Operating speed limits for services

[santiago15]

Folks,

My, albeit limited, experience has shown that services such as gear and flaps have IAS operating limits but not mach number limits. Why is this, or is this just the types I have flown?

Thanks in advance

[refplus20]

G550 has 0.60M limit for flap and 0.70M for landing gear operations published in its limitations section.

[arba]

To lower the gear 270 KIAS or .82 Mach maximum.
For Flaps, may be because the Max. Operating Altitude, that only IAS is concerned.

[santiago15]

I did wonder whether the limits on the services would be a factor of EAS, and not mach. Given that at altitude, when mach no would become a player, your IAS would always be higher than your EAS thus negating the need for a mach limit. Seems I was wrong though, I suspect your answer, arba, ref Max. Operating Altitude is more along the right lines. Thanks

[Old Smokey]

santiago15,

All aircraft have both IAS (CAS) and Mach Number limits for Gear and laps/Slats. As arba has pointed out, the B737 has a 270 KIAS / M0.82 limit, interestingly the same on the somewhat faster B777.

As has been correctly pointed out, the IAS/CAS limit is representative of an EAS limit, but as we unfortunately don't have EAS indicators, IAS is the only available substitute. (Gear and Flaps are typically operated only at low altitudes where CAS and EAS, for all practical purposes, are the same).

Most manufacturers cunningly avoid having to impose a Mach Number limit on Flaps/Slats, by imposing a maximum operating altitude for them (commonly 20,000 feet in my experience), at, and below which the CAS limit is either equal to or less than the Mach Number limit. Somewhere above that maximum operating altitude Mach Number would be limiting if the maximum operating altitude were not imposed. Boeing do this, Douglas used to provide the Mach Number limit for Flaps/Slats as well as CAS - either system works.

Perhaps for the aircraft that you've flown, the maximum operating altitude is less than that at which Mach Number would be limiting. In arba's example, if the maximum operating altitude were less than 36,513 feet (where 270 CAS = M0.82), only the CAS limit would be required.

One of the reasons why the Mach Number limit is provided for the Gear, but not the Flaps/Slats, is to provide for the Gear being a secondary means of high speed drag during emergency descent.

Regards,

Old Smokey