View Full Version : Safety Altitude V MSA

Matt 999
28th Mar 2016, 17:17
Standards Doc 10 ver 7 asks the following:

Explain the meaning of the terms “safety altitude” and “minimum safe altitude” (MSA). Explain the relevance of safety altitude and/or MSA for a VFR flight.

I also came across the following in Instrument Flying by David Hoy pg 95

An Examiner's View - Safety Altitude

"Recent questioning of applicants has revealed that few have any understanding of IFR outside CAS. Specifically, the application of safety altitude is misunderstood. Many applicants are using the MEF or MSA figures on their charts for calculation of SA, rather than complying with the requirements of their operations manual (of which they also have little knowledge). The use of MEF or MSA might be sensible when executing an unplanned diversion, but is not acceptable for a pre-planned IFR flight"

So my questions are:

What is the SA in VFR
What is the SA in IFR
How is it calculated, and
What information is contained in the operations manual re an SA (specifically what and where is this information for the DA42 tdi)


RD Campbells's book which is clearly dealing with VFR states "the purpose of determining a safety altitude during the preparation stage is to be able to assess the minimum safe altitude if a decision to descend is required in the event of a lowering cloud base." pg 18-16 to 18-17

I can only guess that this factors in a 1000' clearance from land based objects i.e MEF +1000', (and if above 3,000amsl require a vertical distance from cloud of 1000, making a cloud base of 2000 agl a minimum) PLUS some self imposed safety margin.

However RD Campbell does not appear to say anything else on the matter.

Help in this matter would be appreciated.

28th Mar 2016, 21:19
What is the SA in VFRThere is no such thing, you avoid visually taking into account SERA.5005 Visual flight rules. You may wish to make additional allowance for obstacles in reduced visibility hence the concept of a Minimum Safe Altitude.
What is the SA in IFRHistorically it was one of the UK Rules of the Air which has now been replaced by SERA.5015 Instrument flight rules (IFR) — Rules applicable to all IFR flights
How is it calculated, (b) Minimum levels
Except when necessary for take-off or landing, or except when specifically authorised by the competent authority, an IFR flight shall be flown at a level which is not below the minimum flight altitude established by the State whose territory is overflown, or, where no such minimum flight altitude has been established:

(1) over high terrain or in mountainous areas, at a level which is at least 600 m (2 000 ft) above the highest obstacle located within 8 km of the estimated position of the aircraft;

(2) elsewhere than as specified in (1), at a level which is at least 300 m (1 000 ft) above the highest obstacle located within 8 km of the estimated position of the aircraft.
What information is contained in the operations manual re an SA (specifically what and where is this information for the DA42 tdi)That depends upon which company wrote the manual. It is not likely to be aircraft specific.

See also ORS No 1124 (http://publicapps.caa.co.uk/docs/33/ORS4No1124.pdf)

28th Mar 2016, 21:35
Minimum safe altitude is a bit of a misnomer, it is a confusion of Safety Altitude and minimum sector altitude. MSA is usually referecened to an airport and protects you within 25 miles of the fix, giving you 1000' clearance above the highest obstacle.

Safety altitude is route related (we use MORAs) and is 1000' above the highest obstacle (up to 5000' and 2000' above that) 10 miles either side of the route centreline and 10 miles beyond the end of the route segment.

28th Mar 2016, 23:05
Er I always understood 'MSA' referred to 'Minimum Sector Altitude' not Minimum Safe Altitude; on some TAPs this may be depicted as SSA - Sector Safe Altitude.
MSA/SSA in this context is the safety altitude in that quadrant ie 0 deg to 090 deg etc

29th Mar 2016, 09:59
No wonder there is confusion!

30th Mar 2016, 08:17
In the context of the question I think “safety altitude” and “minimum safe altitude” mean the same thing.

Certainly for PPL's on UK VFR charts I always get them to check terrain and obstacles up to ten miles either side and arc at departure and destination.
Add 300 feet for unknowns, then add 1000 feet to get a minimum safe altitude, then round up to nearest 100.
Then cross check with the CAA own maximum elevation figure MEF printed on chart.
MEF are ok for the IFR pilot (just add 1000 feet), but for VFR it's nice to know where the obstacles are (within the bounds of the lat log rectangle) as they can be good visual points.

Then I point that the student can fly at any altitude he wants below MSA so long as he doesn't break the low flying rules.
The MSA can then be used as minimum cloud base check to make a go-no go, turn back decision tool.

30th Mar 2016, 08:53
There is no regulatory definition of 'Safety Altitude' in the UK - it is a military concept that drifted into civil aviation many years ago and has since become generally accepted terminology. Guidance on the calculation of en-route Safety Altitude is included in the UK Military Rules of the Air (RA 2307).

The only relevant 'official' terminology in civil aviation is Minimum Sector Altitude (MSA), Minimum Obstacle Clearance Altitude (MOCA) and Minimum En-route Altitude (MEA), the definitions of which are all in ICAO Doc 8186 (PANS-OPS).

The calculation and use of 'Safety Altitude' in the civil world is at the discretion of individual operators and will vary accordingly. I have seen operations manuals that state that safety altitude is not relevant to VFR operations and others that require it to be calculated for all VFR cross-country flights.

Note: The above applies only to the UK and other states may have different terminology. For example, in the US Minimum Safe Altitudes are defined in 14 CFR 91.119

3rd Apr 2016, 07:06
We could argue about the semantics but I see a difference between deciding a minimum safe altitude to fly when considering a VFR flight (this would be used and compared to the forecast in order to decide whether or not to depart) and "safety altitude" when flying IFR. If you are planning to fly VFR both should be calculated and entered on the flight log notwithstanding that the pilot may or may not have an instrument qualification.

As an example when planning to fly VFR, assuming no flight over congested areas, with respect to the 500 ft rule the white contours on a half million chart could (possibly) have terrain at 499 feet with an obstruction on the summit 299 feet high (generally only obstructions at 300 ft or above agl are shown). For me that's near enough 800 feet plus 500 feet = 1,300 feet. You would also need to consider individual high masts etc. For inexperienced pilots (students) one might consider 1,000 feet as a more sensible add on. It also depends on how confident one is about maintaining track - 5 miles either side, 10 miles either side - you choose unless specified in Ops Manuals/Flight Order Books.

But for IFR operation, notwithstanding the low flying regs and/or flight over congested areas, we are, legally, required to be a 1,000 feet above the highest obstacle within 5 nm of the a/c. Once again how accurately can you navigate?

Of course there are other factors to consider such as wind over higher terrain/mountains etc.

In extremis a student/pilot without an instrument qualification may have to fly below the planned minimum altitude for VFR in order to maintain VFR but rarely should this occur if an up to date weather forecast is obtained before flight. However an instrument qualified pilot with a suitably equipped a/c would, generally speaking, climb to the IFR "safety altitude" to continue flight.