MINIMUM SAFE ALTITUDE (MSA)

Consider at all times the Minimum Safe Altitude (MSA) of the terrain you are operating over, in relation to an engine-out case and the aircraft’s drift down profile down to the stabilising altitude, on every leg of a flight as you pass a waypoint, particularly as you approach mountainous terrain.

19.3.1 Departing

On departure, you must not only note the MSA to 25 nautical miles but must also consider the MSA all the way to the Top of Climb. For example, when departing Zurich for Milan, the 25 miles MSA is 6000 feet, becoming 14,100 feet at 29 miles, only 4 miles further along! It is therefore vital to consider the MSA beyond the 25 mile range of the SID plate, up to Cruise altitude.

19.3.2 En route

Remain similarly aware at all times of your MSA as you pass a Waypoint, particularly if there are mountains ahead as you travel towards high ground below you. A Drift-down and Stabilising altitude graph or table, should be readily available to hand at all times, so that it can be consulted immediately should an engine failure occur at any time. It should indicate the target IAS to fly at, with remaining engine(s) set to Maximum Continuous Power (MCP). Doing this will achieve the graph’s profile and show how rapidly the aircraft will descend immediately after the failure, before the descent flattens out and stabilising altitude is reached and also how far the stabilising altitude is, ahead of the aircraft.

This allows a decision to be made.

If the failure occurs immediately above the highest ground, say Mont Blanc when Northbound on the way to Northern Europe somewhere, with Geneva on the other side virtually in sight, then it is safe to continue past the mountain range.

Only then can you drift-down, arrive at stabilising altitude, provided it is above MSA after Mont Blanc, then continue down into the Geneva 25-mile MSA area on the other side. Now you could divert to it for an en-route landing, certainly if on a twin-engine aircraft. If the failure occurs well before Mont Blanc, depending on how far before, if stabilising altitude happens to be below MSA and will be reached before the mountain range, then a turn back is indicated for a landing at an airfield South of the Alps, such as Milan or Turin.

Remain aware that even with MCP selected on the live engine(s), the aircraft drift-down profile is not a straight line. It starts as a rather steep curve that flattens-out as stabilising altitude is approached. This needs to be considered whenever the distance to stabilising altitude data is consulted, when approaching a mountain range, to establish whether the aircraft can safely descend and successfully negotiate passing over the mountains, to stabilise at the MSA beyond it.

But there could well be an urgent need to turn back, because stabilising altitude is below MSA for the over-flight of the high ground that will be reached before getting to it.

Green-coloured MSA contour envelopes showing the safe altitude one can maintain, is a preferred method of showing high terrain, as developed by the British Airways Aerad (now Thales) team, many years ago. Within the envelope, is a 2 or 3 digit number such as 25 or 103, meaning the MSA is respectively, 2,500 or 10,300 feet high. This method of depicting terrain and how low one can safely fly above this, is approved in the ICAO Annex 4 Cartographers’ supplement, as an alternative to terrain contours and ‘spot’ heights, to which a pilot must then add the appropriate minimum clearance margins, to obtain the MSA. This latter presentation is more time consuming to use and less readily interpreted. It therefore is more pilot-unfriendly.

19.3.3 Arriving

A radar arrival chart, centred on a VOR in the terminal area close to or on the destination airfield, with MSA contours shown thereon, together with a 50 mile range 'dartboard' to show the VOR quadrants and 10 nautical mile range circles, is a most useful tool to confirm an aircraft position whilst approaching an airfield in a mountainous Terminal Area. Aircraft position can be established on a distance and bearing basis from the VOR, allowing the crew to readily plot position visually, to check a descent instruction and confirm it is safe to descend to the given altitude; thus keeping control of the descent profile on the flight deck. Yet again, the BA Aerad team developed it. An old example for Zurich airport is illustrated below.

Had such a chart been available in the Barcelona area many years ago, a Dan Air Comet 4 aircraft might not have

crashed in the mountains to the North of the airport. The pilots accepted an instruction to descend, in the mistaken belief that the controller had them on radar and knew it was safe for them to descend to the given altitude, at the time ATC gave the instruction.

19.3.4 Overflying high terrain

Basic MSA calculation: MSA shown includes an additional minimum safe over flight increment of 1000 feet added to the spot height of the obstacle, for obstacles up to 5,000 feet high, or a 2,000 feet increment, for spot heights above 5,000 feet AMSL, depending upon the height of the terrain.

For general application: The Basic calculation applies. To wit, where the terrain or obstacle is 5,000 feet above Mean Sea Level (AMSL) or lower, the MSA is 1,000 feet above, the highest terrain or obstacle within 20 nm of the route centre line. Where the terrain or obstacle is higher than 5,000 feet AMSL, the MSA is 2,000 feet or more above the highest terrain or obstacle within 20 nm of the route centreline.

For flight in Controlled Airspace: Where the Track is well defined by two separate aids, the MSA is 1,000 feet above the highest terrain or obstacle within 10 nm of the route centre line. Where the highest terrain or obstacle within 10 nm of the route centre line is higher than 5,000 feet AMSL, the MSA is 2,000 feet or more above that terrain or obstacle. When the sector length between navigational aids is such that the aircraft could be more than five nautical miles from the centre line due to inherent errors in the system used to define an airway, the limit of protection must be increased by the extent to which the divergence exceeds 5 nm.

For radar controlled flights within 25 nm of the aerodrome of departure or intended landing: The MSA is 1,000 feet above the highest terrain or obstacle within 5 nm of the intended track. Commanders are instructed to monitor all radar instructions by reference to other aids and are reminded that, when under radar control, it is their individual responsibility to ensure adequate terrain clearance. MSA within 25 nm of aerodromes are referred to as Minimum Sector Altitudes.

The use of Flight Guides: An operator may use MSAs and Minimum Sector Altitudes in a recognised Flight Guide, provided that the basis of the publisher’s calculations will give at least an equal standard to that required by this section.

If necessary, corrections can be made and promulgated in the Operations Manual, so that the prescribed vertical separation is maintained.

19.3.5 Corrections to planned MSAs for flights over high ground

When the selected cruising altitude, or flight level, or one-engine-inoperative stabilising altitude is at or close to the calculated minimum safe altitude and the flight is within 20 nm of terrain with a maximum elevation exceeding 2000 feet, any previously calculated MSA must be increased as follows:

HEIGHT INCREASE FOR FLIGHT OVER HIGH GROUND

Elevation of terrain Wind speed in Knots

0-30 31-50 51-70 Over 70

2000-8000 ft 500 ft 1,000 ft 1,500 ft 2,000 ft

Above 8000 ft 1,000 ft 1,500 ft 2,000 ft 2,500 ft

Operations Manuals must include a reference to the effect of mountain waves on the maintenance of vertical separation, instructing commanders to take suitable precautions when such conditions are reported or forecast. Relevant instructions must be included in the Manual.

Adequate allowances to calculated minimum safe altitudes must be made when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. When the ambient temperature is lower than International Standard Atmosphere (ISA) -15°C, the following additions to minimum safe altitude must be made:

Lower than ISA - 15°C Not less than 10%

Lower than ISA - 30°C Not less than 20%

Lower than ISA - 50°C Not less than 25%

Note: For any route, the maximum altitude obtainable with all power units operating, or the appropriate stabilising altitude with one or more engines inoperative must be greater than the calculated minimum safe altitude (MSA) for that route.

Antonov An-225 -Wikimedia Commons

19.4 ILLUSTRATIONS

19.4.1 A local German airport as example for limited MSA sectors NOT FOR OPERATIONAL USE

With thanks to Lufthansa Systems AG, Marketing Communications, Am Weiher 24, 65451 Kelsterbach, Germany, Tel. +49 (0)69-696 90000, Fax +49 (0)69-696 95959,

As mentioned on the chart, the MSA sector is not 25 nm but is limited by the boundary of the territory, which makes it sometimes very hard to determine.

19.4.2 EAG Aerad Terminal area Radar Arrival Chart with DME Dartboard & MSA Contour envelopes NOT FOR OPERATIONAL USE

The aircraft position (distance and bearing from the Kloten VOR) can be readily ascertained whilst approaching Zurich, by looking at the 10 nm grid of concentric circles radiating from the VOR and the quadrant radials shown every 20 . A clearance to descend can therefore be easily checked, to confirm that the MSA is such as to permit safe flight at the cleared altitude in the approaching area. The darker the green tint, the higher the MSA (which is shown as a two-figure group within the coloured contour).

MSA shown includes an additional minimum safe overflight increment of 1000 feet added to the spot height of the obstacle, for obstacles up to 5000 feet high, or a 2000 feet increment for spot heights above 5000 feet AMSL, depending upon the height of the terrain.

19.4.3 EAG Aerad - Instrument Approach chart with MSA contour envelopes NOT FOR OPERATIONAL USE

The darker the green tint, the higher the MSA, which is shown as a two figure group within the coloured contour. The larger digit indicates a safe altitude in thousands of feet (on the QNH altimeter setting, that is above Mean Sea Level) and the smaller appended number giving the hundreds of feet; for example, 23 (means 2,300 feet AMSL). The Minimum Sector Altitude and MSAs shown, include pre-calculated minimum safe overflight tolerances of 1000 or 2000 feet, depending upon the height of the terrain.

The 25 nm range Minimum Sector Altitude is given in the circle printed, in this case, on the left hand side of the let-down plate above, showing it to be 2,200 feet to the North-West, 2,100 feet in the South-West and 2300 feet to the East of the 180 / 360 delineator.

19.5 DRIFT- DOWN TO A STABILISING ALTITUDE AFTER ENGINE FAILURE