737 Fuel Flow switch and AIR COND questions
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737 Fuel Flow switch and AIR COND questions
Hi!
Undergoing the type rating, I've had a couple questions that the SFI's havn't been able to answer fully. They may not be entirely necessary to know, but it would be nice to get a definitive answer (preferably with reference to official documents).
1. One of the procedures we use before transfering fuel is to check for possible leakage by switching the fuel flow switch to USED and crosscheck if it is correct in relation to what was planned for that stage (Xcheck w/ PLOG). In the lighter aircraft I used to fly, we had the same kind of switch, but it would show us a figure that was derived from the fuel flow transducers, and not the difference in actual tank quantity, which would mean that using this feature wouldn't indicate a leakage even if there was one. So does anyone know where the fuel flow switch gets its information from when moved to the USED position in the 737?
2. When checking takeoff N1 settings, we must turn APU bleed air on in order to not get BLEEDS OFF takeoff figures. This does not make sence entirely to me, as you could very well take off with the APU supplying bleed air with engine bleed air off, and therefore thrust not being affected. What actually prompts the FMC to indicate BLEEDS ON N1 settings? Someone told me that it was when the manifold was pressurized, but the person telling me this wasn't entirely sure. Anyone have an idea?
Undergoing the type rating, I've had a couple questions that the SFI's havn't been able to answer fully. They may not be entirely necessary to know, but it would be nice to get a definitive answer (preferably with reference to official documents).
1. One of the procedures we use before transfering fuel is to check for possible leakage by switching the fuel flow switch to USED and crosscheck if it is correct in relation to what was planned for that stage (Xcheck w/ PLOG). In the lighter aircraft I used to fly, we had the same kind of switch, but it would show us a figure that was derived from the fuel flow transducers, and not the difference in actual tank quantity, which would mean that using this feature wouldn't indicate a leakage even if there was one. So does anyone know where the fuel flow switch gets its information from when moved to the USED position in the 737?
2. When checking takeoff N1 settings, we must turn APU bleed air on in order to not get BLEEDS OFF takeoff figures. This does not make sence entirely to me, as you could very well take off with the APU supplying bleed air with engine bleed air off, and therefore thrust not being affected. What actually prompts the FMC to indicate BLEEDS ON N1 settings? Someone told me that it was when the manifold was pressurized, but the person telling me this wasn't entirely sure. Anyone have an idea?
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#1. An interesting procedure. Not heard of that one. I have my own method of leak determination before balancing. And I never flew with an airline that had an SOP for such. They rather naively, in today's market, relied on airmanship. And loom where that got the A330.
To answer the direct question; is there not a fuel totaliser computer gizmo thingie?
#2. The bleed switches, or one bleed switch, should be in the planned take off position to give the correct % expected to be seen on the N1% gauge and cross checked with the FMC Takeoff indication.
If I remember correctly the FMC receives the Bleeds On/Off information from the Bleed switch. At this stage of flight preparation one would expect to have the APU running for cabin air con'. Thus the manifold is pressurised, but I still think it is from the Bleed switch with pressure or not.
To answer the direct question; is there not a fuel totaliser computer gizmo thingie?
#2. The bleed switches, or one bleed switch, should be in the planned take off position to give the correct % expected to be seen on the N1% gauge and cross checked with the FMC Takeoff indication.
If I remember correctly the FMC receives the Bleeds On/Off information from the Bleed switch. At this stage of flight preparation one would expect to have the APU running for cabin air con'. Thus the manifold is pressurised, but I still think it is from the Bleed switch with pressure or not.
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1. It's basically information from a fuel summation unit, that integrates the fuel flow over time. Essentially what you see there is how much the fuel that has passed the HMU. You could have a leak before that, that wouldn't show on the FUEL USED, so good old airmanship is still required.
2. The N1 percentage on N1 LIMIT / TAKE OFF 1 page is displayed with regards to the actual pressure in the manifold, pack switches and engine bleed air switches positions.
2. The N1 percentage on N1 LIMIT / TAKE OFF 1 page is displayed with regards to the actual pressure in the manifold, pack switches and engine bleed air switches positions.
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1) The fuel used is measured at the engine by HMU. Thus you can see how much fuel has been consumed and compare that to how much fuel has left the tank. A (large) discrepancy can indicate a leak. This also explains the discrepancy when adding fuel used and fuel in tank together as the fuel between tank and engine is not accounted for and when engine shuts down will return to tank (small increase after shutdown in each tank)
2) There is a solenoid called the FMC "C" Solenoid Discreet which signals to the FMC the position of the BLEED AIR VALVE (not the BLEED AIR switch position on the overhead panel), resultant of the indicated position the FMC indicates N1% for the respective side. There is NO need to have bleed air on for calculations of N1%, as a simple reduction for bleed demand can be made when bleed air is switched off (-0.8 for full thrust, -0.7 for reduced thrust and -0.6 for assumed thrust).
A failure of the Solenoid C can have either of two faults: failed in ON or failed in OFF position (referring to BLEED AIR VALVE ON/OFF).
Imagine a situation here FMC "thinks" packs are ON thus reduces N1% to be set by auto-throttle, which in case of BLEED OFF performance requirement would thus be insufficient. Result is that FMC Solenoid C has failed only PACK ON departures can be done.
More critical is the alternative situation where pack are thought to be OFF thus higher N1 are set through FMC signal to auto-throttle, thus potential for over boosting the engine exists. In this scenario it is thus forbidden to use the auto-throttle during FULL power on engines, such as Maximum Continuous Thrust, Take Off, Go Arounds and Climb, thus these operational limits apply.
Identifying the correct failed signal is vital and can easily be done to look in MAINT pages for APU > INPUT MONITORING and look at PACK SWITCH position sensed by FMC and comparing it when cycling through the positions on overhead panel, they should agree when making changes. If solenoid C discreet signal to FMC is correct but FMC indicates pack ON or OFF N1 continuously when switching ON/OFF PACK switches with a pressurised system, it is most likely a sticky Bleed Air Valve requiring some mechanical attention (lubricating)
2) There is a solenoid called the FMC "C" Solenoid Discreet which signals to the FMC the position of the BLEED AIR VALVE (not the BLEED AIR switch position on the overhead panel), resultant of the indicated position the FMC indicates N1% for the respective side. There is NO need to have bleed air on for calculations of N1%, as a simple reduction for bleed demand can be made when bleed air is switched off (-0.8 for full thrust, -0.7 for reduced thrust and -0.6 for assumed thrust).
A failure of the Solenoid C can have either of two faults: failed in ON or failed in OFF position (referring to BLEED AIR VALVE ON/OFF).
Imagine a situation here FMC "thinks" packs are ON thus reduces N1% to be set by auto-throttle, which in case of BLEED OFF performance requirement would thus be insufficient. Result is that FMC Solenoid C has failed only PACK ON departures can be done.
More critical is the alternative situation where pack are thought to be OFF thus higher N1 are set through FMC signal to auto-throttle, thus potential for over boosting the engine exists. In this scenario it is thus forbidden to use the auto-throttle during FULL power on engines, such as Maximum Continuous Thrust, Take Off, Go Arounds and Climb, thus these operational limits apply.
Identifying the correct failed signal is vital and can easily be done to look in MAINT pages for APU > INPUT MONITORING and look at PACK SWITCH position sensed by FMC and comparing it when cycling through the positions on overhead panel, they should agree when making changes. If solenoid C discreet signal to FMC is correct but FMC indicates pack ON or OFF N1 continuously when switching ON/OFF PACK switches with a pressurised system, it is most likely a sticky Bleed Air Valve requiring some mechanical attention (lubricating)
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Original poster: You are perhaps in danger of overcomplicating your thoughts during what is essentially a straight forward type rating. Look at it in practical terms, a fuel leak is usually represented by an imbalance and the QRH is Boeings most condensed method to address this issue. I have never come across any company procedure to check fuel used prior to crossfeed without being directed the NNC or some strange anomaly.
Regarding taking off with APU bleed supply the packs as opposed to engine bleeds, again this is a relatively rare and the performance tools you use to calculate the N1 should have the option for " Bleeds OFF" to give the appropriate target N1.
The APU bleed cannot supply the same volume of air as both engine bleeds.
Regarding taking off with APU bleed supply the packs as opposed to engine bleeds, again this is a relatively rare and the performance tools you use to calculate the N1 should have the option for " Bleeds OFF" to give the appropriate target N1.
The APU bleed cannot supply the same volume of air as both engine bleeds.
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Hi!
1. One of the procedures we use before transfering fuel is to check for possible leakage by switching the fuel flow switch to USED and crosscheck if it is correct in relation to what was planned for that stage (Xcheck w/ PLOG). In the lighter aircraft I used to fly, we had the same kind of switch, but it would show us a figure that was derived from the fuel flow transducers, and not the difference in actual tank quantity, which would mean that using this feature wouldn't indicate a leakage even if there was one. So does anyone know where the fuel flow switch gets its information from when moved to the USED position in the 737?
1. One of the procedures we use before transfering fuel is to check for possible leakage by switching the fuel flow switch to USED and crosscheck if it is correct in relation to what was planned for that stage (Xcheck w/ PLOG). In the lighter aircraft I used to fly, we had the same kind of switch, but it would show us a figure that was derived from the fuel flow transducers, and not the difference in actual tank quantity, which would mean that using this feature wouldn't indicate a leakage even if there was one. So does anyone know where the fuel flow switch gets its information from when moved to the USED position in the 737?
