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A cracked windshield in flight will decrease the performance. To what extent can it disrupt the flight? Can flakes of glass be ingested on tail-mounted engines? Is it possible to blow out the windshield because of the high speed?

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The biggest restriction will be the reduced flight speed and cabin pressure which need to be observed with a damaged windshield. No pressurization will restrict the flight altitude to 10,000 ft or less. The crack will not increase drag, but force the pilot to continue flight at a less efficient speed and altitude.

Why high altitude is good for engine efficiency can be found here.

How to find the most efficient flight speed can be read here.

If pieces break off, the aircraft should not continue the flight, but land immediately.

To answer the other questions: Yes, sure, tail-mounted engines are known for ingesting stuff which has broken off from the fuselage. Ice from a leaking lavatory has caused engine damage and the imbalance made the engine shake so hard that it fell off. The whole windshield or substantial fractions of it could have this effect, but I doubt that flakes will damage the engine.

The aerodynamic pressure at the windshields tends to press them into the fuselage. Suction only starts above of the windshield with the curvature of the fuselage contour. However, with cabin pressurization added, the windshield is indeed forced out if it fails. One accident which was caused by using the wrong screws to secure the windshield is BA Flight 5390. The captain was sucked out of the opening left by the window and only his ankles, which got stuck in the flight controls, prevented him from falling out entirely.

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This depends on how the windshield is damaged. Aircraft windscreens are often multi-layer and can withstand cracks in the outermost layer. If this is the only damage then the flight can operate as normal and write up the airplane so that maintenance looks at it before returning the airplane to service. If the inner layers are cracked then the things mentioned in Peter's answer come into effect.

Specifically in the EMB-145 I operated:

  • if only the outer layer is cracked, no action is necessary
  • if the inner layer is cracked
    • descend to 10,000 or MEA and maintain pressurization at $\Delta$P at 1 PSI
    • If both windshields are impaired
      • depressurize
      • max airspeed 140 KIAS.

In the case the inner layer is cracked, this will likely turn into a diversion coordinated with maintenance to either swap an airplane or wait for repairs to be made. With both windshields cracked and completely obscured landing becomes a lot more interesting.

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