24 May 2025, 17:32 [ UTC - 5; DST ]
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Post subject: Re: Jets with normal cruise of 0.7 mach or better?  Posted: 15 Oct 2022, 19:21 |
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Joined: 03/28/17
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Username Protected wrote: Don, are you referring to the Baltic Sea 551 crash? That plane was climbing at 1000 FPM through FL 220 and was cruising at FL 360 and 360 KT ground speed for hours until it ran out of fuel. A 551 can't do that on one engine. https://globe.adsbexchange.com/?icao=44 ... 1660909871No, just referring to my own experience. Didn’t really think about it at the time, lots of alarms going off. Just wondered if the cabin altitude would stay safe.
The old Lears with the GE 610 engines didn't have a lot of extra bleed air for pressurization up in the 40's flight levels. The Lear 35's with the Garrett 731 fan engines had a lot of reserve bleed air, and as I recall could maintain a 14,000 foot cabin altitude up in the 40's with a cabin window blown out, directing all the bleed air to the cabin.
It varies, but most jet engine flame outs at high altitude don't re-light very well until drifting down to 25 or 30 in the re-light envelope.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 15 Oct 2022, 21:22 |
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Joined: 12/03/14
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Username Protected wrote: The Lear 35's with the Garrett 731 fan engines had a lot of reserve bleed air, and as I recall could maintain a 14,000 foot cabin altitude up in the 40's with a cabin window blown out, directing all the bleed air to the cabin. There's no chance that is correct. A hole the size of a window with 6 PSI drop across it (14,000 ft to FL400) would require abut 25,000 SCFM of air, or about one ton of air per minute. Just no way the engine can deliver that much air, maybe not even out the tailpipe. Mike C.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 15 Oct 2022, 21:32 |
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Username Protected wrote: It varies, but most jet engine flame outs at high altitude don't re-light very well until drifting down to 25 or 30 in the re-light envelope. My JT15D-5A airstart envelope goes up to FL350 per the manual. At the higher altitudes, the minimum airspeed does increase (160 KIAS at FL350). Untested by myself, and I hope to never need it. Mike C.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 15 Oct 2022, 22:21 |
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Joined: 08/07/09
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Username Protected wrote: The baby bus (320) is decidedly pokey. .80, maybe. you can select speed .99 in the 320 It’ll stop at .82 though. But it can do that no problem in cruise. If you ask.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 16 Oct 2022, 00:22 |
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Username Protected wrote: The Lear 35's with the Garrett 731 fan engines had a lot of reserve bleed air, and as I recall could maintain a 14,000 foot cabin altitude up in the 40's with a cabin window blown out, directing all the bleed air to the cabin. There's no chance that is correct. A hole the size of a window with 6 PSI drop across it (14,000 ft to FL400) would require abut 25,000 SCFM of air, or about one ton of air per minute. Just no way the engine can deliver that much air, maybe not even out the tailpipe. Mike C.
I don't know; I doubt that the statement passed down to me has ever been demonstrated, but I'm not ready to call it impossible; big engines, little windows. 
As for the tail pipe statement, at roughly 3700 pounds of thrust with a roughly 3:1 bypass ratio creating about 2600 pounds of thrust for two engines, 1300 pound of thrust each out the tail pipe, it would seem to be enough volume to blow more windows out, but I'm not an engineer, and we are talking about maximum available bleed air flow for two engines.
If your calculations are correct about required cubic feet per minute, each engine would need to produce 12,000 CFM emergency bleed air, and I have no idea if they could. The actual size of a Lear 35 cabin window would have to be known for the calculation.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 12:23 |
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Username Protected wrote: The other difference between 100% and EMER is that EMER Is 100% O2 under slight pressure so that, when you breath in, O2 is “forced” in to the lungs. That pressure is quite small and probably negligible physiologically. The EMER mode is stated to assure you do not take in any external air from the cabin, and thus used in cases of cabin smoke or contamination. It is not necessary for keeping you alive oxygen wise. My mask as a way to vent that into my smoke googles for the same reason, to exclude smoke. Mike C.
The EMER/pressure mode is used to keep you alive at very high altitudes. The difference is not negligible at all.
You do not have enough O2 pressure in your lungs at extreme altitudes to create an oxygen exchange across the membranes of the lungs. The mask pressure is required to create the differential necessary for O2 exchange.
Even on 100% O2, above 35,000 ft the reduction in rate of O2 exchange into your capillaries is measurable. The rate drops below the consciousness-sustaining limit somewhere in the 40s, depending on the subject.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 13:15 |
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Username Protected wrote: The Lear 35's with the Garrett 731 fan engines had a lot of reserve bleed air, and as I recall could maintain a 14,000 foot cabin altitude up in the 40's with a cabin window blown out, directing all the bleed air to the cabin. There's no chance that is correct. A hole the size of a window with 6 PSI drop across it (14,000 ft to FL400) would require abut 25,000 SCFM of air, or about one ton of air per minute. Just no way the engine can deliver that much air, maybe not even out the tailpipe. Mike C.
Typically in the propulsion business, flow rates are measured in mass/time. Here in the US, lbm/sec. The outside air pressure at FL 400 is about 2.7 psi, and at FL 140 the pressure is about 8.6 psi. So there is a pressure ratio of about 3.2, so the flow would be choked. At those conditions the flow would be about 1.55 (lbm/sec)/in^2. If the window was 10" by 10" that would mean 155 lbm/sec of air would be exiting the aircraft.
The Honeywell 731 engine has a total mass flow of about 115 lb/sec. Two of them so a grand total of 230 lbm/sec. Only a small percentage is used for bleed. Even 10% bleed which would be quite high, only yields 23 lbm/sec. In other words, you will have a difficult time breathing without oxygen.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 15:02 |
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Username Protected wrote: The EMER/pressure mode is used to keep you alive at very high altitudes. The difference is not negligible at all. That's not how the mask manual states to use it. EMER is for contaminated air in the cabin, not high altitude flight. --- • In Emergency mode the regulator supplies an oxygen overpressure. • It prevents smoke and harmful gases from entering into the mask, allowing the user to breathe safely. --- My manual says to use 100% mode above FL250 and to not use the masks continuously above FL370. The presumption is that if you are at FL450, the masks gives you enough time to descend to FL370 before you would run out of oxygen. Quote: You do not have enough O2 pressure in your lungs at extreme altitudes to create an oxygen exchange across the membranes of the lungs. The mask pressure is required to create the differential necessary for O2 exchange. The mask is not designed to allow you to remain at FL450, but does provide enough oxygen to keep you awake long enough to descend to at least FL370. Quote: Even on 100% O2, above 35,000 ft the reduction in rate of O2 exchange into your capillaries is measurable. The rate drops below the consciousness-sustaining limit somewhere in the 40s, depending on the subject. You don't need to sustain high oxygen levels at high altitudes, just enough to provide the 2 minutes you need to get lower where 100% mode will work long term. Mike C.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 17:01 |
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Username Protected wrote: My manual says to use 100% mode above FL250 and to not use the masks continuously above FL370. Serious question: How does that mesh with 91.211? Does your SPE prohibit flight above FL 370, or is that something that slipped through the cracks between Cessna and the FAA?
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 18:10 |
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Username Protected wrote: My manual says to use 100% mode above FL250 and to not use the masks continuously above FL370. Serious question: How does that mesh with 91.211? Does your SPE prohibit flight above FL 370, or is that something that slipped through the cracks between Cessna and the FAA?
The manual is talking about cabin altitudes, not aircraft altitudes. You can't use the pilot mask when the cabin is above FL370 continuously.
Wearing the mask "normally" at FL450 aircraft altitude is when the cabin is at 8000 ft cabin altitude. That's fine.
The weird part is that you want the mask in 100% mode in case the cabin depressurizes, but the oxygen is lesser used if it is in normal or diluter mode when cabin is at 8000 ft.
These oxygen masks confuse me. What I would want is a mask that has no modes at all, and uses no oxygen until the ambient pressure drops to above 14,000 ft, then properly manages between normal and 100% modes as the pressure further drops. Why isn't that the normal way they work? Also, what happens if the switch is in normal mode when you need it at FL450? Do you pass out or stay awake?
Mike C.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 18:24 |
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Username Protected wrote: What about a flameout event? I was in a LR25 that had one flameout at FL390. (and successfully re-lit) Just curious if most twin jets can maintain a reasonable cabin at that high altitude on one engine?
That was an interesting few minutes, we were on TOP of a T-storm, at night, lots of lightning below us. Some ice before re-start too. My Phenom 300 will hold cabin altitude on one engine at FL450. However, the QRH requires decending to FL360 if on single engine bleed. I experienced a flow control valve failure at FL430 just a few days ago which resulted in a single engine bleed; no pressurization issue.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 18:29 |
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Username Protected wrote: What I would want is a mask that has no modes at all, and uses no oxygen until the ambient pressure drops to above 14,000 ft, then properly manages between normal and 100% modes as the pressure further drops. Why isn't that the normal way they work? Simpler is better for the last safety system in the chain. If the feature doesn't exist, then the feature can't fail. There is no backup if your mask gets stuck in the automatic <14,000 ft mode after depressurization.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 18:33 |
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Username Protected wrote: My Phenom 300 will hold cabin altitude on one engine at FL450. However, the QRH requires decending to FL360 if on single engine bleed. I experienced a flow control valve failure at FL430 just a few days ago which resulted in a single engine bleed; no pressurization issue. My Citation V manual has no similar requirement. I was at FL430 and operated on one bleed for some period of time. Pressurization was maintained at full diff. The only noticeable thing was the ACM wasn't quite able to put out enough heat on one bleed and the cabin was gradually getting colder, but it was very gradual, like 20 minutes to lose 3 C. Mike C.
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Post subject: Re: Jets with normal cruise of 0.7 mach or better? Posted: 17 Oct 2022, 19:23 |
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Username Protected wrote: The baby bus (320) is decidedly pokey. .80, maybe. you can select speed .99 in the 320 It’ll stop at .82 though. But it can do that no problem in cruise. If you ask.
Not at 170-197k weight it wont.
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