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Cruising Altitudes  
User currently offlineKmh1956 From Bermuda, joined Jun 2005, 3324 posts, RR: 7
Posted (7 years 11 months 2 weeks 2 days 2 hours ago) and read 5499 times:

Ok, I may be asking to be flamed here but bear in mind I'm only an aviation enthusiast...not an expert. I wouldn't know the difference between a 734 and a 738 and I'm content to stay that way.

When looking at the specs on various passenger jets, you'll always see maximum cruising altitude. However, is there such a thing as a 'minimum cruising altitude'? For safety and passenger comfort, I would imagine that such a thing exists but it's not something you see in the databases.

Thanks
Kelly


'Somebody tell me why I'm on my own if there's a soulmate for everyone' :Natasha Bedingfield
35 replies: All unread, showing first 25:
 
User currently offlineJkudall From United States of America, joined Mar 2004, 615 posts, RR: 1
Reply 1, posted (7 years 11 months 2 weeks 2 days 1 hour ago) and read 5491 times:

An aircraft's performance doesn't usually dictate a minimum cruising altitude per se. Airliners generally cruise at an altitude where the aircraft will operate most efficiently, where they can save more fuel and fly where the winds are most favorable. Higher altitudes also allow for smoother flights most of the time and allow the aircraft to stay out of most of the weather. As far as minimums go, IFR (Instrument Flight Rules) dictates minimum altitudes you must fly at on any particular route. IFR takes many factors into consideration for a minimum altitude. Some of those factors being terrain, ability to pick up radio signals, and other conflicting airspace.

User currently offlineKmh1956 From Bermuda, joined Jun 2005, 3324 posts, RR: 7
Reply 2, posted (7 years 11 months 2 weeks 2 days 1 hour ago) and read 5476 times:

Quoting Jkudall (Reply 1):

Thank you....but is there an altitude that these aircraft should NOT fly below?



'Somebody tell me why I'm on my own if there's a soulmate for everyone' :Natasha Bedingfield
User currently onlineStarlionblue From Greenland, joined Feb 2004, 17173 posts, RR: 66
Reply 3, posted (7 years 11 months 2 weeks 2 days ago) and read 5463 times:

Quoting Kmh1956 (Reply 2):
Thank you....but is there an altitude that these aircraft should NOT fly below?

Well, there is terrain. If all the mountains are 3000 feet in the area, going under 4000-5000 (just guesstimating here) is probably unwise during cruise in an airliner.

As long as the plane is over the ground, it can fly. Aerobatic pilots routinely fly at just a few feet. If you look at barometric altitude, nothing stops Israeli fighter pilots from flying under sea level over the Dead Sea.

Some other factors:
- ATC restrictions.
- Under 10k you need to follow sterile cockpit rules, which is probably a pain in the ass to do for several hours.
- If you start going really low, danger creeps in since any mistake or mechanical fault becomes critical. I would say this us under a few thousand feet.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSkyman From Germany, joined May 2006, 494 posts, RR: 0
Reply 4, posted (7 years 11 months 2 weeks 2 days ago) and read 5467 times:

Quoting Kmh1956 (Reply 2):
Thank you....but is there an altitude that these aircraft should NOT fly below?

Yes the MRVA (minimum radar vectoring altitude) or MSA (minimum sector altitude). First of all it is not allowed and that is for a good reason: terrain.


User currently offlineThrottleHold From South Africa, joined Jul 2006, 659 posts, RR: 1
Reply 5, posted (7 years 11 months 2 weeks 2 days ago) and read 5462 times:

Quoting Skyman (Reply 4):
or MSA (minimum sector altitude). First of all it is not allowed and that is for a good reason: terrain.

If flight under MSA is not allowed....how do they take off and land?!

Flight below MSA is allowed when on a published procedure or under positive radar control.


User currently offlineAirfoilsguy From , joined Dec 1969, posts, RR:
Reply 6, posted (7 years 11 months 2 weeks 2 days ago) and read 5461 times:

Quoting Kmh1956 (Reply 2):
Thank you....but is there an altitude that these aircraft should NOT fly below?

It is absolutely critical to you future career as a pilot and also to your personal well being that you don't Fly below terrain level

Quoting Skyman (Reply 4):
Under 10k you need to follow sterile cockpit rules, which is probably a pain in the ass to do for several hours.

Also speed restrictions apply plus you will be in Indian country and that is not a fun place to be for airline pilots.

The general rule is that the higher you go the less wind resistance you will encounter. The less wind resistance you encounter the less fuel you will use. This applies as true in the reverse.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 7, posted (7 years 11 months 2 weeks 2 days ago) and read 5460 times:

Airliners take advantage of higher altitudes for two reasons (and I think the other posters stated these above, but I'll re-iterate for clarity  Wink ), fuel effeciency and riding above the majority of the weather. Beyond that, cruising altitude is chosen for other such factors such as the most favorable winds aloft or to minimize the fuel consumption for the flight (by optimizing the fuel needed for climb).

OPNLguy once shared with us some of the details of a 737-300 his company dispatched to PAE for maintenance (from Texas), however it had to cruise at or below 12,000 feet because of maintenance issues (I'm assuming it was pressurization issues  Wink ), and it had to make two fuel stops enroute. Normally, at altitude and with no passengers on board, the plane would be able to make this hop non-stop.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineThrottleHold From South Africa, joined Jul 2006, 659 posts, RR: 1
Reply 8, posted (7 years 11 months 2 weeks 2 days ago) and read 5456 times:

Quoting Airfoilsguy (Reply 6):
The general rule is that the higher you go the less wind resistance you will encounter.

As a general rule, windspeed increases with altitude. Now whether that is a tailwind, crosswind or headwind will depend on the wind velocity relative to your track.

Quoting Airfoilsguy (Reply 6):
The less wind resistance you encounter the less fuel you will use

Less fuel is burnt at altitude due to the air density being much lower. The amount of fuel burnt depends on the amount of air passing throught the engine. Less air at altitude therefore means less fuel so burn goes down.


User currently offlineAndrej From United Kingdom, joined Jun 2001, 1039 posts, RR: 0
Reply 9, posted (7 years 11 months 2 weeks 2 days ago) and read 5442 times:

Hello Kmh1956,

I believe that this varies where you are flying. Each chart will define MSA (I believe it stands for Minimum Sector Altitude). It gives altitudes that one should not fly below. Otherwise there is a risk of flying into mountains and other obstacles.

This MSA is defined in many enroute charts and they may differ from low levels such as 2500' to high flight levels such as FL280.

But as stated above, planes perform better at higher altitudes and therefore I believe that there is no such thing as minimum altitude for specific airplanes. Each airplane has charts and tables where optimum flight levels are defined over specific distance. If you fly lower, your fuel consumption will increase. (Unless headwinds are very strong)

I am not pilot, so I may be wrong.

Cheers,
Andrej

EDIT: When I read your post there were only 3 replies therefore I may repeat what was already said.  Smile

[Edited 2007-01-10 20:40:29]

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (7 years 11 months 2 weeks 2 days ago) and read 5437 times:

Since you mentioned a couple of 737s in your o/p I'll talk about jet airliners and cruise.

The general sort of thinking is that you want to go as high as is safe and practical. Big reasons mostly twofold: One as you go up, fuel flow will decrease some. Second, as you go up, True AirSpeed will increase for a given Indicated Air Speed, which has an upper limit called VMO

This works for us all the way until we encounter the other speed limitation which has been way out in front of us during the climb, the constant mach limit: MMO

So go high and stay there as long as is practical.

When I was training dispatchers I found it very useful to throw in a very short flight to their planning curriculum. For a jet to go a hundred miles or so is a good exercise in planning and one of my favorite students (my daughter) had it reduced to zero nm cruise phase - climb 'til you descend.

After this there are the other considerations: Appropriate flight level for direction of flight, FL restrictions on a given airway segment, coffin corner and so on.

Quoting Starlionblue (Reply 3):
Under 10k you need to follow sterile cockpit rules

Does not apply in cruise flight. The last three words in FAR 121.542 are "except cruise flight."

Quoting Airfoilsguy (Reply 6):
the higher you go the less wind resistance you will encounter.

To clarify an unnoficial phrase: "Wind resistance" is often used by laypeople to describe "drag" which is a word for friction and other effects the passing air has on an airplane. There is another possible interpretation readers might take here and that is the effect of actual atmospheric "wind" itself, also a factor in flight planning.

Drag will decrease with increasing altitude within the normal envelope of the airplane in question.

Wind factor varies hour by hour at any given point in the atmosphere and must be monitored by pilot and dispatcher. Some times climbing higher gives you more favorable (tailwind) winds and some times it does not. Sometimes climbing higher gives you headwind increases that will offset the other benefits of climbing. (decreased fuel flow and increased TAS)



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 11, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5432 times:

Quoting Andrej (Reply 9):
believe that this varies where you are flying. Each chart will define MSA (I believe it stands for Minimum Sector Altitude). It gives altitudes that one should not fly below. Otherwise there is a risk of flying into mountains and other obstacles.

This is for off-airways use, and takes a number of factors into consideration. It tries to be a catch-all, to protect you from bumping into either terrain or man-made objects.

Generally, when operating IFR at lower altitudes (below FL180, or 18,000 feet, at least in the USA), you are on airways except in terminal areas (radar vectors) or when on approach (when you are on defined segments of the instrument approach). En-route on airways, you are subject to the airway's MEA, or Minimum En-route altitude. This is often lower (in some cases by large amounts) than the MSA for the sector on the chart. This can go up or down on a particular airway, due to terrain and/or other considerations. I could go into even more detail (like the Minimum Obstacle Clearance Altitude, or MOCA, and the Minimum Reception Alititude), but unless you are trying to get an instrument rating, this is most likely knowledge you don't need  Wink

Of course, if you are operating VFR (which I don't think any airliner ever would do  Wink except when cleared for a visual approach and maybe for traffic separation upon departure on a VFR day), none of this applies, and you use your two eyeballs for terrain separation.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently online113312 From United States of America, joined Apr 2005, 576 posts, RR: 1
Reply 12, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5430 times:

In general, most jet airliners are better off cruising above 28,000 feet MSL. At about this altitude, the True Airspeed is highest with relation to a typical cruise MACH speed limit as well as indicated airspeed. As altitude increases, air temperature decreases. As MACH number is related to temperature, MACH number becomes the indicated speed limit at higher altitudes. However, with a fixed MACH number, indicated and true airspeed both decrease as altitude increases(temp decreases up to the Tropopause). Below 28,000 feet, indicated airspeed usually becomes limiting and true airspeed also decreases along with an increase in fuel consumption.

For economy (miles per pound of fuel) high altitude is best unless there is substantial head wind. Maximum altitude is generally limited by the weight of the plane and aerodynamic buffet boundaries. For speed, 28,000 feet plus or minus a thousand or so is optimum for pounds of fuel burned vs. miles per hour. Below that altitude, fuel consumption increases and speed decreases.


User currently offlineKmh1956 From Bermuda, joined Jun 2005, 3324 posts, RR: 7
Reply 13, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5430 times:

Quoting Airfoilsguy (Reply 6):
It is absolutely critical to you future career as a pilot and also to your personal well being that you don't Fly below terrain level

 rotfl 

Quoting SlamClick (Reply 10):
coffin corner

Very informative...thank you.
"Coffin corner"...explain?


Thanks to all of you for your detailed and well-thought-out responses.



'Somebody tell me why I'm on my own if there's a soulmate for everyone' :Natasha Bedingfield
User currently offlineThrottleHold From South Africa, joined Jul 2006, 659 posts, RR: 1
Reply 14, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5424 times:

Quoting SlamClick (Reply 10):
Does not apply in cruise flight. The last three words in FAR 121.542 are "except cruise flight."

Ah yes, but the whole world doesn't operate under FAR's you know!  Wink
Our SOP's say a sterile cockpit at all times when below FL100.


User currently onlineStarlionblue From Greenland, joined Feb 2004, 17173 posts, RR: 66
Reply 15, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5422 times:

Quoting SlamClick (Reply 10):
Quoting Starlionblue (Reply 3):
Under 10k you need to follow sterile cockpit rules

Does not apply in cruise flight. The last three words in FAR 121.542 are "except cruise flight."

Argh. I forgot about that one. Still, I imagine the potential for ramming some poor unsuspecting 172 in the tailpipe limits conversation topics a little.  Wink



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5417 times:

Quoting ThrottleHold (Reply 14):
Our SOP's say a sterile cockpit at all times when below FL100.

Fine, but what do the regulations under which you conduct flight operations have to say?

Quoting ThrottleHold (Reply 14):
Ah yes, but the whole world doesn't operate under FAR's you know

I think we are all aware of that. Care to post something useful from another set of flight regulations?

edit: I understand that the original poster is not from the United States. However, the question was posed to a forum of people of varying expertise and the majority of which are from the US. Anyone wishing to know the rule in Europe or over the Indian Ocean usually will frame his question to single out responses relevant to those places. Only about two thousand hours of my flying time was outside the US and all of that under US civil and/or military rules. I post from my own knowledge and experience and, if it actually seems important, will so state. If I speak from a point of view that is ex-USA I will state that it is my guess, opinion, or something that was told to me.

[Edited 2007-01-10 21:26:17]


Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 17, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5410 times:

Quoting Kmh1956 (Thread starter):
However, is there such a thing as a 'minimum cruising altitude'?

500'-1000' AGL, depending.  Smile

Quoting Airfoilsguy (Reply 6):
The general rule is that the higher you go the less wind resistance you will encounter.

Not to mention the higher the altitude you can request over the radio the better the pilot you are.  Smile . I bet those SR-71's made some airline pilots jealous.
"Cleared direct to FL650, have a good day"  Smile

Quoting Starlionblue (Reply 15):
ramming some poor unsuspecting 172 in the tailpipe

Don't hit me.  duck 


User currently offlineThrottleHold From South Africa, joined Jul 2006, 659 posts, RR: 1
Reply 18, posted (7 years 11 months 2 weeks 1 day 23 hours ago) and read 5398 times:

Quoting SlamClick (Reply 16):
Fine, but what do the regulations under which you conduct flight operations have to say?

To be honest, I haven't a clue what the JAR's say. If our SOP's say that, then they'll be as, if not more conservative than the legal regulations.
Not that anyone's ever talked about "that house down there" when on 5 mile finals....

Quoting SlamClick (Reply 16):

I think we are all aware of that. Care to post something useful from another set of flight regulations?

Tongue-in-cheek 'twas....


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 19, posted (7 years 11 months 2 weeks 1 day 22 hours ago) and read 5380 times:

Quoting Futurecaptain (Reply 17):
500'-1000' AGL, depending. Smile

Yeah, helicopter pilots and agricultural pilots start to get nosebleed above those altitudes  duck 



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 20, posted (7 years 11 months 2 weeks 1 day 22 hours ago) and read 5377 times:

Quoting KELPkid (Reply 19):
Yeah, helicopter pilots and agricultural pilots start to get nosebleed above those altitudes

There are cropdusters in California's Imperial Valley who never get up to sea level all season long. Sea level used to be painted on the side of the elevators at a local sugar company. The line was way up there.

In a helicopter if you are more than thirty seconds off the ground you just might not make it down in time. Couple friends of mine had a transmission chip light. They did a full-power descent, listening to the groaning from the hellhole. When the skids hit the ground the transmission siezed and they did a half turn to the left before coming to a stop. They lost all further interest in the top half of their altimeter dial!



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 21, posted (7 years 11 months 2 weeks 1 day 21 hours ago) and read 5355 times:

Quoting SlamClick (Reply 20):
There are cropdusters in California's Imperial Valley who never get up to sea level all season long. Sea level used to be painted on the side of the elevators at a local sugar company. The line was way up there.

I've seen those driving through on Intersate 8...you don't want to drive a clean white car through the Imperial Valley. The front of your car will get absolutely splattered with dead yellow butterflies Big grin



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineAirfoilsguy From , joined Dec 1969, posts, RR:
Reply 22, posted (7 years 11 months 2 weeks 1 day 20 hours ago) and read 5340 times:

Quoting ThrottleHold (Reply 8):
As a general rule, windspeed increases with altitude

You got me, had a brain skip, I meant air.


User currently offlineHighFlyer9790 From United States of America, joined Jul 2005, 1241 posts, RR: 0
Reply 23, posted (7 years 11 months 2 weeks 1 day 19 hours ago) and read 5326 times:

Quoting KELPkid (Reply 11):
Of course, if you are operating VFR (which I don't think any airliner ever would do except when cleared for a visual approach and maybe for traffic separation upon departure on a VFR day), none of this applies, and you use your two eyeballs for terrain separation.

Airlines are not allowed to fly VFR. IFR only for these guys. they can, however, be granted a visual approach/separation on a VFR day as you stated.

highflyer



121
User currently offlineSkyman From Germany, joined May 2006, 494 posts, RR: 0
Reply 24, posted (7 years 11 months 2 weeks 1 day 10 hours ago) and read 5249 times:

Quoting ThrottleHold (Reply 5):
If flight under MSA is not allowed....how do they take off and land?!

Of course you are right but he was asking about cruising altitudes and not departures.
In general for Germany I would say IFR don´t fly below FL100 (or FL150 in the Alpsregion) to keep clear of airspace E. Normal is FL300 and above unless the destination is to close.


25 Post contains links BAe146QT : Heard a story about a SR-71 crew requesting FL800 from a civillian controller who was unaware of their AC type. The controller laughed and said, "Sur
26 Airfoilsguy : Very interesting, but wasn't the altitude and speed performance of the SR 71 classified? wouldn't the pilots get in trouble for doing something like
27 IAHFLYR : For the FYI Dept. The weather doesn't have to be VFR to accept visual separation, only able to see the traffic until possible conflict is resolved...
28 BAe146QT : Dunno, but mabe - and that's why I had it tagged as a probably urban legend. Having said that... to what altitude is a ground-based aviation RADAR ac
29 Skyman : Do you mean in respect of other traffic? Because in Germany it is not allowed anymore over FL100. Probably due to TCAS. Most civil radar works with m
30 SlamClick : There must be thousands of air traffic controllers who have a pretty good idea of what this plane could do in terms of speed over CONUS. Altitude was
31 IAHFLYR : Not sure of how the clearances have been given so it may not have included traffic.
32 SlamClick : When we do something like that we are not flying VFR as our flight plan is filed as IFR and we may not even be authorized VFR flight under our OpsSpe
33 Post contains images Iahflyr : That is what I actually meant to say, IFR but taking a VFR climb in essence maintaining visual with the rock. Thanks for making my post clear, almost
34 Turkee : I've got to tell you - I went for a jumpseat ride in one of our company aircraft the other day, and on short final into PER, the pilot-flying turns t
35 Post contains images Starlionblue : On landing at ARN when I was in the jumpseat, all the pilots were talking about the rabbit the previous plane landing had hit, and whether the carcas
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