AVIATION: Altimeter settings

We never know what will come up in WAIS.  The topic du jour is altimeter settings, about wiuch I know nothing. Mike Sullivan, an expert on these matters, writes: Actually Flight Levels come in 1000 ft increments and not 100 ft increments as reported in the previous email.   All altimeters are set to 29.92 inches or the corresponding QNH for those aircraft using millibars above 18,000 ft and reset to the local altimeter setting in inches or millibars descending through 18,000 ft.  

Flight levels don't start until above 18,000 ft.  The high altitude jet route structure starts at FL290 (29,000 ft).  All aircraft on headings between 001 degs and 180 degs fly at altitudes of FL290, FL330, FL370, FL410 etc being separated by 4,000ft increments.  Aircraft flying between 181 degs and 360 degs fly at FL310, FL350, FL390, FL430 etc.  There is at least 2,000 ft separation between all aircraft anytime they have converging flight paths in the high altitude jet route structure so when you see aircraft or contrails above or below you are very safe.   All aircraft are required to have transponders above 18,000ft which give exact altitude readouts to the air traffic controllers on the ground and if your more than 200 ft off altitude they'll call you and tell you to check your altitude!

When you start your aircraft you set the altimeter to the actual field elevation which should be close to the altimeter setting that ground control gives you. You should note the error, usually very minute, and when you get to your destination you crank in that minute error to the altimeter setting you're given for landing.  The error is usually under a 200 ft which doesn't matter as you are visual by that time and use your eyes to land the aircraft. However, in instrument weather you rely on your radar altimeter to give you exact height above the ground when you're below 500 ft.  In the F/A-18 Hornet or the AV-8B Harrier I would select radar altimeter below 18,000 ft vice using the barometric altimeter for the most precise altitude readouts.  You set a "bug" on the "radalt" to 200 ft which is the minimum descent altitude without having a visual on the landing area.  When you arrive at 200 ft AGL the"bug" activates an audio warning which says,"Altitude, altitude" and if you can't see the runway or the deck of the aircraft carrier you must initiate a waive off and go around for another approach or go to your alternate.   

RH: This posting raises a philosophical question. Mankind, sorry peoplekind, has gone from riding a donkey to Mike's aircraft. I graduated from a bicycle to a car, closer to the donkey than to Mike's aircraft. Or take writing. We have progressed from the quill pen to the computer.  I use a computer, but I am awed by people who understand all its complexities. We have progressed from the simple cures of folk medicine to the incredibly complicated medical science of today.   Wherever you look, simplicity has given way to complexity.  We live in an age of experts and non-experts. It is a replay of the times when there was a division between scribes and the illiterate. Denunciations of the  scribes and the pharisees appealed to the far more numerous illiterates.  Current denunciations of intellectuals, universities and liberals have a similar appeal. Of course, the scribes and the pharisees deserved criticism, and so do today's experts.

George Sassoon writes: Perhaps I should clarify the need for units needed for altimeter settings, inches Hg in the U.S. or millibars in Britain.  The altimeter must be set to a given pressure to read the required altitude correctly, the settings being given over the radio.  There are three settings needed:  

1.  QFE  ("from earth") used when flying locally to or from an airfield.  When set to this, the altimeter reads height above ground at that airfield.
2.  QNH.  When set to this pressure, the altimeter reads height above sea level.  On a long-distance flight there are "regional QNH's", so the setting must be changed between regions.
3.  Flight level (FL).  This is used generally above 24,000 feet and increasingly at lower altitudes.  The altimeter is set to 1013 millibars so does not read height above any particular datum. The important thing is avoiding collisions, so all altimeters must read the same.  Flight levels come in units of 100 feet, so if you are told to climb to FL80, you should set your altimeter to 1013 mb and climb till it reads 8000 feet.

I have an aircraft altimeter on my desk reading 400 ft. at 1010 mb, 400 being my approx. height above sea level, so it is set to QNH. If I turn the knob till it reads zero height, the pressure scale reads 995 mb, QFE, and if I set the pressure to 1013 mb it reads 470 feet and flight level 4.7, not that decimals are used for flight levels. So you can see the importance of these settings, and the difficulties that might be caused if they change the units for political, not aeronautical reasons. What if they change flight levels to 30 meters spacing, which is 98.425 feet?This is a difference of 157.5 feet at flight level 100 (10,000 ft.).  The GPS system is not accurate enough to be used for altitudes, though good enough for positions.  Also, it can be turned off or the accuracy reduced if there is a war somewhere.

RH: Aviation worldwide is a WAISworthy subject. It involves many things, including weight. They once took the weight of passengers. In Brazil I was weighed and told that I weighed 40 lbs.  I protested, saying I weighed 200 lbs. The official said "40 lbs is all we have available".  I was relieved to reach my destination safely. I boasted about being an old dog who has learned new tricks. However, after reading George's account, I decided that this new trick is not for me.

*George Sassoon wrote: Distances are in nautical miles, which is very convenient for quick calculations in the cockpit (or flight deck), speeds are in knots, and heights in feet.  The GPS system is not accurate enough to be used for altitudes, though good enough for positions.  Also, it can be turned off or the accuracy reduced if there is a war somewhere. Gene Franklin says: Two comments. It is interesting that both the kilometer and the nautical mile are based on measurements of the earth. The nautical mile is one minute of  an arc, there being 60 minutes in a degree and 360 degrees around  a great circle. There are 10,000 kilometers in a quarter of a great circle or 40,000km  all the way around. Because the earth is not exactly spherical, these are only approximate; the exact values being, I believe, defined in terms of the wavelength of an atomic radiation.  As for GPS, I'm a bit surprised by George's comment that it is not accurate enough for altitude since there have been very successful demonstrations of completely blind landings of a DC-3 using only measurements from a version of GPS.

RH: Since the earth is pear--shaped, the calculations must be horribly complicated.

Once in Brazil, in the good old times when passengers were weighed, I was told that I weighed 40 lbs. because that was the amount of weight which was still available. Randy Black comments: I wish that I weighed only 40 lbs. as did Professor Hilton once in Brazil! What a great, Soviet-sounding story. Weights and balances are an integral part of aviation. More than a few times a year, a plane crashes, usually on takeoff, due to being overloaded or loaded improperly, with too much cargo in the nose or toward the tail.
Part of every pilot‚s training, at all levels from amateur to commercial, involves those calculations. Every plane on earth has load limits. Plus the load has to be properly balanced between the front and the rear of the aircraft. Too much in the tail section and it‚s hard to keep the plane‚s nose angled properly on takeoff and the plane may stall (quit flying). It happens all the time in smaller planes but rarely in commercial jets, since they‚re generally overpowered .
Recently, I noted a news item that said the American air carriers were updating their weights and balances standards for people that for years had
"averaged" the weights of passengers at approximately 160 lbs for men and less for women. They were adjusting their averages upward about 20 pounds per person. The last time this change took place was evidently in the 1950s. And it‚s no mystery that we are bigger and fatter at the same time, just as those persons in the 50s were larger than their ancestors were 50 years earlier.

RH: I seem to recall that on the Brazilian flight they sat me half way between the nose and the tail.  It also suited my ideology. If people are getting fatter, airline expenses will rise, leading to bankruptcies.  There have been lawsuits involving fat passengers.

Randy Black said: All aircraft operating in "controlled airspace" which, for instance includes approximately anything within about 50 miles of Dallas/Fort Worth International Airport, must have working transponders.

Mike Sullivan comments: Randy is correct if your operating in controlled airspace you must have an operating transponder but controlled airspace is normally designated in high density, air traffic control areas around major metropolitan areas or military operating areas.  

The altitudes below FL 290 are assigned odd and even  altitudes in 2,000 ft separation levels using the same heading requirements as aircraft operating in the high altitude route structure.  So there is always 1,000 ft separation for aircraft converging on instrument flight plans (IFR) below FL 290.  Above 18,000 ft you must be on an instrument flight plan and  visual flight rules are not permitted.

Problems can arise below 18,000 ft where most light, civil aircraft, like Cessnas 172s, fly as they are on visual flight plans. They are required to conform to the same east-west, north-south altitude rules as IFR flight plans plus they add 500 ft.   If they are not conforming to these altitude rules they can cause near misses or mid air collisions with other aircraft.   Operating without a transponder makes it very difficult for air traffic controllers to see the aircraft or know its altitude to warn other aircraft in the vicinity.  It is recommended that all aircraft have transponders, and they are required, as Randy says, in controlled airspace, but they are realitively expensive so many light aircraft owners don't have them.  I predict that in the not too distant future all aircraft flying in US airspace will be required to have an operating transponder.

Randy Black writes: General Sullivan may not be aware that all aircraft operating in „controlled airspace‰ which, for instance includes approximately anything within about 50 miles of Dallas/Fort Worth International Airport, must have working transponders. This rules affects all aircraft, small or large, from the ground up, not simply from 18,000. Further, flight levels are assigned below 18,000 feet but with smaller separation limits that at the „commercial/military‰ levels.
Further, the FAA changed the flight level separation requirements on January 20 regarding the FL290-410 altitudes to 1,000 foot vertical separation. This is the first separation change, to my knowledge, in years.
Source: http://www.faa.gov/ats/ato/drvsm/default.asp
(I have held a private pilot license since 1977. While I no longer am an every month flyer, I continue to maintain interest in the rules of flying.)
But I can assure Mike that when I‚m flying from Dallas to west Texas in a Cessna 172, below 10,000 feet, I am at an FAA approved flight level, with a working transponder, since that flight path goes through numerous controlled airspace first, around Dallas, then around Carswell in west Ft. Worth, then the military training airspace around Weatherford all the way to Abilene where the B-1s are prevalent, and on into miles and miles of west Texas.
For more on restrictions regarding flight levels below 18,000, and there is much, go to:

I boasted about being an old dog who has learned new tricks. However, after reading George Sassoon's account of the problems of flying, I decided that this new trick is not for me.  George rebukes me: Come on Ronald, California is a wonderful place for flying!  Just turn up at any local airfield and ask for an instructor, and he will take you up and let you have a go at the controls.  The costs are very reasonable compared to Europe.  I have done this several times in the U.S. and flown over Reagan's ranch, the Grand Canyon, and Mt. St Helens.  I didn't have my papers with me - the medical was all they asked for otherwise I could have gone solo and taken my wife, not that I wanted to because I was too busy with the camera.  With the instructor, she could come anyway even though I was officially taking instruction. As regards age, I read recently of a U.S. pilot 100 years old who held a full pilot's license with no medical provisos.  Being too young is no bar either, as long as you can reach the rudder pedals. PS. Things may have changed since 9/11 - you may have to produce some proof that you are not an Arab terrorist!

RH: I have been called all kinds of things, but never an Arab terrorist. I no longer drive my car. I have returned to my childhood, when my range of movement was limited. Slowly it grew until I flew around the world.  Then the decline set in, and now I rarely wander beyond my habitat.

Mike Sullivan, an expert on aviation matters, wrote:  Flight Levels come in 1000 ft increments and not 100 ft increments as reported in the previous email by George Sassoon.   All altimeters are set to 29.92 inches or the corresponding QNH for those aircraft using millibars above 18,000 ft and reset to the local altimeter setting in inches or millibars descending through 18,000 ft.  George coimments: I am most grateful to Mike Sullivan for his expert comments on this.  I agree that flight levels are usually given in 1000 ft. intervals, though they are in units of 100 ft. If you take up my suggestion of going flying with an instructor, I would not recommend helicopters.  Their mode of flight is most unnatural compared to fixed-wing, the controls all interact, and they require continual attention if lethal accidents are to be avoided.  The person who got me started on flying was an ex-WWII pilot called Richard "Brad" Bradbury, who had a Tiger Moth and a Hiller 360 12C helicopter, the one with the hanging stick which went straight up to the cyclic control swash plate.  After a few hours on this he allowed me to take him and his girl-friend Pam for a local flight around Cambridge, the Hiller having 3 seats abreast. We were going along the river at about 500 ft., and came to my Alma Mater college.  I thought: "this is fun, what about a photograph?" Then I saw that Brad and Pam were playing chess on a portable set. So I banked the aircraft to get them and the chess-set into view, and took a photo.  Later I sent this to the local paper which printed it over the caption: "Richard Bradbury counters a threat to his queen.  King's College Chapel is visible in the background." Unfortunately this effort caused me to lose control of the thing, so I shouted to Brad to take over, which he did, and as a result, lost his queen.  So I don't recommend helicopters for a first flight.

Later, this Hiller was written off by another of his students who overpitched and landed on top of a hedge.  The wreckage was in the hangar for a long time, then Brad sold it to the makers of the James Bond films who used it.  A worthy end I suppose. It is still shown occasionally and I watch it with a touch of nostalgia. I did much better with Brad's Tiger Moth, a fixed-wing biplane, and soon we were blowing over land-yachts on a local disused airfield.  We landed to apologise, but they were most amused and asked us to go up and do it again.  After all this, my more recent experience in Cessnas seems rather office-bound and boring.  But not the YaK-52! I am glad to hear that you are not excessively skinny.  Myself I weigh in at 80 kg (~168 lb.) and height 6' 2", but I was told that if I did National
Service in the RAF I would not be able to fly fighters due to my height. They reckon that the nerve impulses take longer in tall persons so it slows the reactions.  Also I would find it difficult to get into the things.  Anyway this is no obstacle to non-military flying!

RH: I have flown in a helicopter only once, and I was uneasy. Does the RAF prefer pygmies?  Possibly chimps would do.

Ronald Hilton 2004


last updated: February 2, 2005