer supplying air to pressurise the cabin - the related pack valve automatically closes = no air going into pack so therefore no air coming out of pack. With a single PACK TRIP OFF the other pack can provide sufficient air to keep the aircraft pressurised.
If both packs suffer a PACK TRIP OFF at the same time (a "dual pack trip off") then there is no airflow coming into the cabin to keep the aircraft pressurised. The pressurisation outflow valve will modulate towards closed to try to maintain pressurisation but all aircraft have small leaks and eventually the aircraft will be depressurised - ie a descent to a lower altitude will be required, maybe even an emergency descent if it is an older aircraft with a high leak rate.
The PACK TRIP OFF non-normal checklist calls for a WARMER temperature to be selected (so the pack is not "working so hard") and pressing the TRIP RESET button (on the pneumatic control panel). If the temperature inside the pack has cooled sufficiently then the PACK TRIP will be cleared and the pack will resume operation. If the temperature inside the pack has not cooled sufficiently then the pack will not reset.
A PACK TRIP OFF does not mean that air coming out of the pack has exceeded a temperature limit - that would be a DUCT OVERHEAT - see the two DUCT OVERHEAT amber lights on air conditioning panel :
Air conditioning panel (overhead panel)
Pressurisation outflow valve (lower right fuselage below the aft service door)
Q: dual pack tripoff - what is it please?
PACK TRIP OFF Light
(two amber lights on pneumatic control panel - see photo in my previous post)
Illuminated (amber) -
(two amber lights on pneumatic control panel - see photo in my previous post)
Illuminated (amber) -
- indicates pack temperature has exceeded limits
- related pack valve automatically closes and mix valves drive full cold
- requires reset.
This usually occurs when the pack is "working hard" - ie trying to reduce the temperature of the air a lot.
A PACK TRIP OFF could happen for a number of reasons - most usually on a very warm day with a cold cabin temperature selected :
--> very warm air into pack + very cold air out of pack = high temperature inside the pack = PACK TRIP OFF <--
It could also happen due to a reduced ram airflow for cooling - eg because the RAM AIR DOORS are not FULL OPEN when they should be FULL OPEN, or because the turbofans are not working when they should be working. A faulty flap position sensing switch could cause these kinds of problems.
The problem with a PACK TRIP OFF is that the affected airconditioning pack is no longer supplying air to pressurise the cabin - the related pack valve automatically closes = no air going into pack so therefore no air coming out of pack. With a single PACK TRIP OFF the other pack can provide sufficient air to keep the aircraft pressurised.
If both packs suffer a PACK TRIP OFF at the same time (a "dual pack trip off") then there is no airflow coming into the cabin to keep the aircraft pressurised. The pressurisation outflow valve will modulate towards closed to try to maintain pressurisation but all aircraft have small leaks and eventually the aircraft will be depressurised - ie a descent to a lower altitude will be required, maybe even an emergency descent if it is an older aircraft with a high leak rate.
The PACK TRIP OFF non-normal checklist calls for a WARMER temperature to be selected (so the pack is not "working so hard") and pressing the TRIP RESET button (on the pneumatic control panel). If the temperature inside the pack has cooled sufficiently then the PACK TRIP will be cleared and the pack will resume operation. If the temperature inside the pack has not cooled sufficiently then the pack will not reset.
A PACK TRIP OFF does not mean that air coming out of the pack has exceeded a temperature limit - that would be a DUCT OVERHEAT - see the two DUCT OVERHEAT amber lights on air conditioning panel :
Air conditioning panel (overhead panel)
Pressurisation outflow valve (lower right fuselage below the aft service door)
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CY - thanks for the correction
, I knew that really - honest! I have gone back and edited my post to correct to avoid confusing the lovely lady! Excellent piccies, BTW.
Any ideas why our particular 'event' happened'? There would be no need for any excessive pack activity to warm the cabin, as the mix valve should just add more 'hot'? Could there have been an icing problem somewhere in the packs due to the extra pre-cooling?
, I knew that really - honest! I have gone back and edited my post to correct to avoid confusing the lovely lady! Excellent piccies, BTW.Any ideas why our particular 'event' happened'? There would be no need for any excessive pack activity to warm the cabin, as the mix valve should just add more 'hot'? Could there have been an icing problem somewhere in the packs due to the extra pre-cooling?
Last edited by BOAC; 20th Jul 2004 at 08:33.
