As a result, it is possible to establish a flight profile, with which the aircraft must always remain, taking into account the above-mentioned oxygen requirements. This profile depends on the installed oxygen system: 

Chemical system: Fixed profile (published in the FCOM). 

Gaseous system: Customized profile (depends on the number of oxygen bottles and obstacle location). 

This flight profile represents the maximum level that can be flown with respect to the oxygen system’s capability. As an example, the following Figure (D13) shows the descent profile of a 22 minute oxygen system. 

For example, the above profile shows that 7 minutes after the cabin depressurization, the aircraft must fly at or below FL250. 

3.1.3.2. Performance limitation 

The above descent profile only depends on the oxygen system’s capability, and not on the aircraft’s performance capability. 

Nevertheless, this doesn’t mean that the aircraft is always able to follow the oxygen profile, particularly in descent. As a consequence, the performance profile must be established, and this profile must always remain below the oxygen profile. The calculation is based on the following assumptions: 

.

Descent phase: Emergency descent at MMO/VMO. Airbrakes can be extended to increase the rate of descent, if necessary. 

Cruise phase: Cruise at maximum speed (limited to VMO). 

As a result, for a given initial weight and flight level, the oxygen profile, function of the time, is transformed into a performance profile, function of the distance (Figure D14). 

Note: When establishing this performance profile, it is always assumed that the aircraft is able to fly at MMO/VMO. Cases where speed should be decreased (structural damage, turbulence…) have not to be taken into account. 

3.1.4. Minimum Flight Altitudes 

The minimum flight altitudes must be selected as follows: 

“FAR 121.657 

(c) No person may operate an aircraft under IFR, […] in designated mountainous areas, at an altitude less than 2,000 ft above the highest obstacle within a horizontal distance of five miles from the center of the intended course.” 

“JAR-OPS 1.250 

(a) An operator shall establish minimum flight altitudes and the methods to determine those altitudes for all route segments to be flown […]. 

 

(b) Every method for establishing minimum flight altitudes must be approved by the authority.” 

To assist JAA operators in their choice, guidance material is provided in IEM OPS 1.250, where the most common definitions of published minimum flight altitudes are recalled: 

MOCA (Minimum Obstacle Clearance Altitude) and MORA (Minimum Off-Route Altitude). They correspond to the maximum terrain or obstacle elevation, plus: 

1,000 feet for elevation up to and including 5,000 feet (or 6,000 feet)1. 

2,000 feet for elevation exceeding 5,000 feet (or 6,000 feet) rounded up to the next 100 feet. 

MEA (Minimum safe En route Altitude) and MGA (Minimum safe Grid Altitude). They correspond to the maximum terrain or obstacle elevation, plus: 

1,500 feet for elevation up to and including 5,000 feet. 

2,000 feet for elevation above 5,000 feet and below 10,000 feet. 

10% of the elevation plus 1,000 feet above 10,000 feet. 

As a result, the minimum flight altitude above 10,000 feet considered acceptable to carry out studies, is equal to the highest obstacle elevation plus 2,000 feet. 

3.1.5. Obstacle Clearance – Cabin Pressurization Failure 

A net flight path is not required in the cabin pressurization failure case. The net flight path shall be understood as a safety margin, when there is a risk that the aircraft cannot maintain the expected descent performance (engine failure case). 

In case of cabin depressurization, any altitude below the initial flight altitude can be flown without any problem as all engines are running. Therefore, the standard minimum flight altitudes apply and the descent profile must, therefore, clear any obstacle by 2,000 feet (Figure D15). 

1 Depends on the method: Jeppesen (5,000 feet) or KSS (6,000 feet) 

4. ROUTE STUDY 

As a general rule, failures (engine or pressurization) must always be expected to occur at the most critical points of the intended route. Nevertheless, as descent profiles differ, the critical points may differ between the two failure cases. It is important to notice that regulations don’t require to consider performance tocope with both failures simultaneously.