Cruising Altitudes: Definitions for IFR and VFR
Selection of Cruising Altitudes
When to Start Descent for Landing
Altitudes on Approach to Landing

Cruising Altitudes: Definitions for IFR and VFR

I have had a few requests with respect to altitude usage when flying IFR, in particular while on approach, both with and without ATC online. First, I'll start by attempting to clarify cruising altitudes and the selection of those.

The Canadian Aviation Regulations specifies which altitudes may be used for cruise flight, and direction exists there for IFR and VFR, as well as direction of flight. Some definitions for the newbies might come in handy.

Altitude: When refering to cruising flight, these are heights measured Above Sea Level (ASL) as indicated on a standard aircraft barometric altimeter that has been corrected for local barometric pressure by using the local station pressure.

Flight Level: In Canada and the US, above 18,000 feet ASL, we set our altimeters to standard pressure (29.92 inches of mercury) and maintain altitudes as indicated on the same altimeter. This is our "Transition Altitude". As you climb above 18,000 feet, set your altimeter from the current station pressure you were given on departure with your clearance or your taxi instructions to the standard pressure setting. When descending through FL180, set it to the current station pressure for the destination aerodrome. Flight Levels are also used over the ocean and in other areas where there are few weather stations capable of reporting a barometric pressure for use in aircraft, such as the Canadian far north. Note that when set to 29.92, your altimeter will only show your altitude above sea level when you are flying over a point where the barometric pressure is actually 29.92. This means that you may be higher or lower than your altimeter is indicating. The key here is that the other aircraft that are flying near you will also be using an altimeter with approximately the same amount of error, so you will still have 1,000 or 2,000 feet between your flight levels as you pass.

I'll assume you know IFR and VFR. Do you know CVFR? Controlled VFR flight is still accomplished in visual weather conditions with visual reference to the ground, but it takes place in Class B airspace. You must be in contact with ATC and receive authorization to operate within Class B airspace. This authorization is to be treated just like an IFR clearance. The difference is that you are still responsible for keeping out of cloud and seeing the ground for navigational reference. If the clearance issued by ATC and accepted by the pilot will have the aircraft operating in cloud, the pilot must advise ATC as soon as possible to obtain a modified clearance that will permit the aircraft to remain in visual conditions. Climbs and descents must be requested and approved before the commencement of either, and routing changes (including turns) must also be approved. ATC provides IFR separation between IFR and CVFR aircraft, so they have to know what you're doing. They also have to have some control over your flight path to be able to provide this separation, hence the requirement for a clearance. Currently in Canada, Class B airspace is defined as, "Controlled low level airspace above 12,500 up to and including 18,000 feet."

Low level airspace is up to, but not including 18,000 feet, while high level airspace is 18,000 feet and above. FL180 is considered to be above 18,000 feet, and therefore part of the high level structure. 18,000 feet doesn't exist as a cruising altitude, since that's where we transition to Flight Levels in Canadian (and US) airspace. The transition level varies around the globe, so check you local regulations.
 

Aircraft track is determined by magnetic track in the Southern Domestic Airspace, and by true track in the Northern Domestic Airspace. Perhaps a topic for another week? I didn't include altitudes above FL600, as you are now in uncontrolled airspace, and so few aircraft can actually reach those altitudes. If you look at the table above you'll notice that there is 2,000 feet of vertical separation between flight levels above FL290. There is a new advancement in avionics and now procedures allow, in certain airspace, Reduced Vertical Separation Minima. With RVSM, there is only a requirement for 1,000 feet between aircraft. Special considerations must be met for aircraft to receive RVSM certification, but in VatSim, we consider all aircraft to be certified. Within RVSM airspace and the RVSM Transition airspace, outside of organized and structured traffic flows, the pattern for the lower flight levels continues. Even flight levels are westbound and odd levels are eastbound. RVSM only extends up to FL410, so the normal designation of altitudes continues from there. RVSM and RVSM Transition airspace includes all of Canada and the Gander oceanic airspace, with the exceptions of the extreme southern portions of Alberta, Saskatchewan, and Manitoba, most of Ontario and southwestern Quebec. Unfortunately, my scanner couldn't do the diagram in the AIP justice. The paper was too thin.

Selection of Cruising Altitudes

Determining which altitude to use depends largely on the type of aircraft you are planning to fly, but also on the weight of your load and the distance you need to travel. Other basic issues include, but are not limited to, Minimum Enroute Altitudes on airways (MEAs), terrain and obstructions in the flight path, and restricted airspace along the route of flight. Advanced issues to consider include temperature, winds aloft, pressure patterns and so forth. Generally speaking, the higher you go, the more fuel efficient your aircraft will be. There are limitations to this statement, though, perhaps better left to another weekly topic. For a short hop, you may be better off staying low for the cruise, rather than fighting hard to climb just to begin descent when you reach your cruising altitude. I'll give a few examples of cruising altitudes with distances (ignoring other factors mentioned above) for a typical Dash 8.
 

Note that this is a guideline only. There could be many reasons why some of these may not be acceptable. Short hops through the Rockies would preclude the use of 3,000 feet, for example, since even many of the aerodromes are above 3,000. Strong westerly winds aloft might require a westbound flight to stay lower than an optimal altitude to get out of the headwinds. Turbulence may dictate a higher or lower altitude, as could icing. A heavy load could keep a Dash 8 on a long haul flight out of the higher levels like FL250. The altitudes mentioned above are simply observations of preferred altitudes based on flight plans filed in the real world in the Moncton FIR.

Another interesting note comes for Flight Levels near the transition altitude, including mainly FL180, 190, and, though seldomly, 200. When the barometric pressure is below standard, there would be less than 1,000 feet of separation between an aircraft operating at 17,000 with his altimeter set to a local station pressure and an aircraft cruising at FL180 with his altimeter set to 29.92. If the barometric pressure is below 29.92, FL180 is no longer useable and ATC will not clear an aircraft to operate at that altitude. Additionally, if the pressure goes below 28.92, as it does on occasion, FL190 also becomes unavailable by the same logic. If it drops incredibly low, below 27.92, FL200 would also be out the window. This rarely happens in Canada, as pressures below 28.92 generally only accompany a very strong low pressure system which means very bad weather and very high winds. 27.92 would be low enough to stir hurricane force winds and nasty weather, and we just don't see that very often in Canada. So when you see a barometric pressure below 29.92, expect that clearance request to operate at FL180 to be shot down, and you'll most likely be offered 16,000 or FL200 if you're heading westbound.

When to Start Descent for Landing

When flying, eventually you have to descend. When should you start down? How low can you descend?

To sort of answer the first question, there are several rules of thumb used for determining when to descend. Which rule to use comes more from knowing your aircraft than anything else. Many people use the guideline of 3 Nautical Miles per thousand feet. If you're up at 15,000 feet, and your destination aerodrome is at 1,000 feet ASL, subtract 1,000 from 15,000, and you find that you have to descend 14,000 feet. This would mean 14 thousand feet times 3 NM equals a "Top of Descent" at around 42 NM from touchdown.

While researching the Canadair Challenger 600 for a model I was building, a real-world pilot gave me their guideline: Twice the altitude in thousands of feet, plus 10 NM. So if you're at FL330 and your airport is at 1,000 feet, you have to descend 32,000 feet, so 32 x 2 = 64, and adding the 10 NM gives you 74 NM. At about 74 NM from destination, they would throttle back and commence a descent at about 3,000 feet per minute.

Yet another method of determining when to start down is based also on altitude to lose, but this time on ground speed as well. For example, your airport is at 1,000 feet, and you're at FL240, doing a groundspeed of 240 knots. This speed translates to 4 miles per minute. You have 23,000 feet to lose(24,000 - 1,000 = 23,000), and you plan to adjust power to descend at 1,000 feet per minute. At 1,000 fpm, descent would take you 23 minutes. In that time you would fly 92 NM (23 minutes times 4 NM per minute). Using this method you would begin descent at about 92 NM.

Remember that if ATC is online, or if other traffic is in the way, you may have to adjust this. Simply requesting a descent doesn't mean you're going to get it. So my suggestion is that you request descent a little early, perhaps even advising ATC when you want to start down. "Center, ACA123, we'd like to start descent in about 5 minutes," or you could specify a distance from destination, "ACA123 requesting descent at 100NM from destination." Something like that. This way ATC can advise you if there is traffic or some other reason why they can't issue a descent clearance right away, allowing you to alter your planned descent rate, or your forward speed once you commence your descent. Stating that you're requesting descent right away is fine. Typically, a flight requesting descent without any further information will be regarded as being ready for descent right away. Upon receipt of a descent clearance ATC will expect you to begin descent promptly at a normal rate of descent for your aircraft type unless you ask otherwise, so you should complete any pre-descent checklists before asking.

ATC may use the term, "At your discretion" with a descent clearance. When you receive a clearance like this, you may commence descent at any time you are ready, and you can even level off at imterim altitudes on the way down. An interim altitude is just an altitude above that which you are cleared for. For example, you're at FL250, cleared "pilot's discretion" to 10,000 feet, and you may want to level off at 15,000 for a few minutes. If you level off, you still have a clearance to the lower altitude and require no further permission from ATC to continue your descent. The only rules that apply are:

1. While descending, you must descend at a normal rate of descent for your type of aircraft unless you ask for and obtain approval for something else, like a cruise descent
2. If you choose to level off at an interim altitude, you are supposed to report leveling off, and to report resuming descent.

Altitudes on Approach to Landing

If ATC is online, you'll generally be given an altitude to descend to. A pet peave of mine online is the controller who will state, "Cross Oshawa VOR at 8,000 or below," without actually giving me an altitude to descend to. How low can I go in that case? A proper descent clearance would include an altitude, and, if required, a restriction to reach a certain altitude by a time or a fix, similar to what I gave in my example. But what if ATC is not online? What if they're online and they just say something like, "Air Canada 123 is cleared to the Sydney airport for an approach"?

You would commence your descent at an appropriate time when conditions permit. How low you go depends on several factors. As an IFR aircraft, you are required to observe a Minimum IFR Altitude at all times in flight. This means 1,000 feet above the highest terrain or obstacle within a 5 NM radius of the aircraft. To aid the pilot in this respect, minimum IFR altitudes have been established in many airspaces, and it's just a matter of picking the right one. For example, if you're descending on an airway, don't descend below the MEA, or Minimum Enroute Altitude, until you find another published altitude to allow you lower. For example, you could use a 100NM Safe Altitude if you're within 100NM from the fix designated, and this information is located on every approach plate for Canadian airports. Or you could use a "quadrantal" altitude from an NDB or other type of fix specified on an approach plate. This is generally the Final Approach Fix, and this is presented to you in a circle, typically in the lower right-hand portion of the approach plate. You have to ensure that you're within the distance specified (standard is 25 NM) from the fix named, and also that you're within a given quadrant for use of the specific altitude. If you transition from an airway to the localizer for an ILS using a DME arc, there are minimum segment altitudes published along the arcs as well. Have a look at the approach plate for the ILS 24 at CYHZ. I've circled the 100 and 25 NM safe altitudes.

Generally, you use an altitude that will serve your purpose. For example, the 100 NM Safe Altitude from the Fredericton, NB, NDB (FC) is something over 7,000 feet simply because it has to be above the highest terrain or obstacle within 100 NM of the fix. Not very useful when landing at an airport whose elevation is 67 feet. So look around on the IFR charts and approach plate you're using for other lower altitudes to get you down. There are airway altitudes that you can use if you're within the airspace for the airway and GASAs (Geographical Area Safe Altitudes, found on LO charts in Canada) that are based on widely spaced latitude and longitude blocks, as well as other sources. As long as you remain at or above an altitude until you find one that's lower, you're safe. Another trick we use in Moncton Center is the 100 NM safe altitudes of other nearby aerodromes. For example, because of the 100 NM Safes from YQI, YZX, YYG, and YQY, we know that everywhere in the Maritime provinces south of YFC to F9 (Fredericton to Miramichi, NB) 3,000 feet is safe. In fact, 2,800 feet is even acceptable within these areas except toward YQY. The key is finding a published altitude and ensuring you're in the airspace covered. You are not likely to have the time to review many approach plates to find an altitude that's nice and low thanks to a nearby airport while you're on approach to an entirely different one. This is where your flight planning comes into play: Using the IFR charts to review the locations of restricted airspace, high terrain and so forth while you're planning your route of flight. Many FIR websites contain downloadable files which include approach plates for the airports within the FIR.

While we're on the topic of altitudes, you may be given altitudes that are not published when you're being vectored by ATC. For example, on the ILS RWY 24 at Halifax International (CYHZ), The approach plate will show you that a safe sector altitude from the FAF is 2,000 feet, and the RNAV STARs will show an altitude of 3,100 feet from LEROS on final. ATC should be vectoring you to final for the ILS 24 at 2,200 feet. Where do they find this altitude? At airports where we vector often, we have Minimum Radar Vectoring Altitudes estabished. These MRVAs include all kinds of things neither controllers nor pilots have time (or information) to calculate. Things like obstacles and terrain, the base of controlled airspace, and even altitude corrections for cold weather altimeter errors are factored in where applicable. Last time I checked, radar coverage wasn't actually included in this definition directly, but it is considered. This is largely because varying weather conditions can change radar coverage significantly. I once saw a helicopter operating from a oil rig from 160 NM out. We had him in good, solid radar contact out of 100 feet because of a strong inversion (a weather pattern in which temperature contravenes the normal of getting colder with altitude and actually warms up instead). These MRVAs may even be lower than published altitudes, since the areas may be much more confined than other published altitudes. Terrain or obstacles 29 NM from the FAF that affect the 25 NM safe altitudes don't affect the radar vectoring area if it only covers 20 NM from the same fix, for example. Another factor which comes into play, especially in the example of the ILS 24 at CYHZ is the Glide Path Intercept altitude. On the approach plate, the aircraft on final will be on the glidepath over the FAF at an altitude of 2,180 feet. We have to ensure that we vector you at such an altitude that you intercept the glidepath at or outside the final approach fix. Hence the 2,200 foot altitude while being vectored for RWY 24.
 

I hope I've covered this topic in reasonable detail without too much reading. Anyone who knows me knows that I tend to be wordy at best. I hope you found this information helpful, and will return for the next topic, or perhaps even ask about something for yourself. If you have anything to say, good or bad, about my presentation, the information provided or lack thereof, or anything else, please feel free to e-mail me at moxner@nbnet.nb.ca . Once again, my goal is to help, and feedback always helps.