MSFL

[Radista]

Now we all know that Minimum Safe Flight Level is the Saftey Altitude corrected for pressure, but what is the origin of this? I cannot find it in ICAO documentation - they talk about MSA (Minimum Sector Altitude) based upon NAVAIDS! Is it purely a military thing?:ugh:

[orgASMic]

AFAIK it is a military thing. JSP 552 201.195 refers. It is a safe and expeditious way of homing aircraft to the overhead/NAVAID with minimal fuss. It used to be called the MSFL (Initial Approach) which makes more sense. The lowest usable FL is at least 500ft above the Transition Level.

It is calculated to give pilots the required obstacle and terrain clearance within 25nm of the facility so, when you call for recovery we can give you the appropriate MSFL for the quadrantal and home you to the overhead without having to do any other pressure calculations. The various recovering callsigns are then already separated by 500ft (if coming from different quadrants) and can then be vectored for recovery according to the situation.

[Pontius Navigator]

RADISTA did orgasmic answer your question? MSFL also applies en route on any track and is especially useful in a drift-down situation.

[Radista]

Thanks to you both for the replies. The answer to PN is no, not really - I am still looking for an "official" publication if one exists.

[Pontius Navigator]

I find lots of references to parts of JSP 552 on Google; it should be possible to find the document. This is of course essentially terminal aspect rather than general off-route navigation.

The documentary trail originated with AMFOs - Air Ministry Flying Orders - a slim and readable document that specified safety altitude procedures and from that derived the minimum safe flight level.

That was replaced by a weightier tomb, MOD Flying Regulations or JSP 318. A befitted a tri-Service document it was over 3 times the size of of predecessor and much easier to ignore.

That in turn was replaced by the JSP 550 series for which you needed a H&S risk assessment and a secure (wouldn't fall over) book cabinet to store it. Then it went digital and became completely unaccessible.

The other source that should certainly have MSFL and the SA AP 3456 series.
RAF Cranwell - AP3456 (http://www.raf.mod.uk/rafcranwell/aboutus/ap3456.cfm)

[GOLF_BRAVO_ZULU]

Before throwing good money at AP3456, it only refers to MSFL and not its calculation;
Vol 5, Aircraft Operation.
TERMINAL PROCEDURES AND TECHNIQUES

Preparation for the Approach

77. A good approach at an airfield requires pre-flight study. Amongst other factors, aircrew must anticipate:
a. Speed limit points on the Terminal Approach Procedure (TAP).
b. Altitude check heights in the descent. Initial descent clearance may be given en route. Further descent clearance (down to minimum safe flight level or safety altitude) may be given in the terminal hold.

[FantomZorbin]

I seem to remember it was in the UK Air Pilot RAC section ... but that was a very very long time ago so creeping brain-rot may have got the better of me.

[orgASMic]

Radista

As far as I (a Royal Air Force ATCO) am concerned, Joint Service Publication 552 is very much an official publication.

I cannot find any mention of MSFL in the UK Aeronautical Information Publication (aka the Air Pilot and Civil Air Publication 032) on the NATS website, so it may well just be a British military thing, which is not unusual. I do not have a copy of UK Mil AIP to hand.

[Pontius Navigator]

The more I look the more I feel that MSFL is a taught military procedure and not a rule.

This defines the difference between altitude and level:

Transition altitude
At low altitudes the true height of an aircraft relative to an object on the ground needs to be known. For this reason, the transition altitude (TA) is defined. When operating at or below the TA, aircraft altimeters are set to show the altitude above sea level. The pressure setting to achieve this is called QNH or "altimeter setting" and is available from various sources, including air traffic control and the local METAR-issuing station.

The lowest usable flight level above the TA is called the transition level (TL). Because the transition altitude is fixed and the atmospheric pressure varies, the TL varies from time to time. It is therefore possible to have a valid flight level of 30 in the UK when the atmospheric pressure is above 1013.25 hPa. Note that vertical separation is not guaranteed between an aircraft flying at the transition altitude and one flying at the transitional level. For example, in the UK with a transition altitude of 3,000 ft and a QNH of 996, the Transition Level is FL35; equivalent to an altitude of less than 3,100 ft. (See Manual of Air Traffic Services Part 1 Appendix A)

Flights above transition altitude being directed by air traffic control are generally assigned flight levels. The vertical region extending from the TA to the TL is known as the transition layer.

but takes no account of terrain height. Under the rules you can simply climb above TA, set 1013, cruise at FL 30 or 35 and CFIT. To avoid the lumpy bits a prudent aviator calculated a Safety Altitude. SA is based on 'company' orders.

A different version may be Minimum Obstacle Clearance Altitude, or MOCA, is the lowest published altitude in effect between fixes on VOR airways, off-airway routes, or route segments that meets obstacle (like a building or a tower) clearance requirements for the entire route segment. Within the United States, this altitude also assures acceptable navigational signal coverage only within 22 nm of a VOR. The MOCA seen on the NACO en route chart, may have been computed by adding the required obstacle clearance (ROC) to the controlling obstacle in the primary area or computed by using a TERPS chart if the controlling obstacle is located in the secondary area. This figure is then rounded to the nearest 100 foot increment, i.e. 2,049 feet becomes 2,000, and 2,050 feet becomes 2,100 feet. An extra 2,000 feet is added in mountainous areas, 1,000 in non-mountainous. The MOCA is based upon obstacle clearance over the terrain or over manmade objects, adequacy of navigation facility performance, and communications requirements. The MOCA is always at or below the Minimum enroute altitude (MEA), and may put an aircraft below air traffic control RADAR coverage and also below Minimum reception altitude (MRA) for navigation aids; as a result, it is typically used only in emergencies, especially to get below icing.

and

Minimum safe altitude(MSA) is a concept used in planning and executing aircraft flights. It is an altitude which allows adequate vertical clearance from nearby terrain and manmade obstacles, and allows proper navigational functions.

So MOCA is a hard base. If you go below that flight stops.
MSA or SA is a greater altitude that will provide (or should provide) clearance from the terrain and allows for navigation errors and instrument and pressure errors. The actual SA is calculated using the rules. Some countries simply add 1000 feet to the MOCA for low terrain and 2000 feet for flight over mountains, the additional allowance is to cater for turbulence and mountain waves.

It follows that the MSFL for one operator may be different from that of another if the rules for calculating MSA/SA are different.

The rules in the RAF have varied between a planning assumption of 30 miles in early days and poor navigation aids down to 10 miles now. The rule was the highest ground plus 10 percent plus 1500 feet. This was then modified to the highest contour (or obstruction) plus contour interval plus 10% plus 1500 feet. As this SA is generally greater than TA it is necessary to calculate the lowest FL above SA and that is, by definition, MSFL. In other words, the key is company SA rules and MSFL simply follows from that.

PS

I found this for B1 Ops:

2.7.4.3. MSAs and RAAs will be a minimum of 1,000 feet above the highest obstacle/terrain (rounded up to the next 100 feet) within the lateral limits of the route or operational area, but in no case less than 5 NM either side of planned route corridor.

[Radista]

Thanks for all the input, really appreciated. I think I have enough now to move forward on this.:ok:

[[email protected]]

I believe the origin was from FJ teaching and had to do with assessing what minimum height to eject at following power loss IMC at altitude. Since the pilot above 3000' would be flying on 1013, it made sense to have predetermined what the safety altitude equated to on that pressure setting. It could equally be used as an absolute 'no-go below' altitude, again when faced with an emergency.

[Pontius Navigator]

Crab, 'no go below level' and I will buy that one.

On the regulatory aspects of safety altitude - the track width and safety margin - we see that the B1 allows for a 5 mile track error and a 1000 foot clearance.

When I was trained, when we had 'rapid' fixing aids, a fix every 6 minutes' and a groundspeed of 3 miles per minute 30 miles track error was realistic. At the same time a V-bomber used the same 30 miles when it had true rapid fixing and high-definition radar and a groundspeed of 4 miles per minute. This applied over the sea as well so we had a minimum SA of 1500 feet. The only way we could descend below SA was an instrument approach at an airfield.

In contrast a Nimrod clear of land was cleared to 300 feet. On an AEW Shack we had a similar clearance but less reliable aids and once almost penetrated a CuGr off Scotland - it wasn't on the Nav's chart.

[orgASMic]

Crab

MSFL has nothing to do with the safe minimum height for abandoning your aircraft. It is the lowest usable FL that ensures terrain separation within 25nm of the primary NAVAID at the airfield for which it is calculated.

Similarly, Safety Altitude (either en route or in a given sector around an airfield) is the lowest altitude that ensures terrain separation in that given area so that, in case of disorientation, you can hit the SA and not the ground.