Why does the 172RG have fuel pumps?
- Thread starter dmspilot
- Start date
Greg Bockelman
Touchdown! Greaser!
- Joined
- Feb 23, 2005
- Messages
- 11,096
- Location
- Lone Jack, MO
- Display Name
Display name:
Greg Bockelman
Might possibly be because of the higher horsepower engine requiring a tad higher fuel flow at takeoff power settings.
Capt. Geoffrey Thorpe
Touchdown! Greaser!
- Joined
- Jun 7, 2008
- Messages
- 15,643
- Location
- DXO124009
- Display Name
Display name:
Light and Sporty Guy
The RG's have a 180HP engine and constant speed but it's not injected (O-360).
tinerj
Cleared for Takeoff
- Joined
- Jul 1, 2012
- Messages
- 1,350
- Display Name
Display name:
tinerj
When a C150 is converted from 100hp to 150hp, the mod requires a fuel pump.
The reason is that it still has the same size diameter fuel lines and the smaller diameter requires a fuel pump, although it isn't actually needed, but makes the FAA happy.
Probably the same with the RG vs reguar 172.
The reason is that it still has the same size diameter fuel lines and the smaller diameter requires a fuel pump, although it isn't actually needed, but makes the FAA happy.
Probably the same with the RG vs reguar 172.
gismo
Touchdown! Greaser!
Probably got a pump due to problems with vapor lock during certification testing.
Pilawt
Final Approach
- Joined
- Sep 19, 2005
- Messages
- 9,481
- Display Name
Display name:
Pilawt
I seem to remember that the presence of the nose wheel well in the 172RG makes the arrangement of carburetor and fuel lines in the engine compartment different from fixed-gear 172s, making gravity feed alone unreliable at high power and high AOA.
All Cardinals, even the 150 hp fixed-gear version, have aux fuel pumps, as their tanks are lower and further aft relative to the engine than in the 172.
Skylane81E
Final Approach
172RGs have side draft carbs so the float bowl isn't as low,
Add that to the list
Though my money is on certification, a lower max fuel flow vs max consumption is allowed if you have a pump. 150% for pumps and 200% gravity feed IIRC
Add that to the list
Though my money is on certification, a lower max fuel flow vs max consumption is allowed if you have a pump. 150% for pumps and 200% gravity feed IIRC
At the FSDO in 2006 during my CFI checkride Oral..the chain-smoking Saftey Inspector asked me this very question...I had no answer..
He explained that Cessna and the FAA wanted the 172RG to have everything that every other Complex trainer on the market had...which at the time was all low wing stuff that required a fuel pump...basically it was a for show item...*according to the LAS FSDO*
He explained that Cessna and the FAA wanted the 172RG to have everything that every other Complex trainer on the market had...which at the time was all low wing stuff that required a fuel pump...basically it was a for show item...*according to the LAS FSDO*
poadeleted21
Touchdown! Greaser!
- Joined
- Aug 18, 2011
- Messages
- 12,332
It's type certified on the 175's certificate...dunno if that means anything.
And to be clear for all a 172RG is a Cessna 175 fuselage...if you look close you can easily tell its different
hindsight2020
Final Approach
- Joined
- Apr 3, 2010
- Messages
- 6,733
- Display Name
Display name:
hindsight2020
You beat me to it. I was going to throw that theory on here in jest and lookie lookie, turns out it's true. Further proof these things have no practical economy over checking a box on a certificate application.
hindsight2020
Final Approach
- Joined
- Apr 3, 2010
- Messages
- 6,733
- Display Name
Display name:
hindsight2020
I thought it was only the wings that were 175 typed. The cowling always did seem longer and shaped different than the welded gear 172, so perhaps there are other differences indeed.
dmspilot
Final Approach
- Joined
- Oct 20, 2006
- Messages
- 5,867
- Display Name
Display name:
DISPLAY NAME
No consensus it seems, but some very interesting ideas. Thanks everyone.
Did he give you a pink slip or a white slip? I'm actually getting ready for my CFI checkride. Hopefully next month.
Did he give you a pink slip or a white slip? I'm actually getting ready for my CFI checkride. Hopefully next month.
...prepare and your fine.
Pilawt
Final Approach
- Joined
- Sep 19, 2005
- Messages
- 9,481
- Display Name
Display name:
Pilawt
Aft of the firewall, and other than the wheel wells, where is the 172RG fuselage any different from that of a C-172N? For that matter, how was a 1962 C-175C fuselage any different aft of the firewall from a 1962 C-172C? Or a fuselage of a '63 C-P172D (which was the re-named carry-over from the C-175 series) from a '63 C-172D? (Yes, there were substantial differences in C-172 and C-175 fuselages before 1961.)
172RG is six inches longer overall than 172N. One or two inches of that are the spinner, which has to fit over a C/S prop hub; the rest are the cowl. Retracted nose gear and associated mechanism ain't gonna fit in a stiffleg 172's cowl.
Last edited:
dmspilot
Final Approach
- Joined
- Oct 20, 2006
- Messages
- 5,867
- Display Name
Display name:
DISPLAY NAME
I just checked and you're right: a straight-leg 182 does not have fuel pumps, but a 182RG does. So there's a pattern.
Pilawt
Final Approach
- Joined
- Sep 19, 2005
- Messages
- 9,481
- Display Name
Display name:
Pilawt
Exactly, and that is because the carbs of the 172RG and R182 are different from those of their fixed-gear counterparts, in order to make room for the retracted nosewheels.
[By the way, while the retractable 182 is a Model R182, the retractable 172 is a Model 172RG (likewise Model 177RG). The Model R172 is the fixed-gear version with Continental IO-360 engine (T-41B, Reims Rocket, Hawk XP). The only consistent thing about Cessna's model number protocol is inconsistency.]
Nice story, but I don't buy it. They would not unnecessarily complicate the fuel system with pump, extra plumbing, gauge, circuit breaker, etc., just for "show."
If they wanted a fuel pump switch only as a procedure training device, they could have installed a dummy $5 rocker switch on the panel. That's not an unprecedented concept; dummy landing gear switches were factory options on early fixed-gear Beech Musketeers and Champion Lancers.
Huh. I didn't know that straight leg 182's didn't have fuel pumps.
Well, the R182 has a different engine than the straight leg. It uses a Lycoming O-540 instead of the Continental that the other 182s used at the time. That is one of the reasons why it has such odd parts on it.. like I have a Continental mag on it that has some Lycoming fittings. IIRC the carb is the same way.
nddons
Touchdown! Greaser!
Injected 172-Rs and -SPs are injected and have fuel pumps, but are used only for priming the engine for starting.*
EDIT: And also on the emergency checklist for engine failure during flight.
Last edited:
Capt. Geoffrey Thorpe
Touchdown! Greaser!
- Joined
- Jun 7, 2008
- Messages
- 15,643
- Location
- DXO124009
- Display Name
Display name:
Light and Sporty Guy
No engine driven pump?
Which is why it's on the 175 type certificate and not the 172 one.
nddons
Touchdown! Greaser!
Yes they do. I was talking about the auxiliary fuel pump (electric) used for priming the engine, and for emergency if there is a loss of engine during flight.
Well that all according to SI Marty Kay at the Las Fsdo ...I'd have to think he's somewhat credible
Dan Thomas
Touchdown! Greaser!
- Joined
- Jun 16, 2008
- Messages
- 10,781
- Display Name
Display name:
Dan Thomas
That 540 has its carb on the back of the sump, considerably higher than the bottom-mounted carbs on many other Lycs or Continentals. Besides that, the old bottom-mounted carbs (MA3's and MA-4's) were designed for gravity flow or low-pressure pumps, and had a large float valve to offer less restriction to low-pressure fuel flow. I think the HA-6 used on the 182RG has a smaller float valve, like most automobile carbs that were designed for 7 psi or so of fuel pressure, so it would need a fuel pump, and any engine-driven pump needs a backup pump.
Dan
GCA319
Cleared for Takeoff
Isn't it only to be used when fuel pressure gets below a certain level (.5 psi rings a bell)?
Dan Thomas
Touchdown! Greaser!
- Joined
- Jun 16, 2008
- Messages
- 10,781
- Display Name
Display name:
Dan Thomas
At the density of gasoline, and the approximate 19" of fuel head in a 172 when in the Vx climb, I calculate a static gravity fuel pressure of about 0.4 psi. Under dynamic flow conditions that pressure will be lower. That valve had better be able to flow plenty of fuel at full throttle.
Dan
Pilawt
Final Approach
- Joined
- Sep 19, 2005
- Messages
- 9,481
- Display Name
Display name:
Pilawt
Yes. From the C-172RG POH:
[FONT=Verdana, sans-serif]The auxiliary fuel pump switch is located on the left side of the switch and control panel and is a rocker-type switch. It is labeled AUX FUEL PUMP. When the pump is operating, it will maintain fuel pressure to the carburetor. It should be used whenever the indicated fuel pressure falls below 0.5 PSI, but is not required when gravity flow and/or the engine- driven fuel pump can maintain indicated pressures above 0.5 PSI.[/FONT]
GCA319
Cleared for Takeoff
Thanks.
spinfire
Pre-takeoff checklist
- Joined
- Mar 15, 2011
- Messages
- 413
- Display Name
Display name:
Dan
I did my complex training in a 172RG Cutlass. According to the flight school's checklist and SOP I only activated the aux pump once per flight, during the run-up. This was to test the aux pump for functionality by looking for a slight rise in fuel pressure while the pump was momentarily on. In theory the aux pump would be used only as needed if the engine driven pump failed to operate adequately.
Yep. IIRC my 182RG reads about 3-4 PSI at idle during the runup. The aux pump brings it up (with a horrible noise I might add, is that normal?) and holds it up until I turn it off. End aux fuel pump.
nddons
Touchdown! Greaser!
I agree, that is a nasty sound.
I'll look next time.
I'm glad it's not just my airplane that makes it. It sounds like I ran over a cat or something.
Skylane81E
Final Approach
Try a new injected 172, or the high speed setting on the dukes pump in our diamond.